TO ALL TEACHERS. SCKOOZ. BOOKS. SMILEY'S GEOGRAPHY AND ATLAS, and SACRED AND ANCIENT GEOGRAPHY FOR SCHOOLS. The above works -will be foiind useful and very valuable as works of refer- ence, as well as for schools. The Maps, composing the Atlases, will be fouml equal in execution and correctness to those on the most extensive scale. The author has received numerous recommendations, among which are the fol- lowirg : Dear Sir — I have looked over your " Easy Introduction to the Study o/" GeogYaphy^''^ together with your " Improved Atlas.'''' I have no hesitation m declaring, that I consider them works of peculiar merit. They do honour to your industry, research, and talent, and I am satisfied, will facilitate the im- provement of the student in geographical science. With sentiments of sincere consideration, I am yours truly, WM. STAUGHTON, D. D. President of Columbia College^ District of Columbia, Mr. Thomas Smiley. Philadelphia, Sept. 1, 1823. Extract from the Minutes of the Philadelphia Academy of Teaehen. J^ovember 1, 1823. Resolved unanimously, That the Academy of Teachers highly approve the superior merits of Mr. Smiley's " Easy Introduction to the Study of Geogra- phy,'''' and the accompanying Atlas, and cordially recommend them to the pa- tronage of the public. B. MAYO, President. 1. 1. Hitchcock, Secretary, THE NEW FEDERAL CALCULATOR, or SCHOLAR'S ASSISTANT. Containing the most concise and accurate Rules for performiug the operations in common Arithmetic 5 together with numerous Examples under each of the Rules, varied so as to make them contormable to almost every kind of business. For the Use of Schools and Counting Houses. By Thomas T. Smilej, Teacher: author of An Easy Intro- duction to the Study of Geography. Also, of Sacred Geo- graphy for the Use of Schools. Among the numerous recommendations received to the work, are the following; Mr. John GricxG. Phila. March 8, 1825. Sir — I have examined with as much care as my time would admit, " The New Federal Calculator," by Thomas T. Smiley. It appears to me to be a treatise on Arithmetic of considerable merit. There are parts in Mr. Smiley 's work which are very valuable ; the rules given by him in Barter, Loss and Gdin, and Exchange, are a great desideratum in a new system or treatise on Arithmetic, and renders his book superior to any on the subject now in use ; and when it is considered that the calculations in the work are made in Fede- ral Money, the only currency now known in the United States, and that ap- propriate questions ioliow the difTprent rules, by which the learner can be ex- ercised as to his understanding of each part as he progresses ; I hesitate not to say, that, in my opinion, it is eminently calculated to promote instruction in the science on which it treats. Mr. Smiley deserves the thanks of the public and the encouragement of teachers, for his attempt to simplify and improve the method of teaching Arithmetic. I am yours respectfully, WM. P. SMITH, Preceptor of Mathematics and Natural Philosophy^ No. 132, South Tenth Street. Sir — I have carefully examined " The New Federal Calculator, or Scho- lar's Assistant," by Thomas T. Smiley, on which you politely requested my opinion ; and freely acknowledge that I think it better calculated for the use of the United States schools and counting-houses than any book on the sub- ject that I have seen. The author's arrangement of the four primary rules is, in my opinion, a judicious and laudable innovation, claiming the merit of im- provement ; as it brings together the rules nearest related in their nature and uses. His queetions upon the rules throughout, appear to me to be admirably calculated to elicit the exertions of the learner. But above all, the preference he has given to the currency of his own country, in its numerous examples, has stamped a value upon this little work, which I believe Ijias not fallen to the lot of any other book of the kind, as yet offered to the American public I am, sir, yours respectfully, JOHN MACKAY. Charleston, [S. C.) March 29, 1825. From the United States Gazette. Among the numerous publications of the present day, devoted to the im- provement of youth, ^^e have noticed a new edition of Smiley's Arithmetic, just published by J. Grigg. The general arrangement of this book is an improvement upon the Arith- metics in present use, being more systematic, and according to the affinities of different rules. The chief advantage of the present over the first edition, is a correction of several typographical errors, a circumstance which will render it peculiarly acceptable to teachers. In referring to the merits of this little work, it is proper to mention that a greater portion of its pages are devoted to Federal calculation, than is generally allows in primary works in thjs branch of study. The heavy tax of time and patience which our youth are now compelled to pay to the errors of their ancestors, by performing the various operations of pounds, shillings, and pence, should be remitted, and we are glad to notice that the Federal computation is becoming the prominent practice o^ school arithmetic. In recommending Mr. Smiley's book to the notice of parents and teachers, we believe that we invite their attention to a work that will really prove au " assistant" to them, and a "gwirfe" to their interesting charge. The Editors of the New York Telegraph, speaking of Smiley's Arithmetic, observe that they have within a few days attentively examined the above Aritlimetic, and say, " We do not hesitate to pronounce it an improvement upon every work of the kind previously before the public ; ' and as such, re- commend its adoption in all our Schools and Academies." A KEY to the above Arithmetic, in which all the Examples ne- cessary for a Learner are wrought at large, and also Solutions given of all the various Rules. Designed principally to facili- tate the labour of Teachers, and assist such as have not the opportunity of a tutor's aid. By T. T. Smiley, author of tlie New Federal Calculator, Sac. &c. TORREY'S SPELLING BOOK, or First Book for Children. I have examined Mr. Jesse Torrey's " Familiar Spelling Book." I think it a great improvement in the primitive, and not least important branches of education, and shall introduce it into the seminaries under my care, as one su- perior to any which has yet appeared. IRA HtLL, A. M. Boonsboroughi Feb. 2, 1825. The increasing demand for this work is the best evidence of its merits. A PLEASING COMPANION FOR LITTLE GIRLS AND BOYS, blending Instruction with Amusementj being a Selec- tion of Interesting Stories, Dialogues, Fables, and Poetry. Designed for the use of Primary Schools and Domestic Nur* series. By Jesse Torrey, jr. To secure the perpetuation of our republican form of government to future generations, let Divines and Philosophers, Statesmen and Patriots, imite their endeavours to renovate the age, by impressing the minds of the people with the importance of educating their little boys and girls. S. Adams. lY Report of the Committee of the Philadelphia Academy of Teachers : adopted J^ov. 6, 1824. The Committee, to whom was referred Mr. Jesse Torrey's " Pleasing, Companion for Little Girls and Boys," beg; leave to report, That they have perused the " Pleasing Companion," and have much plea- sure in pronouncing as their opinion, that it is a compilation much better cal- culated for the exercise and improvement of small children in the art of read- ing, and especially in the more rare art of understanding what they read, than the books in general use. All which is respectfully submitted. I. IRVINE HITCHCOCK, PARDOxN DAVIS, CHARLES MEAD, Committee. A true copy from the miHutes of the Academy. C. B. Trego, Secretary. Nw. 22, 1824. THE MORAL INSTRUCTOR AND GUIDE TO VIRTUE, by Jesse Torrey, Jr. Among the numerous recommendations to this valuable School Book, are the following : — Extract of a note from the Hon. Thomas Jefferson, late President of the United States. " I thank you, sir, for the copy of your ' Moral Instructor.^ I have read the first edition with great satisfaction, and encouraged its reading in my family." Extracts of a Letter from the Hon. James Madison^ late President qf the United States. " Sir — I have received your letter of the 15th, with a copy of the Moral Instructor. " I have looked enough into your little volume to be satisfied, that both the- original and selected parts contain information and instruction which may be useful, not only to juvenile but most other readers. " With friendly respects, JAMES MADISON. Dr. Torrey. From Roberts Vaux^ President of the Controllers of the Public Schools in Philadelphia. " The Moral Instructor" is a valuable compilation. It appears to be well adapted for elementary schools, and it will g^ve me pleasure to learn that the lessons which it contains are furnished for the improvement of our youth ge- nerally. Respectfully, , ROBERTS VAUX. Philadelphia, 5th month, 8 1823. HISTORY OF ENGLAND, from the First Invasion by Julius Caesar, to the Accession of George the Fourth, in eighteen hundred and twenty: comprising every Political Event worthy of remembrance; a Progressive View of Religion, Language, and Manners; of Men eminent for their Virtue or their Learn- ing; their Patriotism, Eloquence, or Philosophical Research; of the Introduction of Manufactures, and. of Colonial Esta- blishments. With an interrogative Index, for the use of Schools. By William Grimshaw, author of a History of the United States, &c. HISTORY OF THE UNITED .STATES, from their first settlement as Colonies, to the cession of Florida, in 1821: comprising every Important Political Event; with a Progres- sive View of the Aborigines; Population, Religion, Agricul- ture, and Commerce; of the Arts, Sciences, and Literature; occasional Biographies of the most remarkable Colonists, Writers, and Philosophers, Warriors, and Statesmen; and a Copious Alphabetical Index. By William Grimshaw, author of a History of England, &c. Also, QUESTIONS adapted to the above History, and a KEY, adapted to the Questions, for the use of Teachers. " University of Georgia, Athens, June 4, 1825. "Dear Sir, " With grateful pleasure, I have read the two small volumes of Mr. Grim- shaw, (a History of England, and a History of the United States) which you some time since placed in my hands. On a careful perusal of them, I feel no difficulty in giving m^ opinion, that they are both, as to style and sentiment, works of uncommon merit in their kind ; and admirably adapted to excite, in youthful minds, the love of historical research. " With sincere wishes for the success of his literary labours, " I am very respectfully, your friend, " M. Waddel, President. '^E. Jacksobt, Esa." D. Javpoit presents his respectful compliments to Mr. Grimshaw, and is VI much obliged by his polite attention, and the handsome compliment of his History of the United States with the Questions and Key. " Mr. J. has been in the use of this book for some time ; but anticipates still more pleasure to himself, and profit to his pupils, in future, from the help and facility which the questions and key will afford in tne study of these in- teresting pages. " October 10th, 1822." Golgotha, P. Edwd. Fa. Sep. 26, 1820. " Dear Sir, " Mr. Grimshaw's * History of the United States,' &c. was some time ago put into my hands by Mr. B , who requested me to give you my opinion as to the merits of the work. The history of the late war is well manag- ed by your author: it has more of detail and interest than the former part; and I consider it much superior to any of the many compilations on that subject, with which the public has been favoured. It may be said of the entire per- formance, that it is decidedly the best chronological series, and the chastest historical narrative, suited to the capacity of the juvenile mind, that has yet appeared. Its arrangement is judicious ; its style neat, always perspicuous, and often elegant ; and its principles sound. " American writings on men and things connected with America, have been long needed for the young ; and I am happy to find, that Mr. Grimshaw has not only undertaken to supply this want, but also to Americanise foreign his- tory for the use of our schools. In a word, sir, I am so fond of American fa- brics, and so anxious to show myself humbly instrumental in giving our youth American feelings and character whilst at school ; that 1 shall without hesita- tion recommend Mr. Grimshaw's Works to my young pupils, as introductory to more extensive historical reading. In fine, the work is so unobjectionable, end puts so great a mass of necessary information witliin the reach of school- boys, at so cheap a rate, that I fee] the highest pleasure in recommending it to the public, and wish you extensive sales. Yours respectfully, "William Branch, Jr. " Me. Bxhtjamin Warner, " Philadelphia." *'' History of the United States, from their first settlement as Colonips^ to the Peace of Ghent., Sec. By William Grimshaw, pp. 312, 12mo. " This is the third time, within the space of two years, that we have had occasion to review a volume from the hand of Mr. Grimshaw. He writes with gi-eat rapidity ; and improves as he advances. This is the most cor- rectly written of all his productions. We could wish that a person so well formed for close, and persevering study, as he must be, might find encourage- ment to davote himself to the interests of literature." " Mr. G. has our thanks for the best concise and comprehensive history of the United States which we have seen." Theological Review, October^ 1819. vu " History of England, from the first Invasion by Julius Cczsar^ to the Peace qf Ghent, Sec. For the use of Schools. By William Grimshaw. Philadel- phia, 1819. Benjamin Warner. 12mo. pp. 300. ■ We have copied so much of the title of this work, barely to express our wcided approbation of the book, and to recommend its general introduction into schools. It is one of the best books of the kind to be found, and is in- structive even to an adult reader. We should be pleased that teachers would I ;iak it among tlieir class-books ; for it is well calculated to give correct im- j)ressi6ns, to its readers, of the gradual progress of science, religion, govern- ment, and many other institutions, a knowledge of which is beneficial in the present age. Among the many striking merits of this book, are, the perspi- cuity of the narrative, and chasteness of the style. It is with no little pleasure we have learned, that the author has prepared a similar history of the United States; a work Icvng wanted, to fill up a deplorable chasm in the education of American youth." Analectic Magazine^ October^ 1819. •" Philadelphia^ 23 June, 1819. " Sir — I have read with pleasure and profit your History of England. I think it is written with perspicuity, chasteness, and impartiality. Well writ- ten history is the best political instructor, and under a government in which it is the blessing of the country that the people govern, its pages should be con- stantly in the hands of our youtli, and lie open to the humblest citizen in our wide-spread territories. Your book is eminently calculated thus to diffuse this important knowledge, and therefore entitled to extensive circulation ; which I most cordially wish. With much respect, " Your obedient servant, "IvANGDON ChEVBS. " William Grimshaw, Esa." A NEW METHOD OF TEACHING THE ART OF BOOK- KEEPING, by the use of 1. Necessary Definitions and Uni- versal Rules ; 2. Interrogatory Exercises, or Oral Journaliz- ing ; 3. Practical Exercises, accompanied by blank books and directions for using them ; 4. Instructions mr the adjustment and closure of the Leger, the re-opening of the accounts in the old books, and the transfer of them to new ones : ac- companied by a Key, by the assistance of which Instructors are enabled to teach this art with facility and success to youth of proper age and capacity, and adult persons to acquire a knowledge of it without the help of a teacher. . The whole comprised in fifteen Lessons, and the Rules and Instructions exemplified in two sets of books kept by Double Entry. To wliich are added, (in the Ke;y^,) Specimens showing the forma of the most important auxiliary Books connected, as such» witli the preceding sets. By I. Irvine Hitchcock, Accountant and Teacher. via The following are extracts of letters concerning the above entitled work by Accountants and Teachers of the first respectability and eminence in the prin- cipal cities of the United States. " I do not hesitate to pronounce Mr; Hitchcock's System of Book-keeping, in my opinion, superior to any other treatise I have yet sden." " We are convinced that in point of utility it is superior to any other that kas hitherto appeared." " It appears to me to excel in ease and perspicuity every other system with which I am acquainted, and I presume it needs only to be known to be gene- rally adopted in our schools." " It is preferable to any other treatise which has fallen under my notice." " We are decidedly of opinion, that it is better calculated to give correct in- struction in that most useful art than any other work which we have seen." " I give your work a decided preference over any other which has come to my knowledge." " It is so simple that most persons, even without an instructor, may, by di- ligence, acquire a competent knowledge of the art." " I deem it a valuable acquisition for the use of schools." "I freely say, that both the plan and the execution of the work (Hitch- cock's Book-keeping) have my entire approbation. I consider it a useful and valuable acquisition to our seminaries of learning, and highly worthy of their patronage. J. V. N. YATES, Acting Swperintendant of Common Schools^ for fhe State of New York." " This system has, after repeated trials and comparisons, been pronounced superior to any other heretofore published. The ease with which the learner finds all the complex parts of Book-keeping explained, has astonished the most incredulous, and banished the idea that a boy must attend for months to learn so simple, but necessary a branch of education," Literary Register, THE UNITED STATES SPEAKER, compiled by T. T. Smiley — preferred generally to the Columbian Orator and Scott's Lessons, and works of that kind, by teachers who have examined it. GOLDSMITH'S HISTORY OF GREECE, improved by Grimshaw, with a Vocabulary of the Proper Barnes con- tained in the work, and the Prosodial Accents, in conformity with the Pronunciation of Lempriere — with Questions and a Key, as above. GRIMSHAW'S ETYMOLOGICAL DICTIONARY AND EXPOSITOR OF THE ENGLISH LANGUAGE. CONVERSATIONS ON NATURAL PHILOSOPHY, IN WHICH TBB ZaZiSXHXENTS OF THAT SCXENOS ARE FAMILIARLY EXPLAINED. BY THE AUTHOR OF CONVERSATIONS ON CHEMISTRY, &C. WiTII CORRECTIONS, IMPROVEMENTS, AND CONSIDERABLE ADDITIONS. IN THE BODY OF THE VSTORK ; Appropriate (Questions, ants a (HSilomavs: BY DR. THOMAS P. JONES, PROFESSOR OF MECHANICS, IN THE FRANKLIN INSTITUTf OF THE STATE OF PENNSYLVANIA. PHILADELPHIA I PUBLISHED AND SOLD BY JOHN GRIGG, NO. 9 NORTH FOURTH STREET. Stereotyped by L. Johnson. 1823. £astern District of Pennsylvania^ to toit : Be it remembered, that, on the twenty-fourth day of April, in the Fiftieth year of the Independence of the United States of America, A. D. 1826, John Grigg, of the said District, hath deposited in this office the title of a book, the right whereof he claims as proprietor, ia the words fol- lowing, to wit : "' Conversations on Natural Philosophy, in which the Elements of that Sci- ence are familiarly explained. Illustrated with Plates. By the Author of Conversations on Chemistry, &zc. With Corrections, Improvements, and considerable Additions, in the Body of the Work; appropriate Questions, and a Glossary : By Dr. Thomas P. Jones, Professor of Mechanics, in the Franklin Institute, of the State of Pennsylvania." In conformity to the Act of the Congress of the United States, entitled, '* An Act for the Encouragement of Learning, by securing the Copies of Maps, Charts, and Books, to the Authors and Proprietors of such Copies, during the times therein mentioned ;" — And also to the Act, entitled, " An Act supplementary to an Act, entitled, ' An Act for the Encouragement of Learning, by securing the Copies of Maps, Charts, and Books, to the Au- thors and Proprietors of such Copies during the times tjjerein mentioned,' and extending the benefits thereof to the arts of designing, engraving, and etching, historical and other prints." D. CALDWELL, Clerk of the Eastern District of Pennsylvania. PREFACE. Notwithstanding the great number of books which are written, expressly for the use of schools, and which embrace every subject on which in- struction is given, it is a lamentable fact, that the catalogue of those which are well adapted to the intended purpose, is a very short one. Al- most all of them have been written, either by those who are without experience as teachers, or by teachers, deficient in a competent knowledge of the subjects, on which they treat. Every intelli- gent person, who has devoted himself to the in- struction of youth, must have felt and deplored, the truth of these observations. In most instances, the improvement of a work already in use, will be more acceptable, than one of equal merit would be, which is entirely new ; the introduction of a book into schools, being al- ways attended with some difficulty. The "Conversations on Chemistry,'' written by Mrs. Marcet, had obtained a well-merited celebrity, and was very extensively adopted as a school-book, before the publication of her " Con- versations on Natural JPhilosophy." This, also, has been much used for the same purpose ; but, the observation has been very general, among intel- ligent teachers, that, in its execution, it is very in- ferior to the former work. The editor of the edition now presented to the public, had undertaken to add to the work, ques- tions, for the examination of learners ; and notes, where he deemed them necessary. He soon found, however, that the latter undertaking would IV PREFACE. be a very unpleasant one, as he must have pointed out at the bottom of many of the pages, the de- fects and mistakes in the text ; whilst numerous modes of illustration, or forms of expression, which his experience as a teacher, had con- vinced him would not be clear to the learner, must, of necessity, have remained unaltered. He therefore determined to revise the whole work, and with the most perfect freedom, to make such al- terations in the body of it, as should, in his opi- nion, best adapt it to the purpose for which it was designed. Were the book, as it now stands, care- fully compared with the original, it would be found, that, in conformity with this determina- tion, scarcely a page of the latter, remains un- changed. Verbal alterations have been made, er- rors, m points of fact, have been corrected ; and new modes of illustration have been introduced, whenever it was thought that those already em- ployed, could be improved ; or when it was known, that, from local causes, they are not fami- liar, in this country. The editor feels assured, that, in performing this task, he has rendered the book more valuable to the teacher, and more useful to the pupil ; and he doubts not that the intelligent author of it, would prefer the mode which has been adopted, to that which was at first proposed. The judicious teacher will, of course, vary the questions according to circumstances; and those who may not employ them at all, as questions, will still find them useful, in directing the pupil to the most important points, in every page. The Glossary has been confined to such terms of science as occur in the work ; and is believed to include all those, of which a clear definition can- not be found in our common dictionaries. CONTENTS. CONVERSATION I. Page ON GENERAL PROPERTIES OP BODIES. 9 Introduction. — General Properties of Bodies. — Impenetrability. — Exten- sion. — Figure. — Divisibility. — Inertia. — Attraction.— Attraction of Cohe- sion. — Density. — Rarity. — Heat. — Attraction of Gravitation. CONVERSATION II. ON THE ATTRACTION OF GRAVITY. S Attraction of Gravitation, continued. — Of Weight. — Of the Fall of Bodies.- Of the Resistance of the Air. — Of the Ascent of Light BoJies. CONVERSATION III. ON THE LAWS OF MOTION. 32 Of Motion. — Of the Inertia of Bodies. — Of Force to produce Motion. — Di- rection of Motion. — Velocity, absolute and relative. — Uniform Motion. — Retarded Motion.— Accelerated Motion, — Velocity of Falling Bodies. — Momentum. — Action and Reaction equal. — Elasticity of Bodies. — Porosity of Bodies. — Reflected Motion^ — Angles of Incidence and Reflection. CONVERSATION IV. ON COMPOUND MOTION. 46 Compound Motion, the result of two opposite forces. — Of Curvilinear Motion, the result of two forces. — Centre of Motion, the point at rest, while the other parts of the body move round it. — Centre of Magnitude, the middle of a body. — Centripetal Force, that which impels a body towards a fixed central point. — Centrifugal Force, that which impels a body to fly from the centre. — Fall of Bolies in a Parabola. — Centre of Gravity, the point about which, the parts balance each other, A2 vi CONTENTSi \ Page • CONVERSATION V. ^ ON THE MECHANICAL POWERS. 54 .] Of the Power of Machines. — Of the Lever in general. — Of the Lever of the | first kind, having tlie Fulcrum between tlie 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 J the Inclined Plane. Of the Wedge. — Of the Screw. j CONVERSATION VI. i ASTRONOMY. 1 CAUSES OF* THE MOTION OF THE HEAVENLY BODIES. 70 \ \ Of the Earth's annual motion. — Of the Planets, and their motion. — Of the Diurnal motion of the Earth and Planets. ; 1 CONVERSATION VII. \ ON THE PLANETS. 80 j Of the Satellites and Moons. — Gravity diminishes as the Square of the Dis- \ tance. — Of the Solar System. — Of Comets. — Constellatioos, signs of the ^ Zodiac. — Of Copernicus, Newton, &c. 1 CONVERSATION VIII. ^ ON THE EARTH. 91 Of the Terrestrial Globe.— Of the Figure of the Earth.— Of the Pendulum. — Of the Variation of the Seasons, and of the Length of Days and Nights. — Of the Causes of tlie Heat of Summer. — Of Solar, Sideral, and Equal or Mean Time. CONVERSATION IX. j ON THE MOON. 108 \ Of the Moob's Motion.— Phases of the Moon.— Eclipses of the Moon.— ] Eclipses of Jupiter's Moons. — Of Latitude and Longitude. — Of the Tran&its J of the inferior Planets. — Of the Tides. | CONTENTS. VH Pagi CONVERSATION X. HTDROSTATICS. ON THE MECHANICAL PROPERTIES OF FLUIDS. 118 Definition of a Fluid. — Distinction between Fluids and Liquids. — Of Non- Elastic Fluids, scarcely susceptiblq of Compression. — Of the Cohesion of Fluids. — Of their Gravitation. — Of tlieir Equilibrium. — Of their Pressure. — Of Specific Gravity. — Of the Specific Gravity of Bodies heavier than "Water. — Of those of the same weight as Water. — Of those lighter than Water. — Of the Specific Gravity of Fluids. CONVERSATION XI. OF SPRINGS, FOUNTAINS, &C. 128 Of the Ascent of Vapour and the Formation of Clouds. — Of the Formation and Fall of Rain, &c — Of the Formation of Springs. — Of Rivers and Lakes. — Of Fountains. CONVERSATION XII. PNEUMATICS. ON THE MECHANICAL PROPERTIES OF AIR. 136 Of the Spring or Elasticity of the Air. — Of the Weight of the Air. — Experi- ments with the Air Pump. — Of the Barometer. — Mode of Weighing Air. —Specific Gravity of Air. — Of Pumps. — Description of the Sucking J'ump. —Description of the Forcing Pump. CONVERSATION XIII. ON WIND AND SOUND. 146 Of Wind in General. — Of tlie Trade Wind. — Of the Periodical Trade Winds. — Of the Aerial Tides. — Of Sound in General.— Of Sonorous Bo- dies. — Of Musical Sounds.— Of Concord or Harmony, and Melody. CONVERSATION XIV. ON OPTICS.' 157 Of Luminous, Transparent, and Opaque Bodies. — Of the Radiation of Light. — Of Shadows. — Of the Reflection of Light.— -Opaque Bodies seen only by Reflected Light. — Vision Explained. — Camera Obscura. — Image of Objects on the Retina. i Ylll CONTENTS. CONVERSATION XV. OPTICS — continued. OF THE ANGLE OF VISION, AND REFLECTION OF MIRRORS. 168 Angle of Vision. — Reflection of Plain Mirrors. — Reflection of Convex Mir- rors.— Reflection of Concave Mirrors. CONVERSATION XVI. ON REFRACTION AND COLOURS. 179 Transmission of Light by Transparent Bodies. — Refraction. — Refraction by the Atmosphere. — Refraction by a Lens. — Refraction by the Prism. — Of Colour from the Rays of Light. — Of the Colours of Bodies. CONVERSATION XVII. ON THE STRUCTITRE OF THE EYE, AND OPTICAL INSTRUMENTS. 195 Description of the Eye. — Of the Image on the Retina.— Refraction by the Humours of the Eye.— Of the use of Spectacles. — Of the Single Micro- scope. — Of the Double Microscope.— Of the Solar Microscope.— -Magic Lanthorn.— Refracting Telescope. — Reflecting Telescope. Glossary, 205 CONVERSATION I. ON GENERAL PROPERTIES OF BODIES. IJNTRODUCTIOW. — GENERAL PROPERTIES OF BODIES. — IMPENETRABILITY. — EXTENSION. — FIGURE. — DIVISIBILITY. — INERTIA. — ATTRACTION. — ATTRACTION OF COHESION. — DENSITY. — RARUTY. — HEAT. — ATTRAC- TION OF GRAVITATION. EMILY. I MUST request your assistance, my Dear Mrs. B. in a charge which I have lately undertaken: it is that of > instructing my youngest sister, a task, which I find proves more difficult than I had at first imagined. I can teach her the common routine of children's lessons tolerably well ; but she is such an inquisitive little creature, that she is not satisfied without an explanation of every difficulty that occurs to her, and frequently asks me questions which I am at a loss to answer. This morning, for instance, when I had explained to her that the world was round like a ball, instead of being flat as she had supposed, and that it was surrounded by the air, she asked me what supported it. I told her that it required no support ; she then inquired why it did not fall as every thing else did ? This I confess perplexed me ; for I had myself been satisfied with learning that the world floated in the air, without considering how unnatural it was that so heavy a body, bearing the weight of all other things, should be able to support itself. Mrs. B. I make no doubt, my dear, but that I shall be able to explain this difficulty to you ; but I believe that it would be almost impossible to render it intelligible to the -comprehension of so young a child as your sister Sophia. You, who are now in your thirteenth year, may, I think, with great propriety, learn not only the cause of this particular fact, but acquire a general knowledge of the laws by which the natural world is governed. Emily. Of all things, it is what I should most like to learn; but I was afraid it was too difficult a study even at my age. Mrs. B. Not when familiarly explained : if you have patience to attend, I will most willingly give you all the information in my power. You may perhaps find the subject rather dry at first ; 10 GENERAL PROPERTIES OF BODIES. but if I succeed in explaining the laws of nature, so as to make you understand them, I am sure that you will derive not only instruction, but ffreat amusement from that study. Emily. I make no doubt of it, Mrs. B.; and pray begin by explaining why the earth requires no support; for that is the point which just now most strongly excites my curiosity. Mrs. B. My dear Emily, if I am to attempt to give you a general idea of the laws of nature, which is no less than to intro- duce you to a knowledge of the science of natural philosophy, it will be necessary for us to proceed with some degree of regu- larity. I do not wish to confine you to the systematic order of a scientific treatise, but if we were merely to examine every vague question that may chance to occur, our progress would be but very slow. Let us, therefore, begin by taking a short survey of the general properties of bodies, some of which must necessarily be explained before I can attempt to make you understand why the earth requires no support. When I speak of bodies, I mean substances, of whatever na- ture, whether solid or fluid ; and matter is the general term used to denote the substance, whatever its nature be, of which the different bodies are composed. Thus, tlie wood of which this table is made, the water with which this glass is filled, and the air which we continually breathe, are each of them matter. Emily. I am very glad you have explained the meaning of the word matter, as it nas corrected an errroneous conception I had formed of it : I thought that it was applicable to solid bodies only. Mrs. B. There are certain properties which appear to be common to all bodies, and are hence called the essential or inh^ rent properties of bodies ; these are Impenetrability, Extensioii, Figure, Divisibility, Inertia and .Attraction. These are also called the general properties of bodies, as we do not suppose any body to exist without them. By impenetrability is meant the property which bodies have of occupying a certain space, so that where one body is, another can not be, without displacing the former ; for two bodies can not exist in the same place at the same time. A liquid may be more easily removed than a solid body; yet it is not the less substan- tial, since it is as impossible for a liquid and a solid to occupy the same space at the same time, as for two solid bodies to do so. For instance, if you put a spoon into a glass full of water, the water will flow over to make room for the spoon. Emily. I understand this perfectly. Liquids are in reality 1. What is intended by the term bodies? 2. Is the term matter^ restrict- ed to substances of a particular kind ? 3. Name those properties of bodies, which are called inherent. 4. What is meant by impenetrability ? 5. Can a liquid be said to be impenetrable? GENERAL PROPERTIES OF BODIES. 11 as substantial or as impenetrable as solid bodies, and they ap- pear less so, only because they are more easily displaced. Mrs. B, The air is a fluid differing in its nature from liquids, but no less impenetrable. If I endeavour to fill this phial by plunging it into this bason of water, the air, you see, rushes out of the phial in bubbles, in order to make way for the water, for the air and the water can not exist together in the same space, any more than two hard bodies ; and if I reverse this goblet, and plunge it perpendicularly into the water, so that the air will not be able to escape, the water will no longer be able to fill the goblet. Emily. But it rises some way into the glass. Mrs. B. Because the water compresses or squeezes the air into a smaller space in the upper part of the glass ; but, as long as it remains there, no other body can occupy the same place. Emily. A difficulty has just occurred to me, with regard to the impenetrability of solid bodies ; if a nail is driven into a piece of wood, it penetrates it, and both the wood and the nail occupy the same space that the wood alone did before ? Mrs. B. The nail penetrates between the particles of the wood, by forcing them to make way for it ; for you know that not a single atom of wood can remain in the space which the nail occupies ; and if the wood is not increased in size by the addition of the nail, it is because wood is a porous substance, like sponge, the particles of which may be compressed or squeez- ed closer together ; and it is 'thus that they make way for the nail. We may now proceed to the next general property of bodies, extension. A body which occupies a certain space must neces- sarily have extension ; that is to say, length, breadth and depth or thickness ; these are called the dimensions of extension : can you form an idea of any body without them ? Emily. No; certainly I can not; though these dimensions must, of course vary extremely in diflferent bodies. The length, breadth and depth of a box, or of a thimble, are very different from those of a walking stick, or of a hair. But is not height also a dimension of extension ? Mrs B. Height and depth are the same dimension, consider- ed in different points of view ; if you measure a body, or a space^ from the top to the bottom, you call it depth ; if from the bottom 6. How can you prove that air is impenetrable ? 7. If air is impenetra- ble, what causes the Water to rise some way into a goblet, if I plunge it iiito water with its mouth downward? 8. When I drive a nail into wood, do not both the iron and the wood occupy the same space? 9. In how many direc- tions is a body said to bave extension ? ] 0. How do we distinguish the terms height and depth? 12 GENERAL PROPERTIES OF BODIES. upwards, you call it height ; thus the depth and height of a box are, in fact, the same thing. Emily. Very true; a moment's consideration would have enabled me to discover that ; and breadth and width are also the same dimension. Mrs. B. Yes; the limits of extension constitute figure or shape. You conceive that a body having length, breadth and depth, can not be without form, either symmetrical or irregular ? Emily. Undoubtedly ; and this property admits of almost an infinite variety. Mrs. B. Nature has assigned regular forms to many of her productions. The natural form of various mineral substances IS that of crystals, of which there is a great variety. Many of them are very beautiful, and no less remarkable by their trans- parency or colour, than by the perfect regularity of their forms, as may be seen in the various museums and collections of natu- ral history. The vegetable and animal creation appears less symmetrical, but is still more diversified in figure than the mine- ral kingdom. Manufactured substances assume the various arbitrary forms which the art of man designs for them ; and an infinite number of irregular forms are produced by fractures and by the dismemberment of the parts of bodies. Emily. Such as a piece of broken china, or glass ? Mrs. B. Or the masses and fragments of stone, and other mine- ral substances, which are dug out of the earth, or found upon its surface ; many of which, although composed of minute crystals., are in the lump of an irregular form. We may now proceed to divisibility; that is to say, a suscep- tibility of being divided into an indefinite number of parts. Take any small quantity of matter, a grain of sand for instance, and cut it into two parts ; these two parts miglit be again divided, had we instruments sufiiciently fine for the purpose ; and if by means of pounding, ginnding, and other similar methods, we car- ry this division to the greatest possible extent, and reduce the body to its finest imaginable particles, yet not one of the parti- cles will be destroyed, but will each contain as many halves and quarters, as did the whole ^rain. The dissolving of a solid body in a liquid, affords a very striking example of the extreme divisibility of matter ; when you sweeten a cup of tea, for instance, with what minuteness the sugar must be divided to be diffused throughout the whole of the liquid. 11. What constitutes \\\e Jigure^ or form of a body? 12. What is said respecting the form of minerals? 13. What of the vegetable and animal creation? 14. What of artificial, and accidental forms? 15. What is meant by divisibility? 16. What examples can you give, to prove that the parti- cles of a body are minute in the extreme? GENERAL PROPERTIES OF BODlSS» IS Emily, And if you pour a few drops of red wine into a glass of water, they immediately tinge tlie whole of the water, and must therefore be diffused throughout it. Mrs. B, Exactly so ; and tlie perfume of this lavender water will be almost as instantaneously diffused throughout the room, if I take out the stopper. Emily. But in this case it is only the perfume of the laven- der, and not the water itself that is diffused in the room. Mrs. B. The odour or smell of a body is part of the body itself, and is produced by very minute particles or exhalations which escape from the odoriferous bodies. It would be impossi- ble that you should smell the lavender water, if particles of it did not come in actual contact with your nose. Emily. But when I smell a flower, I see no vapour rise from it; and yet I perceive the smell at a considerable distance. Mrs. B. You could, I assure you, no more smell a flower, the odoriferous particles of which did. not touch your nose, than you could taste a fruit, the flavoured particles of which did not come in c(mtact with your tongue. Emily. That is wonderful indeed ; the particles then, which exhale from the flower and from the lavender water, are, I sup- pose, 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 evapo- rate 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 : not- is there any reason to suppose that in nature an atom is ever annihilated. Emily. Yet, when a body is burnt to ashes, part of it, at least, appears to be effectually destroyed : look how small is the resi- due of ashes in the fire place, from all the fuel which has been consumed within it. Mrs. B. That part of the fuel, which you suppose to be de- stroyed, evaporates in the form of smoke, and vapour, and air, whilst the remainder is reduced to ashes. A body, in burning, undergoes no doubt very remarkable changes; it is generally subdivided; its form and colour altered; its extension increased: but the various parts, into which it has been separated by com- 17. What produces the odour of bodies? 18. How do odours exemplify the minuteness of the particles of matter? 19. Can matter be in any way anni- hilated ? 20. What becomes of the fuel, which disappears in our iii-es ? B 14 GENERAL PROPERTIES OF BODIES. bustion, continue in existence, and retain all the essential pro- perties of bodies. Emily. But that part of a burnt body which evaporates in smoke has no figure ; smoke, it is true, ascends in columns into the air, but it is soon so much diffused as to lose all form ; it becomes indeed invisible. Mrs. B. Invisible, I allow ; but we must not imagine that what we no longer see no longer exists. Were every parti- cle 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 : this is a prin- ciple you must constantly remember. Every thing in nature decays and corrupts in the lapse of time. We me, 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 or in- activity ; this \n)rd expresses the resistance which matter makes to a change from a state of rest, to that of motion, or from a state of motion to that of rest. Bodies are equally incapable of chang- ing 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 mo- tion; 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, arises from its inertia. Emily. In playing at base-ball I am obliged to use all tny strength to give a rapid motion to the ball; and when I have to catch it, 1 am sure I feel the resistance it makes to being stopped. But if I did not catch it, it would soon fall to the ground and stop of itself. Mrs. B. Matter being inert it is as incapable of stopping of itself as it is of putting itself into 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 unacquainted, we can not at present investigate its effects. The last property which appears to be common to all bodies is attraction. All bodies consist of infinitely small particles of matter, each of which possesses the power oi attracting or draw- ing towards it, and uniting with any other particle sufficiently 21. How can that part which evaporates, be still said to possess a substan- tial form? 22. What do we mean hy inertia? 23. Give an example to prove that force is necessary, either to give or to stop motion. 24. What ge- neral power do the particles of matter exert upon other particles .'' GENERAL PROPERTIES OF BODIES. 15 near to be within the influence of its attraction ; but in minute particles this power extends 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. With- out this power, solid bodies would fall in pieces, or rather crum ble to atoms. Emily. I am so much accustomed to see bodies firm and so- lid, that it never occurred to me that any power was requisite to unite the particles of which they are composed. But the attrac- tion of cohesion dpes not, I suppose, exist in liquids; for the particles of liquids do not remain together so as to form a body, unless confined in a vessel ? Mrs. B. Fbeg your pardon; it is the attraction of cohesion which holds this drop of water suspended at the end of my fin- ger, and keeps the minute watery particles of which it is com- posed unitedr But as this power is stronger in proportion as the particles of bodies are more closely united, the cohesive attrac- tion of solid bodies is much greater than that of fluids. The thinner and lighter a fluid is, the less is the cohesive at- traction of its particles, because they are further apart ; and in elastic fluids, such as air, there is no cohesive attraction among the particles. Emily. That is very fortunate ; for it would be impossible 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 essen* 20 GENERAL PROPERTIES OF BODIES. a level with the ground, as it does in the case of a liquid, t!ie cohesive attraction of which is not sufficient to enable it to resist the power of gravity. Emily, And some solid bodies appear to be of this nature, as sand, and powder for instance : there is no attraction 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 at- traction of cohesion ; but amongst the separate grains there is no sensible attraction, because they are not m sufficiently close con- tact. Emily. Yet they actually touch each other ? Mrs. B, The surface of bodies is in general so rough and uneven, that when in apparent contact, they touch each other only by a few points. Thus, when I lay this book upon the table, the bmding of which appears perfectly smooth, so few of the par- ticles of its under surface come in contact with the table, that no sensible degree of cohesive attraction takes place ; for you see that it docs not stick or cohere to the table, and I find no diffi- culty in lifting it off. It is only when surfaces, perfectly flat and well polished, are placed in contact, that the particles approach in sufficient num- ber, and closely enough, to produce a sensible degree of cohesive attraction. Here are two plates of polished metal, I press their" flat surfaces together, having previously interposed a few drops of oil, to fill up every little porous vacancy. Now try to sepa- rate them. 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 plates merely owing to the attrac- tion of cohesion ? Mrs, B. There is no force more powerful, since it is by this lliat the particles of the hardest bodies are held together. It would require a weight of several pounds to separate these plates. In the present example, however, much of the cohesive force is due to the attraction subsisting between the metal and the oil which is interposed; as ^vithout this, or some other fluid, the points of contact would still be comparatively few, although we may have employed our utmost art, in giving flat surfaces to the plates. Emily. In making a kaleidoscope, I recollect that the two plates of glass, which were to serve as mirrors, stuck so fast to- gether, that I imagined some of the gum I had been using had by chance been interposed between them; but I am now con- 49. Why do not two bodies cohere, when laid upon each other ? 50. Can two bodies be made sufficiently flat to cohere with considerable force? — 51. What is the reason that the adhesion is greater when oil is interposed? GENERAL PROPERTIES OF BODIES. 21 vinced that it was their own natural cohesive attraction which produced this effect. Mrs. B, Very probably it was so ; for plate -glass has an ex- tremely smooth, flat surface, admitting of the contact of a great number of particles, when two plates are laid upon each other. Emily, But, Mrs. B., the cohesive attraction of some sub- stances is much greater than that of others ; thus glue, gum and paste, cohere witn singular tenacity. Mrs. B. Bodies which differ in their natures in other respects, differ also in their cohesive attraction ; it is probable that there are no two bodies, the particles of which attract each other with precisely the same force. There are some other modifications of attraction peculiar to certain bodies; namely, that of magnetism, of electricity, and of affinity, or chemical attraction ; but we shall confine our atten- tion merely to the attraction of cohesion and of gravity ; the ex- amination of the latter we shall resume at our next meeting. 52. What other modifications of attraction are there, besides those of cohe- sion and of gi-avitation? 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. Mrs. B. Very willingly ; but I did not think you had any taste for studies of this nature, Caroline. Caroline. I confess, Mrs. B., that hitherto I had formed no very agreeable idea either of philosophy, or philosophers; but what Emily has told me has excited my curiosity so much, that 1 shall be nighly 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 scho- lar as Emily; I know that you are much biased in favour of your own opinions. Caroline. Then you will have the greater merit in reforming 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 ob- jections ; but these. I shall willingly admit, as they will afford an opportunity of elucidatin*> the subject. Emily, do you recol- lect tne 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 pro- perties common to all bodies, and of which they cannot be de- prived ; all other properties of bodies are called accidental, be- cause they depend on the relation or connexion of one body to another. 1. What are those properties of bodie? called, which are not common to all? ON THE ATTRACTION' OF GRAVITY. 25 Caroline. Yet surely, Mrs. B. there are other properties which are essential to bodies, besides those you have enumerated. Colour and weight, for instance, are common to all bodies, and 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 tliis table weigh lieavier 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 uiwn their connexion witli each other. Weight is an eftect of the power of attractionj' without which the table and the book would have no weight whatever. Emily. I think I understand you; it is the /attraction of gra- vity which makes bodies heavy. Mrs. B. You are right. I told you that the attraction of gra- vity was proportioned to the quantity of matter which bo dies con- tain: now the earth consisting of a much greater quantity of matter than any body upon its surface, tlie force of its attrac- tion must necessarily be greatest, and must draw every thing so situated towards it; in consequence of whicli, bodies that ai-e unsupported fall to the ground, whilst those that are supported, press upon the object wliich 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 bo- dies, produces their weight also. It was very dull in me not to understand this before, as it is the natural and necessary conse- quence of attraction; but the idea that bodies were not really heavy of themselves, appeared 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 univer- sal space, what would its weight be ? Caroline. That would depend upon its size; or more accu- rately speaking, upon the quantity of matter it contained. 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 2. Why are they so called ? 3. What is the cause of weight in bodies ? 4. What is the reason that all bodies near to the surface of the e*rth, are drawn towards it? 24 ON THE ATTRACTION OF GRAVITY. particles of which the body was composed, would possess the power of attraction ; but thej could exert it only amongst them- selves ; the whole mass havmg nothing to attract, or to be at- tracted 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 to object to co- lours, Mrs. B.; you will not, I think, deny that they really exist in the bodies themselves. Mrs. B. When we come to treat of the subject of colours, I trust that I shall be able to convince you, that colours are like- wise 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 curi- ous to know how that is possible. Mrs. B. Unless we proceed with some degree of order and method, you will in the end find yourself but little the wiser for all you learn. Let us therefore go on regularly, and make our- selves well acquainted with the general properties of bodies be- fore we proceed further. Brnily. To return, then, to attraction, ^which appears to me by far the most interesting of them, since it belongs equa^y 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 matter which bodies contain, and if you consider the difference 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 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. What an idea, Emily! How can the houses and churches be moved, when they are so firmly fixed in the ground ! Mrs. B. Emily's question is not absuid, and your answer, Ca- 5. If attraction is the cause of weight, could you suppose it possible for a body to possess the former and not the latter property? 6. When a stone falls to the ground, in which of the two bodies does the power of attraction exist? 7. If the attraction be mutual, why does not the earth approach the stone, as much as the stone approaches the earth ? 8. If attraction be in proportion to the mass, why does not a hill, draw towards itself, a house placed near it? ON THF. ATTRACTION OF GRAVITY. 25 roline, 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 begin to suspect that bricklayers and carpenters could give but little stability to their buildings, without the aid of at- traction. 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 by the earth, as to resist every other impulse; otherwise they would necessarily move towards the hills and the mountains; but the lesser force must yield to the greater. There are, however, some circumstances in which I 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 mountain ; and this is owing to the lateral, or sideways attraction 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. Mrs. B. Attraction, you must recollect, is in proportion to the quantity of matter, and although that of the mountain, is mucl> less than that of the earth, it may yet be sufficient to act sensi- bly upon the plumb-line which is so near to it. Caroline. Pray Mrs. B. do the two scales of a balance hang • parallel to each other? ! Mrs. B. You mean, I suppose, in other words to inquire whe- ther two lines which are perpendicular to the earth, are parallel to each other ? I believe I guess the reason of your question ; but I wish you would endeavour to answer it without my assist- ance. ^; 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 explained ; you see now the use of your knowledge of parallel lines: had you been ignorant of their pro- perties, 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 natu- , ral philosophy; and if after I have made you acquainted with 9. How can the attraction of a mountain be rendered sensible? 10. Why cannot two lines which are perpendicular to the surface of the earth be pa- '( rallel to each other ? c 26 ON THE ATTRACTION OF GRAVITY. the first elements, you should be tempted to pursue the study, I would advise you to prepare j^ourselves by acquiring some know- ledge 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 perpendicular to the earth, appear parallel r Mrs, jB. Because the sphere is so large, and the scales conse- quently converge so little, that their inclination is not percepti- ble to our senses ; if we could construct a pair of scales whose beam would extend several degrees, their convergence would be very obvious ; but as this cannot be accomplished, let us draw u small figure of the earth, and then we may make a pair of scales of the proportion we please, (fig. 1. pi. I.) Caroline. This figure renders it very clear: then two bodies cannot fall to the earth in parallel lines ? Mrs. B. Never. €aroli7ie. The reason that a heavy body falls quicker than a light one, is, I suppose, because the earth attracts it more {strongly. Airs. 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 everyday see heavy bodies fall quickly, and light bodies slowly? Emily. It strikes me, as it does Caroline, that as attraction is proportioned to the quantity of matter, the earth must necessa- rily attract a body which contains a great quantity more strongly, and therefore brin^ 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 bodies have no natural ten- dency to fall, any more than to rise, or to move laterally, and that they will not fall unless impelled hy some force; now this force must be proportioned to tlie 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 par- ticles. 11. Draw a small figure of the earth to exemplify this, as in fig. 1. plate 1. Plate J-MJ. 2. I'i^. J. # ON THE ATTRACTION OF GRAVITY. 27 Caroline. I do not understand that; for it seems to me, that the heavier a body is, the more easily and readily it falls. Emily. I think I now comprehend it; let me try if I can ex- plain 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 r And if the eartli draws a body of lOOOlbs. weight 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. ? 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 conclusion is absurd; experience every instant contradicts it ; observe how much sooner tliis book reaches the floor than this sheet of paper, when I let them drop together. Emily. That is an objection I cannot answer. I must refer it to you, Mrs. B. ^ Mrs. B. I trust that we shall not find it insurmountable. It is true that, according to the laws of attraction, all bodies at an equal distance from tne earth, should fall to it in the same space of time; and this would actually take place if no obstacle inter- vened 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 all force their way through the air, but dense heavy bodies over- come this obstacle more easily than rarer or lighter ones; be- cause in the same space they contain more gravitating parti- cles. The resistance which the air opposes to the fall of bodies 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 12. If bodies were not resisted by the air, those which are light, would fall as quickly as those which are heavy, how can you account for this ? 13. What then is the reason that a book, and a sheet of paper, let fall from the same height, will act reach the ground in tlie same time ' 28 ON THE ATTRACTION OF GRAVITY. feather of the same size ; yet the one seems to meet with no ob- struction in its fall, whilst the other is evidently resisted and sup- ported 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. Airs. 13, Certainly : observe the manner in which this sheet of paper falls ; it floats awhile in the air, and then gently de- scends 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 resistance : see how much more rapidly it falls. The heaviest bodies ma^ be made to float awhile in the air, by making the extent of their surface counterbalance their weight. Here is some gold, which is one of the most dense bodies we are acquainted with ; but it has been beaten into a very thin leaf, and offers so great an extent of surface in proportion 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 a thin slate sinks in water more slowly tiian a round stone. But, Mrs. B., if the air is a real body, is it not also subjected lo the laws of gravity.^ Mrs. B, Undoubtedly. ' Caroline. Then why does it not, like all other bodies, fall to the ground ? Mrs. B. On account of its spring or elasticity. The air IS an elastic fluid ; and the peculiar property of elastic bodies is to resume, after compression, their original dimensions; and jou must consider the air of which the atmosphere is composed as existing in a state of compression, 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 con- stant tendency to expand itself, so as to resume tne dimensions it would naturally liave, if not under the influence of gravity. The air may therefore be said constantly to struggle with the power of gravity without being able to overcome it. Gravity thus confines the air to the regions of our globe, whilst its elasti- city prevents it from falling, like other bodies, 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 14. What then will be the effect of mcreasing the surface of a body.' — 13. What coulJ you do to a sheet of paper, to make it fall quickly, and why .^ 16. Inform me how a very dense body may be made to float in the air? — 17. The air is a real body, why does it not fall to the ground -" ON THE LAWS OF MOTION. 33 regard to motion or rest, it follows that a body cannot move without beinff put into motion ; the power which puts a body into motion is called/orce",' thus the stroke of the hammer is tlie force which drives the nail ; the pulling of the horse that which draws the carriage, &c. Force th^n is the cause which produces motion. Emily. And may we not say that gravity is the force which occasions the fall of bodies f Mrs. B. Undoubtedly. I have given you the most familiar illustrations in order to render the explanation clear ; but since you seek for more scientific examples, you may say that cohesion is the force which binds the particles of bodies together, and heat that which drives them asunder. The motion of a body acted upon by a single force, is always in a straight line,' and in the direction in which it received the impulse. Caroline. That is very natural ; for as the body is inert, 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 depend upon the degree of force with which it is impelled. Mrs. B. Yes ; the rate at which a body moves, or the short- ness 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 abso- lute and relative velocity. 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 compared 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 witn that of another body ; thus ijt one ship sail three times as far as another ship in thfe same space of time, the velocity of the former is equal to three times that of the latter. 3. What is the consequence of inertia, on a body at rest ? 4. What do we call that which produces motion ? 5. Give some examples. 6. What may we say of gravity, of cohesion, and of heat, as forces ? 7. How will a body move, if acted on by a single force ? 8. What is the reason of this ? 9. What do we intend by the term velocity, and to what is it proportional ? 10. Ve- locity is divided into absolute and relative; what is meant by absolute velo- city ? 11. How is relative velocity distinguished? 54 ON THE LAWS OF MOTION. Mrs, B, The general rule may be expressed thus : the velo- city 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 ? E7mly. I must divide the space, which is one hundred miles, bjr 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 hundred miles, divided by the velocity, five miles per hour, gives twenty hours for the time ? Mrs, B. Certainly; and we may say also that the space is equal to the velocity multiplied by the time. Can you tell me, Caroline, how many miles you will have travelled, if your velo- city is three miles an hour, and you travel six hours ? Caroline. Eighteen miles; for the product of 3 multiplied by 6, is 18. Mrs. B. I suppose that you understand what is 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. B. 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 ball. Caroline. But the motion of a ball is not uniform; its velocity gradually diminishes till it falls to the ground. Mrs. JB. Recollect that the ball is inei-t, and has no more pow- er to stop, than to put itself in motion ; if it falls, therefore, it must be stopped by some force superior to that by which it was projected, and which destroys its motion. Caroline. And it is no doubt the force of gravity which coun- teracts and destroys that of projection ; but ifthere were no such power as gravity,, would the ball never stop? Mrs. B. If neither gravity nor any other force, such as the resistance of the air, opposed its motion, the ball, or even a stone thrown by the hand, would proceed onwards in a right line, and with a uniform velocity for ever. 12. How "30 we measure the velocity of a body? 13. The time? 14. The space ? 15. What is uniform motion ? and give an example. 16. How is uniform motion produced ? 17. A ball struck by a bat gradually loses it* motion; what causes produce this effect i* ON THE LAWS OF MOTION, 35 Caroline. You astonish me ! I thought that it was impossible to produce perpetual motion ? Mrs. B. Perpetual motion cannot be produced by art, be- cause gravity ultimately destroys all motion that human power 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. B. 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 r«nember, will be proportional to the force with which it is projected ; but if there is nothin* 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 tlie harmony of the system of the universe, as the prevalence of it on the surface of the earth, would to the destruction of all our comforts. The wisdom of Providence has therefore or- dained insurmountable obstacles to perpetual motion here below; and though these obstacles often compel us to contend with great difficulties, yet these appear necessary to 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 diminislied 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 stop to rest when you are tired ; but inert matter is incapable of any feeling of fatigue, can never slacken its pace, and never stop, unless retarded or arrest- ed in its course by some opposing force ; and as it is the laws of inert bodies of which mechanical philosophy treats, I prefer your 18. If gravity did not draw a projected body towards the earth, and the resistance of the air were removed, what would be the consequence ? 19. In this case would not a great degree of force be required to produce a continued motion ? 20. What is retarded motion ? 21 . Give some examples. 36 ON THE LAWS OF MOTION. t' illustration of the stone retarded in its ascent. Now Emily, it is your turn; what is accelerated motion? Emily. Accelerated motion, I suppose, takes place when the. velocity of a body is increased ; if you iiad not objected to our giving such active bodies as ourselves as «xaniples, 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 re- tarded by gravity. Mrs. Ja. Not in all cases ; for the power of gravitation some- times produces accelerated motion; for instance, a stone falling from a height, moves with a regularly accelerated motion. Emily. True ; because the nearer it approaches the earth, the more it is attracted by it. Mrs. B. You bave mistaken the cause of its accelerated motion; for though it is true that the force of gravity increases as a body approaches the earth, the difference is so trilling 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 velocity is continually increased. When a stone falls from a height, the impulse which it receives from gravitation in the first instant of its fall, would be sufficient to bring it to the ground with a uniform velocity: for, as we have observed, a body having been once acted upon by a force, will continue to move with a uniform velocity; but the stone is not acted upon by gravity merely at the first instant of its fall ; this power continues to im- pel it during the whole time of its descent, and it is this continu- ed impulse which accelerates its motion. Emily. I do not quite understand that. Mrs. B. Let us suppose that the instant after you have let a stone fall from a high tower, the force of gravity were annihilated; the body would nevertheless continue 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 ob- stacle 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. Emily. That is very clear. Mrs. B. Then y-ou have only to add the power of gravity con- stantly acting on tne 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 at the very first instant of its descent, will continue in force every instant, till it reaches 22. What is accelerated motion? 23. Give an example. 24. Explain the mode in which gravity operates in producing this effect. ON THE LAWS OF MOTIOiV. 37" 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 follovting 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 preceding im- pulses continue, whilst gravity constantly adds new ones, and thus the velocity is perpetually increased. Mrs. B, Yes ; it has been ascertained, both by experiment, and calculations which it would be too difticult for us to enter into, that heavy bodies near the surface of the earth, 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 tlurd second, seven times in the fourth, and so on, regularly in- creasing their velocities in the proportion of the odd numbers 1, 3, 5, 7, 9, &c. 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 in ascending, that it is in descending ? Mrs. B. Exactly; in ascending, the velocity is diminished by the force of gravity; in descending, it is accelerated by it. Caroline. I should then imagine that it would fall, quicker than it rose? Mrs. B. You must recollect that the force with which it is pro- jected, must be taken into the account ; and that this force is overcome and destroyed by gravity, before the bodv begins to fall. Caroline. But the force of projection given to a stone in throw- ing 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 impulse ^iven to the stone is optional; 1 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 de- scent are equal. But supposing 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 throwing a body upwards, is always equal to the action of the force of gravity 25. VV hat number of feet will a heavy body descend in the first second ci' its fall, and at what rate will its velocity increase ? 26. \Miat is tlie differ- ence in the time of the ascent and descent, of a stone, or other body thrown upwards ? 27. By what reasoning is it proved that there is no difference f D S8 ON THE LAWS O^ MOTION. during its descent ; and that whether the body rises to a greater or less distance, these two forces balance each other. I must now explain to you what is meant by the momentum of bodies. It is the force, or power, with which a body in mo- tion, strikes against another body. The momentum of a body is the product oi its quantity of matter, multiplied by its quantity of motion-, in other words, its weight multiplied by its velocity. Caroline. The quicker a body moves, the greater, no doubt, must be the force which it would strike against another body. Emily, Therefore a light body may have a greater momen- tum than a heavier one, provided its velocity be sufficiently in- creased ; for instance, the momentum of an arrow shot from a bow, must be greater than that of 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 therefore difficult to understand, that the whole power, or momentum of a body, must be composed of these two properties, its weight and its velocity: but I do not understand why they should be multiplied, the one by the other; I sliould have supposed that the quantity of mat- ter, should have been added to the quantity of motion ? Mrs. R. 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 by 6, as would, be the case, were tliese figures added, instead of being multiplied together. Emily. I think that I now understand the reason of this ; if tlie quantity of matter is increased three-fold, it must require three times the force to move it with the same velocity; and then if we wish to give it three times the velocity, it will again require three times the force to produce that effect, which is three times three, or nine; which number therefore,* would represent the momentum. Caroline. I am not quite sure that I fully comprehend what is intended, when weight, and velocity, are represented by num- bers alone ; I am so used to measure space by yards and miles, and weight by pounds and ounces, that I still want to associate them together in my mind. Mrs. B. This difficulty will be of very short duration : you have only to be careful, that when you represent weights and velocities by numbers, the denominations or values of the weights and spacesj'^must not be changed. Thus, if we estimate the weight of one body in ounces, the weight of others with which it is com- pared, must be estimated in ounces, and not in pounds : and in 28. What is meant by the momentum of a body ? 29. How do we ascertain the momentum ? aO. How may a li^'ht body have a greater momentum than one wliich is heavier ? 31 . Why must we muUi'ply the weight and velocity together in order to find the momentum ? ON THE LAWS OF MOTION. 39 like manner, in comparing velocities, we must throughQut, pre- serve the same standards both of space and of time ; as for in- stance, the number of feet in one second, or of miles in one hour. Caroline. I now understand it perfectly, and think that I shall never forget a thing which you have rendered so clear. Mrs. B. I recommend it to you to be very careful to remem- ber the definition of the momentum of bodies, as it is one of the most important points in mechanics : you will find that it is from opposing velocity, to quantity of matter, that machines derive their powers. The reaction of bodies, is the next law of motion which I must explain to you. ^'hen 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 reaction 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 strike another on the face with his fist, he surely does not receive as much pain by the reaction, 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 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 conse- quently destroyed. Emily. I 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 communicated to the second body, is lost by the first ; but this loss proceeds from the reaction 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. You see none of the balls except the last, appear to move, this flies off as far as the first ball fell; can you explain this ? 32. When we represent weight and velocity by numbers, what must we carefully observe ? 33. Why is it particularly important, to understand the nature of niomentum ? 34. What is meant by reaction, and what is the rule respecting it? 35. How is this exemplified by the ivory balls represented in plate 1. fig. 3.'' 40 ON THE LAWS OF MOTIttN, 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 reaction of which set it at rest; the action of the third ball must have been destroyed by the reaction of the fourth, and so on till motion was communicated to the last ball, which, not being reacted upon, flies oif. 3Ir§. B. Very well explained. Observe, that it is only when bodies are elastic, as these ivory balls are, and when their masses are equal, that the stroke returned is equal to the stroke given, and that the striking body loses all its motion. I will show you the diifer- ence with these two balls of clay, (fig. 5.) which are not elastic; when you raise one of these, D, out of the perpendicular, and let it fall against the other, E, the reaction of the latter, on account of its not being elastic, is not sufficient to destroy the motion of Jhe former; only part of the motion of D will be communicated to E, and the two balls will move on togetiier to d and e, which IS not so great a distance as that through which D fell. Observe how useful reaction is in nature. Birds in flying strike the air with their wings, and it is the reaction of the air, which enables them to rise, or advance forwards p reaction being always in a contrary direction to action. Caroline. I thought that birds mi^ht be lighter than the air, when their wings were expanded, and were by that means ena- bled to fly. Mrs. Jb. When their wings are spread, this does not alter their weight, but they are better supported by the air, as they c€ver a greater extent of surface; yet 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 reaction of the air, may be able to support that weight; the bird will then lemain stationary. If the stroke of the wings is greater than is required merely to support the bird, the reaction of the air will make it rise ; iV it be less, it will gently descend ; and you may have observed the lark, sometimes remaining with its wings ex- tended, but motionless; in this state it drops quietly into its nest. Caroline. Tliis is indeed a beautiful effect of the law of reac- tion ! But if flying is merely a mechanical operation, Mrs. B., 36. Explain the manner in which the six balls represented in fig. 4, illus- trate thi3 fact. 37. What must be the nature of bodies, in which the whole motion is communicated from one to the other ? 38. What is the result if the balls are not elastic, and how is this explained by fig. 5? 39. How will reaction assist us in explaining the flight of a binl? 40. How must their ■\vings operate in enabling them to remain stationary, to rise, and to descend.^ 0*r THE LAWS OF MOTION. 41 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 attempted, but never with success ; and it is now considered as totally im- practicable. The muscular power of birds, i^ incomparably greater in proportion to their weight, than that of man ; were we there- fore 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, to that on the air, in flying; in rowing, also, you strike the water with the oars, in a direction opposite to that in which the boat is re- quired to move, and it is the reaction of the water on the oars which drives the boat along. Emily. You said, that it was in elastic bodies only, that the whole motion of one body, would be communicated to another; pray what bodies are elastic, besides the air ? Mrs. B. In speaking of the air, I think we defined elasticity to be a property, by means of which bodies that are compressed, return to their former state. If I bend 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 ma3e. Of all bodies, the air is the most eminent for this property, and it has thence obtained the name of an elastic fluia. Hard bodies are in the next degree elastic ; . if two ivory, or hardened steel balls are struck together, the parts at which they touch, will be flattened; but their elasticity will make them instantaneously 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; you cannot, it is true, perceive any mark, because their elasticity instantly destroys all trace of it. Soft bodies, which easily retain impressions, such - Vclay, wax, tallow, butter, &c. have very little elasticity; but of all descrip- tions of bodies, liquids are the least elastic. Emily. If sealing-wax were elastic, instead of retaining the impression of a seal, it would resume a smooth surface, as soon as the weight of tlie seal was removed. But pray what is it that produces the elasticity of bodies? Mrs. B. There is great diversity of opinion upon that point, 41. Why cannot a man fly by the aid of wings? 42. How does reaction operate in enabling us to swim, or to row a boat ? 43. What constitute* elas- ticity ? 44. Give some examples. 45. What name is given to air, and for what reason? 46. What hard bodies are mentioned as elastic? 47. Do elastic bodies exhibit any indentation after a blow? and why not'' D 2 ' 42 ON THE LAWS OF MOTIO^. and I cannot pretend to decide which approaches nearest to the truth. Elasticity implies susceptibility of compression, and the susceptibility of compression depends upon the porosity of bo- dies ; for were there no pores or spaces between the particles of matter of which a body is composed, it could not be com- pressed. Caroline. That is to say, that if the particles of bodies were as close together as possible, tliey could not be squeezed closer. Emily. Bodies then, whose particles are most distant from each other, must be most susceptible of compression, and conse- quently most elastic ; and this you say is the case with air, which is perliaps 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 emi- nent for this property, tnough the latter are certainly of much greater density than the former; elasticity implies, therefore, not only a susceptibility of compression, but depends upon the power possessed by the body, of resuming its former state after com- pression, in consequence of the peculiar arrangement of its par- ticles. Caroline. But surely there can be no pores in ivory and me- tals, Mrs. B.; how then can they be susceptible of compression? 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 porous ; 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 believe, that the p OS COMPOUND MOTION 4$ 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 ex- tremities of the vanes of a windmill move over a much greater space, than the parts nearest the axis of motion ? (plate 3. fi». 1.) The three dotted circles represent the paths in which three differ- ent parts of the vanes move, and though the circles are of differ- ent dimensions, each of them is described in the same space of time. Caroline. Certainly they are; 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 draws a body towards a centre*, round which it moves, is called the centripetal force; and that force, which impels a body to fly from tlie centre, is called the centrifugal force; when a body revolves round a centre, these two mrces constantly balance each other; otherwise the revolv- ing body would either approach the centre or recede from it^ according as the one or the other prevailed. Caroline. When I see any body moving in a circle, I sha] remember, that it is acted on by two forces. Mrs. B. Motion, either in a circle, an ellipsis, or any otlu 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. 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 ri^ht line from the centre ; but in a ri^ht line in the direction in which it was moving, at the instant of its release ; if a stone, whirled round in a sling, gets loose at the point A, (plate 3. fig. 2.) it flies off in the direction A B ; this line is called a tangent, it touches the circumference 13. How is this explained by fig. 1. plate 3 ? 14. What are the two forces called which cause a body to move in a curve ; and what proportion do these two forces bear to each other when a body revolves round a centre ? 15. If the centripetal force were destroyed, how would a body be carried by the centrifugal ? E 50 ON COMPOUND MOTION. of the circle, and forms a right angle with a lirie drawn from that point of the circumference to the centre of the circle C. Emily. You say, that motion In a curve -line, is owing to two forces acting upon "a body; but when I throw this ball in a hori- zontal direction, it describes a curve-line in falling; and yet it is only acted upon by the force of projection; there is no centripe- tal force to confine it, or produce compound motion. Mrs. B. A ball thus thrown, is acted upon by no less than three forces ; the force of projection, which you communicate to it ; the resistance of the air through which it passes, which dimi- nishes its velocity, without changmg 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 tlian any force of projection we can give a body, the latter is gradu- ally 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 observe, describe 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 projection is strongest when the ball is first thrown; this force, as it proceeds, being weakened by the continued resistance of the air, the stone, therefore, begins by moving in a horizontal direction ; but as the stronger powers prevail, 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 difterent 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 suppose it would, it falls somewhere about E. Caroline. It is precisely so ; look Emily, as I throw this ball directly upwards, how gravity and the resistance of the air con- quer projection. Now I will throw it upwards obliquely: see, the force of projection enables it, for an instant, to act in oppo- sition to that of gravity ; but it is soon brought down again. Mrs. B. The curve-line which the ball has described, is call- ed in geometry a parabola; but when the ball is thrown perpen- dicularly upwards, it will descend perpendicularly ; because the 16. Explain what is meant by a tangent^ as shown in fig. 2. plate 3. — 17. What forces impede a body thrown horizontally ? 18. Give the reason why a body so projected, falls in a curve, (fig. 3. plate 3.) ON COMPOUND MOTION. 51 force of projection, and that of gravity, are in the same line of direction. i r • u We have noticed the centres of magnitude, and of motion ; but I have nat jet explained to you, what is meant by the centre of gravity; it is that point in a body, about which all the parts ex- actly balance each other; if therefore that point be supported, the bodv will not fall. Do you understand this r £mily. I tliink 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 the body supported in any other single point ? Caroline. The surrounding parts no longer balancing eacli other, the body, I suppose, would fall on the side at which tlie parts are heaviest. Mrs. B. Infallibly ; whenever the centre of gravity is unsup- ported, the body must fall.. This sometimes happens with an overloaded wagon winding up a steep hill, one side of the road being more elevated than the other ; let us suppose it to slope as is described in this figure, (plate 3. 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 overset ; and 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 there- fore not supported by them. Caroline. I understand that perfectly; but what is the mean- ing of the other point B ? Mrs. B. Let us, in imagination take oif the upper part of ihe load ; the centre of ^'avity 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 wagon 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 npint D is but just within the left wheel ; ^f the riglit wheel was raised, by merely passing over a stone, the point D would be thrown on the outside of the left wheel, and the wagon would upset. Caroline. A wagon, or any carriage whatever, will then be 19. The curve in which it falls, is not a part of a true circle : what is it de- nominated ? 20. What is the centre of gravity defined to be? 21. What re- sults from supporting, or not supporting the centre of gravity? 22. What is intended to be explained by fig. 4. plate 3 ? 23. What would be the effect of taking off the upper portion of the load? $2 ' ON CaMPOUND MOTION. most firmly supported, when the centre of gravity falls exactly between the wheels ; and that is the case in a level road. Mrs. B. The centre of gravity of the human body, is a point somewhere in aline extending perpendicularly through the mid- dle of it, and as long as we stand upright, this point is supported by the feet; if you lean on one side, you will find that you no longer stand firm. A rope-dancer performs all his feats of agility, by dexterously supporting his centre of gravity; whenever he finds that he is in danger of losin» his balance, he shifts the hea- vy pole which he holds in his nands, in order to throw the ueight towards the side that is deficient; and thus by changing Ihe situation of the centre of gravity, he restores his equilibrnim. Caroline. When a stick is' poised on the tip of the finger, is It not by supporting its centre of gravity? Airs. B. Yes ; and it is because the centre of gravity is not -supported, that spherical bodies roll down a slope. A sphere be- ing perfectly round, can touch the slope but by a single point, aT^d that point cannot be perpendicularly under the centre 6f ^Tavity, and therefore cannot be supported, as you will perceive hy examining this figure, (fig. 5. plate 3.) Emily. So it appears : yet I have seen a cylinder of wood roll up a slope ; how is that contrived ? Mrs. B. It is done by plugging or loading one side of the cylinder with lead, as at B, (fig. 5. plate 3.) 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 or near the lead, as that substance is much heavier than wood ; now you may ob- serve that should this cylinder roll down the plane, as it is here situated, the centre of gravity must rise, which is impossible ; the centre of gravity must always descend in moving, and will descend by the nearest and readiest means, which will be by forcing the cylinder up the slope, until the centre of gravity is supported, and then it stops. Caroline. The centre of gravity, therefore, is not always in the middle of a body. Mrs. B. No, that point we have called the centre of magni- tude ; when the body is of an uniform density, and of a regular form, as a cube, or sphere, the c^tres of gravity and of magni- tude are in the same point ; but when one part of the body isx composed of heavier materials than another, the centre of gravity can no longer correspond with the centre of magnitude. Thus 24. When will a carriage stand most firmly ? 25. What is said of the cen- tre of gravity of the human body, and how does a rope dancer preserve his equilibrium ? 26. Why cannot a. sphere remain at rest on an inclined plane ? (fig. 5. plate 3.) 27. A cylinder of wood, may be made to rise to a small dis- tance itp an inclined plane. HoV may this be effected? (fig. 5. plate 3.) ON COMPOUND MOTION. 53 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 of sub- I stance, as wood, stone, or lead, and whose densities are conse- quently uniform, must stand more firmly, and be more difficult to overset, than bodies composed of a variety of substances, of dif- ferent ^nsities, which may throw the centre of gravity on one side. • Mrs, B, That depends upon the situation of the materials ; if those which are most dense, occupy the lower part, the stabili- ty will be increased, as the centre of gravity will be near the base. 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 a little inclined, their centre of gravity is no longer supported, as you may perceive in fig. 6. Caroline, I have often observed with what difficultya person carries a single pail of water ; it is owing, I suppose, to the cen- tre of gravity being thrown on one side; and tlie opposite arm is stretched out to endeavour to bring it back to its original situa- tion; but a pail hanging to each arm is carried with less difficulty, because they balance each other, and the centre of gravity re- mains supported 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 an inflexible rod, tli*.y are to be considered as form- ing but one body ; if the two bodies be of equal weight, the cen- tre of gravitj 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 the weights were unequal, you would hold it nearest the greater weight, to make them balance each other. Emily. And in both cases we should support the centre 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. 28. When do we find the centres of gravity, and of magnitude in different points ? 29. "What influence will the density of the parts of a body exert up- on its stability ? 30. What other circumstance materially affects the firmness of position ? (fig. 6. plate 3.) 31. Why is it more easy to carry a weight in each hand, than in one only ? 32. What is said respecting two bodies united by an inflexible rod? 33. What is fig. 7, plate 3, intended to iUustrate.' Whatfig. 8;whatfig. 9? E2 CONVERSATION V. ' ON THE MECHANICAL POWERS. O.B THE POWER OF MACHINES. — OF THE LEVER IN GENERAL. — OF THE LEVER OF THE FIRST KIND, HAVING THE FULCRUM BETWEEN THE POWER AND THE WEIGHT. — OF THE LEVER OF THE SECOND KIND, HAVING THE WEIGHT BETWEEN THE POWER AND THE FULCRUM. — OF THE LEVER OF THE THIRD KIND, HAVING THE POWER BETWEEN THE FULCRUM AND THE WEIGHT. MRS. B., We may now proceed to examine the mechanical powers; they are six in number : The lever, the pulley, the wheel and axle, the inclined plane, the wedge and the screw; one or more of which enters into the composition of every machine. A mechanical power is an instrument by which the effect of I given force is increased, whilst the force remains the same. In order to understand the power of a machine, there are four things to be considered. 1st. 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. Tlie power must always be superior to the resistance, otherwise the machine could not oe put in motion. Caroline, If for instance the resistance of a carriage was i^t'eater than the strength of the horses employed to draw it, they vould not be able to make it move. Mrs. B. Sdly. We are to consider the support or prop, or ds it is termed in mechanics, the /w/crwm; this you may recollect is the point upon which the body turns when in motion ; and iastly, the respective velocities of the power, and of the resist- ance; Emily. That must in general depend upon their respective distances from the fulcrum, or from tho 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 1 . How many mechanical powers are there, and what are they named ? — 2. What is a mechanical power defined to be ? 3. What four particulars Must be observed ? 4. Upon what will the velocities depend ^ ON THE MECHANICAL POWERS. 55 / lever is an inflexible rod or bar, moveable about a fidcrum, and having forces applied to two or more points on it. For instance, the steel rod to which these scales are suspended is a lever, and the point in which it is supported, the fulcrum, or centre of mo- tion ; 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 where is the centre of gravity of this pair of scales? (fig. 1. plate 4.) 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 supported by the fulcrum F of the lever which unites the two scales, and which is 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 remove 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 ex- tremities of the lever ? for the scales are not an essential part of the machine ; they have no mechanical 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 movea- ble, you see ; I can take it off the beam, 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 that which hangs on the longest side of the lever descends. Mrs. B. The two parts of the lever divided by the fulcrum, are called its arms ; you should therefore say the longest arm, not the longest side of the lever. Your observation is true that the balance is now destroyed ; but it will answer the purpose of enabling you to comprehend the power of a lever, when. the fulcrum is not in the centre. 5. What is a lever ? 6. Give a familiar example. 7. When and why do the scales balance each other, and where is their centre of gravity? (fig. 1. plate 4.) 8. Why would they not balance with unequal weights ? 9. Were the fulcrum removed from the middle of the beam %bat would result? fO. What do we mean by the arma of a lever ? 56 ON THE MECHANICAL POWERS. Emily, This would be an excellent contrivance for those who cheat in the weight of their goods ; by making the fulcrum a lit- tle on one side, and placing the goods in the scale which is sus- pended to the longest arm of the lever, they would appear to weigh more than they do in reality. Mrs, B. You do not consider how easily the fraud would be detected ; for on the scales bein* emptied they would not hang in equilibrium. If indeed the scale on the shorter arm was made heavier, so as to balance that on the longer, they would appear to be true, whilst they were really false.} 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 the momentum in the longest, is greater than in the shortest arm; the centre of gravity, tlierefore, is no longer supported. Mrs. B. You are right, the fulcrum is no longer in the cen- tre of gravity ; but if we can contrive to make the fulcrum in its present situation become the centre of gravity, the scales will again balance each other ; for you recollect that the centre of gi-avity is that point about which every part of the body is in equilibrium. Emily. It has just occurred to me how this may be acoom- plished; put a great weight into the scale suspended to the shortest arm of tlie lever, and a smaller one into that suspended to the longest arm. Yes, I have discovered it — look Mrs. B., the scale on the shortest arm will carry 3lbs., and that on the longest arm only one, to restore the balance, (fig. 3.) Mrs. B. You see, therefore, that it is not so impracticable as you imagined, to make a heavy body balance a light one; and this is in fact the means by which you observed that an imposi- tion in the weight of goods might be effected, as a weight oi ten or twelve ounces, might thus be made to balance a pound of goods. If you measure both arms of the lever, you will find that the length of the longer arm, is three times that of the shorter; and that to produce an equilibrium, the weights must bear the same proportion to each other, and that the greater weight, must be on the sliorter arm. Let us now take oil the scales, that we may consider the lever simply; and in this state you see that the fulcrum is no longer the centre of gravity, because it has been removed from the middle of the lever ; 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. 11. How may a pair of scales be false, and yet appear to be true ? 12. If the fulcrum be removed from the centre of gravity, how may the equilibrium be restored ? 13. How is this exemplified by fig. 3. plate 4 i* 14> What pro- portion must the weights bear to the lengths of the arms ? ON THE MECHANICAL POWERS. ^7 Caroline. The arms of the lever being different in length, it now exactly resembles the steelyards, with which articles are so frequently weighed. Mrs. B. It may in fact be considered as a pair of steelyards, by which J^he same power enables us to ascertain the weight of different articles, by simply increasing the distance of the power from the fulcrum ; you know that the farther a body is from the axis of motion, the greater is its velocity. Caroline, That I remember, and understand perfectly. Mrs. B. You comprehend then, that the extremity of the lon;jest arm of a lever, must move with greater velocity than that of tlie shortest arm, and that its momientum is greater in propor- tion. Emily. No doubt, because it^is farthest from the centre of motion. And pray, Mrs. B., when my brothers play at see-saWj 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 the ends of the lever. And have you not observed that when those who ride are of equal weight, the plank must be supported in the middle, to make the two arms equal ; whilst if the persons differ in weight, the plank must be drawn a little ferther over the prop, to make the arms unequal, and the lightest person, who may be supposed to represent the power, 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 I now understand that it is because he is more distant from the centre of motion. Mrs. B. The greater velocity with which your little bi'other moves, renders his momentum equal to yours. Caroline. Yes ; I have the most weight, he the greatest velo- city; so that upon the whole our momentums are equal. But you said, Mrs. B., that the power should be greater than the re- sistance, to put the machine in motion ; how then can the plank move if the momentums of the persons who ride are equal? Mrs. B. Because each person at his descent touches and pushes against the ground with his feet ; the reaction of which gives him an impulse which produces the motion ; this spring is requisite to destroy the equilibrium of the power and the resist- ance, otherwise the plank would not move. Did you ever ob- serve that a lever describes the arc of a circle in its motion ? 15. On what principle do we Weigh with a pair of steelyards, and what will be the difference in the motion pf the extremities of euch a lever ? 5$ ON THE MECHANICAL POWERS. Emily. No; it appears to me to rise and descend perpendi- cularly ; 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, (fie. 4. plate 4.) 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 pei-pendicularly, to the point A; or for your brother to descend in a straight line, to the point B ; you must in rising, and he in descending, describe arcs of your respective circles. This drawing shows you also how much su- perior 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 saying, that in mechanics, velocity is opposed to weight, 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, wnen compared with the other, that arm is to which the power is ap- plied, the greater will be the effect produced by it; because the greater is the velocity of the power compared to that uf the weight. Levers are of three kinds ; in the first the fulcrum is between the power and the weight. Caroline. This kind then comprehends the several levers you have described. Mrs. B. Yes, when in levers of the first kind, the fulcrum is equally distant from the power and the weight, as in the balance, there will be an equilibrium, when the power and the weight are equal to each other; it is not then a mechanical power, for nothing can in this case be gained by velocity; the two aims of the lever being equal, the velocity of their extremities must be so likewise. The balance is therefore of no assistance as a me- chanical power, although it is extremely useful in estimating the respective weights of bodies. But when (fig. 5.) the fulcrum F of a lever is not equally dis- 16. How is this exemplified by fi^. 4. plate 4? 17. What line is descri- bed by the ends of a lever ? fig. 4. plate 4. 18. How many kinds are there; and in the first how is the fulcrum situated? 19. When may the fulcrum be se situated that this lever ia not a mechanical power, and why ? ON THE MECHANICAL POWERS- 59 tant from the power and the weight, and the power P acts at the extremity of the longest arm, it may be less tnan the weight W ; its deficiency being compensated by its superior velocity, as we obser\'ed 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 t!ie longest arm ? Mrs. B. If the case will admit of your putting the end of the. lever under the resisting body, no fastening will be required; as you will perceive, when a nail is drawn by means of a hammer, which, though bent, is a lever of the first liind; the handle being the longest arm, the point on which it rests, the fulcrum, and the distance from that to the part which holds the nail, the short arm. But let me hear, Caroline, whether you can explain the action of 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 con- struction, you will discover tliat it is the power of tlie 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 power of the fingers is applied to the handles, and the article to be cut, is the resistance ; 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 paste- board or any hard substance, I always make use of that part of the scissors nearest the screw or rivet, and I now understand M'hy it increases the power of cutting; but I confess that I never should have discovered scissors to have been double levers; and pray are not snuffers levers of a similar description ? Airs. B. Yes, and most kinds of pincers ; the great power of which consists in the great relative length of the handles. Did you ever notice the swingle-tree of a carriage to which the horses are attached when drawing ? Emily. O yes ; this is a lever of the first kind, but the ful crum being in the middle, the horses should draw with equal power, whatever may be their strength. Mrs. B. That is generally the case, but it is evident that by making one arm longer than the other, it might be adapted to horses of unequal strength. Caroline. And of what nature are the other two kinds of le- vers ? 20. What is represented by fig. 5. plate 4? 21. Give a familiar example of the use of a lever of the first kind. 22. In what instruments are two such levers combined ? 23. How may two horses of unequal strength, be advan- tageously coupled in a carriage f ©O ON THE MECHANICAL POWERS. Mrs, B, In levers of the second kind, the weight, instead of being at one end, is situated between the power and the ful- crum, (fig. 6.) Caroline. The weight and the fulcrum have here changed places ; and what advantage is gained bj this kind of lever ? Mrs, B, In moving it, tlie velocity of the power must neces- sarily 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 kind, (fi^. 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 th«re is a great differ- ence in the length of the arms of the lever ; for the weight is al- most close to the fulcrum. Mrs. B. And the advantage gained is proportional to this difference. The most common example that we have 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 j but where is the weight to be moved ? ^ Mrs, B. The door is the weight, which in this example occu- pies the whole of the space between the power and the fulcrum. Nut crackers are double levers of this kind : the hinge is the ful- crum, the nut the resistance, and the hands the power. In levers of the third kind (fig. 8.) the fulcrum is again at one extremity, the weight or resistance at the other, and the power is applied between the fulcrum and the resistance. Emily. The fulcrum, the weight, or the power, then, each in its turn, occupies some part of the lever between its extremi- ties. But in this third kind of lever, the weight being farther than the power from the centre of motion, the difficulty of rais- ing 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 raising a ladder in order to place it against a wall J the man who raises it cannot place his nands on the upper part of the ladder^ the power, therefore, is necessarily placed much nearer to the ful- crum than to the weight. Caroline, Yes, the hands are the power, the ground the ful- crum, and the upper part of the ladder the weight. 24. DescribiR a lever of the second kind. (Fig. 6. plate 4.) 25. What is represented in fig. 7. plate 4, and in what proportion does this lever gain power? 26. What is said respecting a door? 27. Describe a lever of the third kind. 28. In what instance do we use this ? «N THE MECHAIflCAL POWERS. 61 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 3ie third kind ; the elbow is the fulcrum, the muscles of the fleshy part of the arm, the power; and as these are nearer to the elbow than to the hand, it is ne- cessary that their power should exceed the weight to be raised. 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 that no doubt which is best adapted to enable it to perform its various functions. There is one rule which applias to every lever, which is this : In order to produce an e thereby produceij ? CAUSES OF THE MOTION OF THE HEAVENLY BODIES. 79 together as forming one vast and magnificent system, worthy o the Divine hand by which it was created. Endly. I can scarcely conceive the idea of this immensity of creation ; it seems too sublime for our imagination ; — and to think t4iat the goodness of Providence extends over millions of worlds throughout a boundless universe — Ah! Mrs. B., it is we only wlio become trjfling and insignificant beings in so magnificent a creation ! Mrs. B. This idea should teach us humility, but without producing despondency. The same Almighty hand which guides these countless worlds in their undeviating course, conducts with equal perfection, the blood as it circulates through the veins of a fly, and opens the eye of the insect to behold His wonders. Notwithstanding this immense scale of creation, therefore, we need not fear that we shall be disregarded or forgotten. But to return to our station in the skies. We were, if you recollect, viewing the earth at a great distance, in appearance a little star, one side illumined by the sun, the other in obscurity. But would you believe it, Caroline, many of the inhabitants of this little star imagine that when that part which they inhabit is turned from the sun, darkness prevails throughout the universe, merely because it is night with them ; whilst, in reality, the sun never ceases to shine upon every planet. When, therefore, these little ignorant beings look around them during their flight, and behold all the stars shining, they cannot imagine why the planets, which are dark bodies, should shine; concluding, that since the sun does not illumine themselves, the whole universe must be in darkness. Caroline. I confess that I was one of these ignorant people ; but I am now very sensible of the absurdity of such an idea. To the inhabitants .of" the other planets, then, we must appear as a little star? Mrs. B. Yes, to those which revolve round our sun; for since those which may belong to other systems, (and whose exist- ence is only hypothetical) are invisible to us, it is probable that we also are invisible to them. Emily. But they may see our sun as we do theirs, in appear- ance a fixed star ? Mrs. B. No doubt ; if the beings who inhabit those planets are endowed with senses similar to ours. By the same rule we must appear as a moon to the inhabitants of our moon ; but on a larger scale, as the surface of the earth is about thirteen times as large as that of the moon. 39. What must be the appearance of the earth to an inhabitant of one of the planets ? 40. What the appearance of the sun to tjie inhabitants of pla- nets in other systems? 40. What the appearance of the earth to an inhabit- ant of the moon ? 80 OF THE PLANETS. Emily. The moon, Mrs. B., appears to mgve in a different direction, and in a different manner from the stars ? Mrs. B. I shall defer the explanation of the motion" of the moon till our next interview, as it would prolong our present lesson too much. CONVERSATION VII. OF THE PLANETS. OF THE SATELLITES OR MOONS. — GRAVITY DIMINISHES AS THE SftUARE OF THE DISTANCE. — OF THE SOLAR SYSTEM. — OF COMETS. — CONSTHL- LATIONS, SIGNS OF THE ZODIAC. — OF COPERNICUS, NEWTON, &C. MRS. B. The planets are distinguished into primary and secondary. Those wliich revolve immediately about the sun are called pri- mary. Many of these are attended in their course by smaller planets, which revolve round them : these are called secondary planets, satellites, or moons. Such is our moon which accom- panies the earth, and is carried with it round the sun, Emily. How then can you reconcile the motion of the secon- dary planets to the laws of gravitation ; for the sun is much larger than any of the primary planets ; and is not the power of gravity proportional to the quantity of matter ? Caroline, Perhaps the sun, though much larger, may be less dense than the planets. Fire you know, is very light, and it may contain but little matter, though of great magnitude. Mrs. B. We do not know of what kind of matter the sun is made ; but we may be certain, that since it is the general centre of attraction of our system of planets, it must be the body which contains the greatest quantity of matter in that system. You must recollect, that the force of attraction is not only 1 , Into what two classes are the planets divided, and how are they distin- ^lished .'* 2. By what reasoning do you prove that the sun contains a greater quantity of matter than aay otber body in the system } OF THE PLANETS. 81 proportional to the quantity of matter, but to the degree of prox- imity of the attractive boily: this power is weakened by being dift'used, and tliminishes as the distance increases. Emily, Then if a phiiiet was to lose one-half of its quantity of matter, it would lose one half of its attractive power; and the same effect would be produced by removing it to twice its former distance from the sun; that I understand. Mrs, B, Not so perfectly as you imagine. You are correct as respects the diminution in size, because the attractive force is in the same proportion as the quantity of matter; but were you to remove a planet to double its former distance, it would retain but one-fourth part of its gravitating force ; for attraction de- creases not in proportion to the simple increase of the distance, but as the squares of the distances increase. Caroline, I do not exactly comprehend what is meant by the squares, in this case, although I know very well what is in ge- neral intended by a square. Mrs. B, By the square of a number we mean the product of a number, multiplied by itself; thus two, multiplied by two, is four, which is therefore the square of two ; in like manner the square of three, is nine, because three multiplied by three, gives that product. Emily, Then if one planet is three times more distant from the sun than another, it will be attracted with but one-ninth part of the force; and if at four times the distance, with but one-six- teenth, sixteen being the square of four ? Mrs, B, You are correct; the rule is, that the attractive force is in the inverse proportion of the square of the distance. And it is easily demonstrated by the mathematics, that the same is the case with every power that emanates from a centre ; as for ex- ample, the light from the sun, or from any other luminous body, decreases in its intensity at the same rate. Caroline. Then the more distant planets, move much slower in their orbits ; for their projectile force must be proportioned to that of attraction ? But I do not see how this accounts for the motion of the secondary, round the primary planets, in preference to moving round the sun ? Emily, Is it not because the vicinity of the primary planets, renders their attraction stronger than that of the sun ? 3. What two circumstances govern the force with which bodies attract each other ? 4. Were a planet removed to double its former distance from the sun, what would be the effect upon its attractive force ? 5. Why would it be re^ duced to one-fourth f 6. What is meant by the square of a number, and what examples can you give ? 7. What then would be the effect of removing it to three, or four times its former distance ? 8. How is the rule upon this subject expressed ? 9. Does this apply to any power excepting gravitation? 10. How is it that a secondary planet revolves round its primary, and is not drawn off by the sun ? 86 OF THE PLANETS, Mrs. B, Exactly so. But since the attraction between bo- dies is mutual, the primary planets are also attracted by th satellites which revolve round them. The moon attracts the earth, as well as the earth the moon ; but as the latter is the smaller body, her attraction is proportionally less; therefor** neither the earth revolves round the moon, nor the moon round the earth ; but they both revolve round a point, which is their common centre oi gravity, and which is as much nearer to the earth than to the moon, as the gravity of the former exceeds that of the latter. Emily. Yes, I recollect your saying, ?that if two bodies were fastened together by a wire or bar, their common centre of gra- vity would be in the middle of the bar, provided the bodies were of equal weight ; and if they differed in weight, it would be near- er the larger body. If then, the earth and moon had no projec- tile force which prevented their mutual attraction from bringing them together, they w^ould meet at their common centre of gr vity. Caroline, The earth then has a great variety of motion, it revolves round the sun, round its own axis, and round the point towards which the moon attracts it. Mrs, B. Just so; and this is the case with every planet which is attended by satellites. The complicated effect of this variety of motions, produces certain irregularities, which, how- ever, it is not necessary to notice at present, excepting to observe that they eventually correct each other, so that no permanent derangement exists. The planets act on the sun, in the same manner as they are themselves acted on by their satellites ; for attraction, you must remember, is always mutual ; but the gravity of the planets (even when taken collectively) is so trifling compared with that of the sun, that were they all placed on the same side of that luminary, they would not cause him to move so much as one-half of his diameter towards them, and the common centre of gravity, would still remain within the body of the sun. The planets do not, therefore, revolve round the centre of the sun, but round a point at a small distance from its centre, about which the sun also re- volves. Emily. I thought the sun had no motion ? Mrs. B. You were mistaken ; for besides that round the com- mon centre of gravity, which I have just mentioned, which is indeed very inconsiderable, he revolves on his axis in about 25 11. What is said respecting the revolution of the moon, and of the earth, round a common centre of gravity ? 12. By what law in mechanics is this explained ? 13. What motions then has the earth, and are these remarks confined to it alone ? 14. What effect have the planets upon the sun, and TThat is said of the common centre of gravity of the system Ficf. 1. Fig. 'J. OF THE PLANETS. 83 days ; this motion is ascertained by observing certain spots which disappear, and reappear regularly at stated times. Caroline. A planet has frequently been pointed out to me in the heavens; but I could not perceive that its motion differed from that of the fixed stars, which only appear to move. Mrs, B, The great distance of the planets, renders their apparent motion so slow, that the eye is not sensible of their progress in their orbits, unless we watch them for some consi- derable length of time : but if you notice the nearness of a planet to any particular fixed star, you may in a few nights perceive that it has changed its distance from it, whilst the stars them- selves ahvays retain their relative situations. The most accu- rate idea I can give you of the situation and motion of t)ie pla- nets in their orbits, will be by the examination of this diagram, (plate 7. fig. 1.) representing the solar system, in which you will find every planet, with its orbit delineated. Emily. But the orbits here are all circular, and you said that they were eliptical. The planets appear too, to be moving round the centre of the sun ; whilst 3'ou told us that they moved round a point at a little distance from thence. Mrs. B. The orbits of the planets are so nearly circular, and the common centre of gravity of the solar system, so near the centre of the sun, that these deviations are too small to be re- presented. The dimensions of the planets, in their proportion to each other, you will find delineated in fig. 2. Mercury is the planet nearest the sun ; his orbit is consequent- ly contained within ours ; his vicinity to the sun, prevents our frequently seeing him, so that very accurate observations cannot be made upon mercury. He performs his revolution round the sun in about 87 days, which is consequently the length of his year. The time of his rotation on his axis is not known ; his distance from the sun is computed to be 37 millions of miles, and his diameter 3180 miles. The heat of this planet is supposed to be so great, that water cannot exist there but in a state of vapour, and that even quicksilver would be made to boil. Caroline. Oh, what a dreadful climate ! Mrs. B. Though we could not live there, it may be perfectly adapted to other beings, destined to inhabit it; or he who created it may have so modified the heat, by provisions of which we are ignorant, as to make it habitable even by ourselves. Venus, the next in the order of planets, is 68 millions of miles from the sun : she revolves about her axis in 23 hours and 21 15. What other motion has the sun, and how is it proved ? 16. How may you observe the motion of a planet, by means of a fixed star ? 17. What is represented by fig. 1. plate 7 ? 18. Why are the orbits represented as circu- lar? 19. In what order do the planets increase in size as represented, fig. 2. plate 7 ? 20. Wliat are we told respecting Mercury ? 84 OF THE PLANETS. minutes, and goes round the sim in 244 days, 17 hours. The orbit of Venus is also within ours ; during nearly one-half of her course in it, we see her before sun-rise, and she is then called the morning star ; in the other part of her orbit she rises later than the sun. Caroline. In that case we cannot see her, for she must rise in the day time ? Mrs. A True; but when she rises later than the sun, she also sets later; so that we perceive her approaching the horizon after sun-set: she is then called Hesperus, or the evening star. Do you recollect those beautiful lines of Milton No-w came still evening on, and twilight gray Had in her sober livery all things clad ; Silence accompanied ; for bea^t and bird, They to their grassy couch, these to their nests Were slunk, all but the wakeful nightingale ; She all night long her amorous descant sung ; Silence was pleas'd ; now glowed the firmament "With living sapphires. Hesperus that led The starry host, rode brightest, till the moon Rising in clouded majesty, at length Apparent queen unveil'd her peerless light, And o'er the dark her silver mantle threw. The planet next to Venus is tlie Earth, of which we shall soon speak at full length. At present I shall only observe that we are 95 millions of miles distant from t!ie sun, that we perform our annual revolution in 365 days 5 hours and 49 minutes; and are attended in our course by a single moon. Next follows Mars. He can never come between us and the sun, like Mercury and Venus ; his motion is, however, very per- ceptible, as he may be traced to different situations in the hea- vens; his distance from the sun is 144 millions of miles ; he turns round his axis in 24 hours and 39 minutes; and he performs his annual revolution, in about 687 of our days: his diameter is 4120 miles. Then follow four very small planets, Juno, Ceres, Pallas and Vesta, which have been recently discovered, but whose dimensions, and distances from the sun, have not been very accurately ascertained. They are generally called asteroids. Jupiter is next in order: this is the largest of all the planets. He is about 490 millions of miles from the sun, and completes his annual period in nearly 12 of our years. He turns round his axis in about ten hours. He is above 1200 times as big as our earth; his diameter is 86,000 miles. The respective proportions 21. What respecting Venus? 22. When does Venus become a morning, and when an evening star ? 23. What is said of the Eaith ? 24. What of Mars ? 25. What four small planets follow next ? OF THE PLANETS. S5 of the planets cannot, therefore, you see, be conveniently deli- neated m a diagram. He is attended by four moons. The next planet is Saturn, whose distance from the sun, is about 900 millions of miles ; his diurnal rotation is performed in 10 hours and a quarter : his annual revolution is nearly 30 of our years. His diameter is 79,000 miles. This planet is sur- rounded by a luminous ring, the nature of which^ astronomers are much at a loss to conjecture : he has seven moons. Lastly, we observe the planet Herschel, discovered by Dr. Herschel, by whom it was named the Georgium Sid us, and which is attended by six moons. Caroline, How charming it must be in the distant planets, to see several moons shining at the same time ; I think I should like to be an inhabitant of Jupiter or Saturn. Mrs. B, Not long I believe. Consider what extreme cold must prevail in a planet, situated as Saturn is, at nearly ten times the distance at which we are from the sun. Then his numerous moons are far from making so splendid an appearance as ours ; for they can reflect only the light which they receive from the sun ; and botli light, and heat, decrease in the same ratio or proportion to the distances, as gravity. Can you tell me now how much more light we enjoy than Saturn ? Caroline. The square of ten is a hundred ; therefore, Saturn has a hundred times less — or to answer your question exactly, we have a hundred times moreli^ht and heat, than Saturn — this certainly does not increase my wish to become one of the poor wretches who inhabit that planet. Mrs. B. May not the inhabitants of Mercury, with equal plausibility, pity us for the insupportable colaness of our situa- tion; and those of Jupiter and Saturn for our intolerable heat? The Almighty power which created these planets, and placed them in their several orbits, has no doubt peopled them with be- ings, whose bodies are adapted to the various temperatures and elements, in which thev are situated. If we judge from the analogy of our own eartli, or from that of the great and univer- sal beneficence of Providence, we must conclude this to be the case. ^ Caroline. Are not comets, in some respects similar to planets ? Mrs. B. Yes, they are ; for by the reappearance of some of them, at stated times, they are known to revolve round the sun; but in orbits so extremely eccentric, that they disappear for a great number of years. If they are inhabited, it must be by a species of beings very different, not only from the inhabitants of 26. What is said of Jupiter? 27. What of Saturn? 28. What of Her- schel ? 29. Why do we conclude that the moons of Saturn afford less light than ours ? 30. In what proportion will the light and heat at Saturn be di- minished, and why ? H 86 OF THE PLANETS. this, but from those of any of the other planets, as they must experience the greatest vicissitudes of heat and cold ; one part of their orbit being so near the sun, that their heat, when there, is computed to be greater than that of red-hot iron ; in this part of its orbit, the comet emits a luminous vapour, called the tail, which it gradually loses as it recedes from the sun; and the comet itself totally disappears from our siffht, in the more dis- tant parts of its orbit, which extends considerably beyond that of the furthest planet. The number of comets belonging to our system cannot be as- certained, as some of them are several centuries before they make their reappearance. The number that are known by their regu- lar reappearance is, I believe, only three, although their whole number is very considerable. Emily. Pray, Mrs. B., what are the constellations ? Mrs, B. They are the fixed stars; which the ancients, in or- der to recognise them, formed into groups, and gave the names of the figures, which you find delineated on the celestial globe. In order to show their proper situations in the heavens, they should be painted on the internal surface of a hollow sphere, from the centre of which you should view them ; you would then behold tiiem as tliey appear to be situated in the heavens. The twelve constellations, called the signs of the zodiac, are those wliich are so situated, that the earth, in its annual revolution, passes directly between them, and the sun. Their names are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sa- gittarius, Capricornus, Aquarius, Pisces ; the whole occupying a complete circle, or broad belt, in the heavens, called the zodiac, (plate 8. fig. 1.) Hence, a right line drawn from the earth, and passing through the sun, would reach one of these constellations, and the sun is said to be in that constellation at which the line terminates : thus, when the earth is at A, the sun would appear to be in the constellation or sign Aries ; when the earth is at B, the sun would appear in Cancer; when the earth was at C, the sun would be in Libra; and when the earth was at D, the sun would be in Capricorn. You are aware that it is the real motion of the earth in its orbit, which gives to the sun this apparent motion through the signs. This circle, in which the sun thus appears to move, and which passes through the middle of the zodiac, is called the ecliptic. Caroline. But many of the stars in these constellations ap- pear beyond the zodiac. 31. What do the comets resemble, and what is remarkable in their orbits? .32. What is said of the number of comets ? 33. What is a constellation ? 34. How are the twelve constellations, or signs, called the zodiac, situated ? 35. Name them. 36. What is meant by the sun being in a sign ? 37. What cause* the apparent change of the sun's place i* |: * !*» mp OF THE PLANETS. sr Mrs. B. We have no means of ascertaining the distance of the fixed stai's. When, therefore, they are said to be in the zodiac, it is merely implied that they are situated in that direc- tion, and that they shme upon us through that portion of the heavens, which we call the zodiac. Emily. But are not those large bright stars, which are called stars of the first magnitude, nearer to us, than those small ones which we can scarcely discern? Mrs. B, It may be so ; or the difference of size and brillian- cy of the stars(may proceed from their difference of dimensions; this is a point which astronomers are not enabled to determine. Considering them as suns, I see no reason why different suns should not vary in dimensions, as well as tlie planets belonging to them. Emily. What a wonderful and beautiful system this is, and how astonishing to think that every fixed star may probably be attended by a similar train of planets ! Caroline. You will accuse me of being very incredulous, but I cannot help still entertaining some doubts, and fearing that there is more beauty than truth in this system. It certainly may be so; but there does not appear to me to be sufficient evidence to prove it. It seems so plain and obvious that the earth is mo- tionless, and that the sun and stars revolve round it ; — ^your solar system, you must allow, is directly in opposition to the evidence of our senses. Mrs. B. Our senses so often mislead us, that we should not place implicit reliance upon them. Caroline. On what then can we rely, for do we not receive all our ideas through the medium of our senses ? Mrs. B. It is true that they are our primary source of know- ledge ; but the mind has the power of reflecting, judging, and deciding upon the ideas received by the organs of sense. This faculty, which we call reason, has frequently proved to us, that our senses are liable to err. If you have ever skilled on the water, with a very steady breeze, you must have seen the houses, trees, and every object on the shore move, while you were sailing. Caroline. I remember thinking so, when I was very young ; but I now know that their motion is only apparent. It is true that my reason, in this case, corrects the error of my sight. Mrs. B. It teaches you, that the apparent motion of the ob- jects on shore, proceeds from your bemg yourself moving, and that you are not sensible of your own motion, because you meet with no resistance. It is only when some obstacle impedes our motion, that we are conscious of moving; and if you were to 38. The stars appear of different magnitudes, by what may this be caused? 39. We are not sensible of the motion of the earth ; what fact is mentioned to illustrate this point? 40. What does this teach us? 88 OF THE PLANETS. close your eyes when you were sailing on calm water, with a steady wind, you would not perceive that you moved, for you could not feel it, and you could see it only by observing the change of place of the objects on shore. So it is with the motion of the earth : every thing on its surface, and the air that surrounds it, accompanies it in its revolution ; it meets with no resistance : therefore, like the crew of a vessel sailing with a fair wind, in a tjalm sea, we are insensible of our motion. Caroline. But the principal reason why the crew of a vessel in a calm sea do not perceive their motion, is, because they move exceedingly slow, while the earth, you say, revolves with great velocity. Mrs. B. It is not because they move slowly, but because they move steadily, and meet with no irregular resistances, that the crew of a vessel do not perceive their motion ; for they would be equally insensible to it, with the strongest wind, provided it it were steady, that they sailed with it, and that it did not ad- tate the water; but this last condition, you know, is not possible, for the wind will always produce waves which offer more or less resistance to the vessel, and then the motion becomes sensible, because it is unequal. Caroline. But, granting this, the crew of a vessel have a proof of their motion, which the inhabitants of the earth cannot nave, — the apparent motion of the objects on shore, or their hav- ing passed from one place to another. Mrs. B. Have we not a similar proof of the earth's motion, in the apparent motion of the sun and stars ? Imagine the earth to be sailing round its axis, and successively passing by every star, which, like the objects on land, we suppose to be moving instead of ourselves. I have heard it observed by an aerial tra- veller in a balloon, that the earth appears to sink beneath the balloon, instead of the balloon rising above the earth. It is a law which we discover throughout nature, and worthy of its great Author, that all its purposes are accomplished by the most simple means ; and what reason have we to suppose this law infringed, in order that we may remain at rest, while the sun and stars move round us; their regular motions, which are explained by the laws of attraction, on the first supposition, would be unintelligible on the last, and the order and harmony of the universe be destroyed. Think what an immense circuit the sun and stars would make daily, were their apparent motions, real. We know many of them, to be bodies more considerable than our earth ; for our eyes vainly endeavour to persuade us, that they 41. Would the slowness, or the rapidity of the motion, if steady, produce any sensible diflference I 42. If we do not feel the motion of the earth, how may we he ronriiired of its reality? OF THE PLANETS. 89 are little brilliants sparkling in the heavens; while science teaches us that they are immense spheres, whose apparent dimensions are diminished bj distance. Why then should these enormous globes daily traverse such a prodigious space, merely to prevent the necessity of our earth's revolvmg on its axis ? Caroline. I think I must now be convinced. But you will, I hope, allow me a little time to familiarise to myself, an idea io different from that which I have been accustomed to entertain. And pray, at what rate do we move? Mrs. B. The motion produced by the revolution of the earth on its axis, is abou^ seventeen miles a minute, to an inhabitant on the equator. Emily. But does not every part of the earth move with the same velocity ? 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 farthest from the axis of motion. But in the earth's revolution round the sun, every part must move with equal velocity ? Mrs. B. Yes, about a thousand 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 the beginning of the sixteenth century it was generally discarded, and the solar system, such as I have shown you, was established by the celebrated astronomer 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, and who demonstrated its truth. Emily. It appears, indeed, far less difficult to trace by obser- vation me motion of the planets, than to divine by what power 43. Were we to deny the motion of the earth upon its axis, what must we admit respecting the heavenly bodies ? 44. What distance is an inhabitant on the equator carried in a minute by the diurnal motion of the earth ? 45. Why is not tlie velocity every where equally great? 46. What distance does the earth travel in a minute, in its revolution round the sun ? 47. What was formerly supposed respecting the motion of all the heavenly bodies ? 48. What do we mean by the Copernican system, and what is said respecting Coper- nicus and Newton ? H2 90 «F THE PLANETS. they are impelled and guided. I wonder how the idea of gra- vitation could first have occurred to sir Isaac Newton ? Mrs, B. It is said to have been occasioned bja circumstance 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 avoia the contagion : when sitting one day in an orchard, he observed an apple fall from a tree, and was led to consider what could be the cause which brought it to the ground. 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 superior genius to find matter for wonder, observation, and research, in circumstances which, to the ordinary mind, appear trivial, because they are common; and with which they are satisfied, because they are natural; without reflecting that nature is our grand field of observation, that with- in it, is contained our whole store of knowledge ; in a word, that to study the works of nature, is to learn to appreciate and ad- mire the wisdom of God. Thus, it was the simple circumstance of the fall of an apple, which led to the discovery of the laws upon ^hich the Copernican system is founded; and 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 liave been sung by the poets. The apple of discord for which the goddesses contended ; the golden apples by which Atalanta won the race ; nay, even the apple which William Tell shot from the head of his son, can- not be compared to this ! 49. What circumstance is said to have given rise to the speculations of Newton, on the subject of gravitation ? CONVERSATION VIII. ON THE EARTH. !>P THE TERRESTRIAL GLOBE. — OF THE FIGURE OP THE EARTH. — OF THE. PENDULUM. OF THE VARIATION OP THE SEASONS, AND OP THE LENGTH OF DAYS AND NIGHTS. — OF THE CAUSES OF THE HEAT OF SUM- MER. — OF SOLAR, SIDERIAL, AND EaUAL OR MEAN TIME. MRS. B. As the earth is the planet in which we are the most particu- larly interested, it is my intention this morning, to explain to you the effects resulting from its annual, and diurnal motions; but for this purpose, it will be necessary 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 informed they were only imaginary divisions, they did not appear to me worthy of much attention, and were soon forgotten. Mrs. B. You supposed, then, that astronomers had been at the trouble of inventmg 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 plate 8. fig. 2. you will find them all deline- ated : and you must learn them perfectly, if you wish to make any proficiency in astronomy. Caroline. I was taught them at so early an age, that I could not understand their meaning ; and I have often heard you say, that the only use of words, was to convey ideas. Mrs. B. A knowledge of these lines, would have conveyed some idea of the manner in which they were designed to divide the globe into parts; although the use of these divisions, might at that time, have been too difficult for you to understand. Child- hood is the season, when impressions on the memory are most strongly and most easily made : it is the period at which a large stock of terms should be treasured up, the precise application of which we may learn when tlie understanding is more developed. It is, I think, a very mistaken notion, that children should be taught such things only, as they can perfectly understand. Had you been early made acquainted with the terms which relate to 92 ON 'mE EARTH figure and motion, how much it would iiu.;e- facilitated your pro- gress in natural philosophy. I have been obliged to confine myself to the most common and familiar expressions, in explain- ing the laws of nature; although I am convinced that appropriate and scientific terms, might have conveyed more precise and ac- curate ideas, had you been prepared to understand them. Emily. You may depend upon our carefully learning the : names of these lines, Mrs. B.; but before we commit them to memory, will you have the goodness to explain them to us ? Mrs. B. Most willingly. This figure of a 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, distinguished by the names of the north and the south pole. The circle C D, which divides the globe into two equal parts between the poles, and equally distant from them, is called the equator, or equinoctial line; that Eart of the globe to the north of the equator, is the northern emisphere; that part to the south of the equator, the southern hemisphere. The small circle E F, which surrounds the north pole, is called the arctic circle; that G H, which surrounds the south pole, the antarctic circle; these are also called polar circles. There are two circles, intermediate between the polar circles and the equator; that to the north I K, called the tropic of Cancer; that to the south, L M, called the tropic 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 tro- pic of Cancer, and southward as far as the tropic of Capricorn, is called the ecliptic. The delineation of the ecliptic on the ter- restrial globe is not without danger of conveying false ideas; for the ecliptic (as I have before said) is an imaginary circle in the heavens, passing through the middle of the zodiac, and situated in the plane of the earth's orbit. Caroline. I do not understand the meaning of the plane of the earth's orbit. Mrs. B. A plane, is an even flat surface. Were you to bend a piece of wire, so as to form a hoop, you might then stretch a piece of cloth, or paper over it, like the head of a drum; this would form a flat surface, which might be called the plane of the hoop. Now the orbit of the earth, is an imaginary circle, sur- rounding the sun, and you can readily imagine a plane extend - 1. What does the line A B, (fig. 2 plate 8.) represent, and what are its ex- tremities called ? 2. What is meant by the equator, and how is it situated ? 3. There are two hemispheres ; how are they named and distinguished ? 4. What are the circles near the poles called? 5. What do the lines I K, and L M, represent ? 6. What circle is in part represented by the line L Ki* 7. Against- what mistake must you guard respecting this line ? 8. What is meunt by a plane, and how could one be represented i* Plaxeh. -^ Ma. 1. ON THE EARTH. 9^ ing from one side of this circle to the other, filling up its whole area : such a plane would pass through the centre of the sun, di- viding it into hemispheres. You may then imagine this plane extended beyond the limits of the earth's orbit, on every side, until it reached those fixed stars which form the signs of the zodiac; passing through the middle of these siens, it would give you the place of that imaginary circle in the heavens, call the ecliptic; which is the sun's apparent path. Let fig. 1. plate 9, represent such a plane, S the sun, E the earth with its orbit, and ABC D the ecliptic passing through the middle of the zodiac. Emily. If the ecliptic relates only to the heavens, why is it described upon the terrestrial globe } 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 bein» described qu the same globe; and the obliquity of the eclip- tic shows(}iow much the earth's axis is inclined to the plane of its orbit. But to return to fig. 2. plate 8. The spaces between the several parallel circles on the terres- trial globe are called zones: that which is comprehended between the tropics is distinguished by tlie name of the torrid zone; the spaces which extend from the tropics to the polar circles, the north and south temperate zones; and the spaces contained with- in the polar circles, the frigid zones. By the term zone is meant a belt, or girdle, the frigid zones, however, are not belts, but circles, extending 231 degrees from their centres, the poles. The several lines winch, you observe to be drawn from one pole to the other, cutting the equator at right angles, are called meri- dians; the number of these is unlimited, as a line passing through any place, directly to the poles, is called tlie meridian of that place. When any one of these meridians is exactly opposite to the sun, it is mid-day, or twelve o'clock in the day, at all the places situated any where on that meridian; and, at the places situated on the opposite meridian, it is consequently midnight. Emily. To places situated equally distant from these two meridians, it must then be six o'clock. 9. Describe what is intended by the plane of the earth's orbit. 10. Ex- tendings this plane to the fixed stars, what circle would it form, and among what particular stars would it be found? 11. What is fig. 1. plate 9, design- ed to represent? 12. The ecliptic does not properly belong to the earth, for what purpose then is it described on the terrestrial globe ? 13. What does the obliquity of the ecliptic to the equator serve to show ? 14. Within what limits do you find the torrid zone? 15. What two zones are there between the torrid, and the two frigid zones ? 16. Where are the frigid zones situat- ed? 17. What is meant by the term zone ; and are the frigid zones properly so called? 18. How do meridian lines extend, and what is meant by the Qieridian of a place ? 19. What is said of the meridian to which the sun ig opposite, and where is it then midnight? 94 ON THE EARTH. Mrs, B. Yes; if they are to the east of the sun's meridian it is six o'clock in the afternoon, because they will have previously passed the sun; if to the west, it is six o'clock in the morning, and that meridian will be proceeding towards the sun. Those circles which divide the globe into two equal parts, such as the equator and the ecliptic, are called greater circles; to distinguish them from those wnich divide it into two unequal parts, as the tropics, and polar circles, which are called lesser circles. All circles, you know, are imagined to be divided into 360 equal parts, called degrees, and degrees are again divided into 60 equal parts, called minutes. The diameter of a circle is a right line drawn across it, and passing through its centre: were you, for instance, to measure across this round table, that would give you its diameter; but were you to measure all round the edge of it, you would then obtain its circumference. Now Emily, you may tell me exactly how niany degrees are contained in a meridian? Emily. A meridian, reaching from one pole to the other, is half a circle, and must therefore contain \ 80 degrees. Mrs, B. Very well; and what number of degrees are there from the equator to one of the poles? Caroline. The equator being equally distant from- either pole, that distance must be half of a meridian, or a quarter of the cir- cumference of a circle, and contain 90 degrees. Mrs. B. Besides the usual division of circles into degrees, the ecliptic is divided into twelve equal parts, called signs, which bear the name of the constellations through which this circle passes in the heavens. The degrees measured on the meridians from the equator, either towards the north, or towards the south, are called degrees of latitude, of which there may be 90; those measured from east to west, either on the equator, or any of the lesser circles, are called degrees of longitude, of whicn there maybe 180; these lesser circles are also called parallels of lati- tude. Of these parallels there may be any number; a circle drawn from east to west, at any distance from the equator, will always be parallel to it, and is therefore called a parallel of latitude. 20. What hour is it then, at places exactly half way between these meri- dians? 21. How are ^eater and lesser circles distinguished? 22. What part of a circle is a degree, an;1 how are these further divided ? 23. What is the diameter, and what the circumference of a circle, and what proportion do they bear to each other? 24. What part of a circle is a meridian ? 23. How many degrees are there between the equator and the poles? 26. Into what parts, besides degrees, is the ecliptic divided ? 27. How are degrees of latitude measured, and to what number do they extend ? 28. On what circles are degrees of longitude meastired, and to what number do they idxtend ? 29. What is a paraUel of latitude ■ IPIr ON THE EARTH. 95 ISmily. The degrees of longitude must then vary in length, according to the dimensions of the circle on wliich they are reck- I onedj those, for instance, at the polar circles, will be considerably smaller than those at the equator? Mrs. B. Certainly; since the degrees of circles of different dimensions do not vary in number, tliey must necessarily vary in leng-th.; 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.^ Mrs. B. Sixty geographical miles, which is equal to 69?' • English statute miles; or about one-sixth more than a common I mile. ' Emily. The degrees of longitude at the equator, must then f'be of the same dimensions, with a degree of latitude. Mrs. B. They would, were the earth a perfect sphere; but it is not exactly such, being somewhat protuberant about the equator, and flattened, towards the poles. This form proceeds from the superior action of the centrifugal power at the equator, and as this enlarges the circle, it must, in the same proportion, increase the length of the degrees of longitude measured on it. Caroline. I thought I had understood the centrifugal force perfectly, but I do not comprehend its effects in this instance. Mrs. B. You know that the revolution of the earth on its axis, must give to every particle a tendency to fly off from the cen- tre, that this tendency is stronger, or weaker, in proportion to the velocity with which the particle moves; now a particle situated near to one of the poles, makes one rotation in the same space of time as a 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 equa- tor than in the polar regions: it gradually decreases as you leave the equator and approach the poles, at which points the cen- trifugal force, entirely ceases. Supposing, therefore, the earth to have been originally in a fluid state, the particles in the torrid zone would recede much farther from the centre than those in the frigid zones; thus the polar regions would become flattened, and those about the equator elevated. As a large portion of the earth is covered with water, the Creator gave to it the form, denominated an oblate spheroid, otherwise the polar regions would have ^en without water, 30. Degrees of longitude vary in length; what is the cause of this? 31. What is the length of a degree of latitude, and why do not these vary ? 32. What causes the equator to be somewhat larger than a great circle passing through the poles, and what effect has this on degrees of longitude measured on the equator ? 33. What is the cause of this ferm being given to the earth i 96 ON THE EARTH. and those about the equator, would have been buried seve; miles below the surface of the ocean. Caroline. I did not consider that the particles in the neii bourhood of the equator, move with greater velocity than th< about the poles; this was the reason 1 could not understand y(Mi. Mrs. B. You must be careful to remember, that those pai 1^ of a body which are farthest from the centre of motion, must move with the greatest velocity: tlie axis of the earth is the centre of its diurnal motion, and the equatorial regions the parts most dis- tant 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 ivi a valley? Mrs. B. Certainly; your head is more distant from the cen- tre 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 mo- tion, the larger is the circle it will describe, and the greater therefore must be its velocity. # Emily. I have been reflecting, that if the earth is not a per- fect circle— Mrs. B. A sphere you mean, my dear: a circle is a round line, every part of which is equally distant from the centre; a 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, but pro- minent at the equator, and depressed at the poles, would not a body weigh heavier at the equator than at the poles? For the earth beina* thicker at the equator, the attraction of gravity per- pendicularly downwards must be 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 attrac- tion, the more strongly it is attracted; because it is then nearest to the whole mass of attracting matter. In regard to its effects, you might consider the whole power of gravity, as placed at the centre of attraction. Emily. But were you to penetrate deep into the earth, would gravity increase as you approached the centre? Mrs. B. Certainly not; I am referring only to any situation on the surface of the earth. Were you to penetrate into the inte- rior, 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 approach the centre; and if you 34. What would have been a consequence of the centrifugal force, had the earth been a perfect sphere ? 35. A body situated at the poles, is at- tracted more forcibly than if placed at the equator, what is the reason .' »F THE FLANETS. 85 of the planets cannot, therefore, you see, be conveniently deli- neated m a diagram. He is attended by four moons. The next phmet is Saturn, whose distance from the sun, is about 900 millions of miles ; his diurnal rotation is performed in 10 hours and a quarter: his annual revolution is nearly* 30 of our years. His diameter is 79,000 miles. This planet is sur- rounded by a luminous ring, the nature of which, astronomers are much at a loss to conjecture : he has seven moons. Lastly, we observe the planet Herschel, discovered by Dr. Herschel, by whom it was named the Georgium Sid us, and which is attended by six moons. Carolme. How charming it must be in the distant planets, to see several moons shining at the same time; I think I should like to be an inhabitant of Jupiter or Saturn. 3Irs. B. Not long I believe. Consider what extreme cold must prevail in a planet, situated as Saturn is, at nearly ten times the distance at which we are from the sun. Then his numerous moons are far from making so splendid an appearance as ours ; for they can reflect only the light which they receive from the sun ; and both light, and heat, decrease in the same ratio or proportion to the distances, as gravity. . Can you tell me now how much more light we enjoy than Saturn } Caroline. The square oi ten is a hundred ; therefore, Saturn has a hundred times less — or to answer your question exactly, we have a hundred times more light and heat, than Saturn — ^this certainly does not increase my wish to become one of the poor wretches who inhabit that planet. Mrs. B. May not the inhabitants of Mercury, with equal plausibility, pity us for the insupportable coldness of our situa- tion ; and those of Jupiter and Saturn for our intolerable heat ? The Almighty power which created these planets, and placed them in their several orbits, has no doubt peopled them with be- ings, whose bodies are adapted to the various temperatures and elements, in which they are situated. If we judge from the analogy of our own earth, or from that of the great and univer- sal beneficence of Providence, we must conclude this to be the case. Carolme. Are not comets, in some respects similar to planets ? Airs. B. Yes, they are ; for by the reappearance of some of them, at stated times, they are known to revolve round the sun ; but in orbits so extremely eccentric, that they disappear for a great number of years. If they are inhabited, it must be by a species of beings very different, not only from the inhabitants of 26. What is said of Jupiter? 27. What of Saturn? 28. What of Her- schel ? 29. Why do we conclude that the moons of Saturn afford less light than ours ? 30. In what proportion will the light and heat at Saturn be di- minished, and why ? H 86 OF THE PLANETS. this, but from those of any of the other planets, as they must experience the greatest vicissitudes of heat and cold ; one part of their orbit being so near the sun, that their heat, when there, is computed to be greater than that of red-hot iron ; in this part of its orbit, the comet emits a luminous vapour, called the tail, which it gradually loses as it recedes from the sun; and the comet itself totally disap|)ears from our si^ht, in the more dis- tant parts of its orbit, which extends considerably beyond that of the furthest planet. The number of comets belonging to our system cannot be as- certained, as some of them are several centuries before they make their reappearance. The number that are known by their regu- lar reappearance is, I believe, only three, although their whole number is very considerable. Emily. Pray, Mrs. B., what are the constellations.^ Mrs. B. They are the fixed stars; which the ancients, in or- der to recognise them, formed into groups, and gave the names of the figures, which you find delineated on the celestial globe. In order to show their proper situations in the heavens, they should be painted on the internal surface of a hollow sphere, from the centre of which you should view them ; you would then behold them as they appear to be situated in the heavens. The twelve constellations, called the signs of the zodiac, are those which are so situated, that the earth, in its annual revolution, passes directly between them, and the sun.'» Their names are Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sa- gittarius, Capricornus, Aquarius, Pisces ; the whole occupying a complete circle, or broad belt, in the heavens, called the zodiac, (plate 8. fig, 1.) Hence, a right line drawn from the earth, and passing through \\\(^ sun, would reach one of these constellations, and the sun is said to be in that constellation at which the line terminates : thus, when the earth is at A, the sun would appear to be in the constellation or sign Aries ; when the earth is at B, the sun would appear in Cancer ; when the earth was at C, the sun would be in Libra; and when the earth was at D, the sun would be in Capricorn. You are aware that it is the real motion of the earth in its orbit, which gives to the sun this apparent motion through the signs. This circle, in which the sun thus appears to move, and which passes through the middle of the zodiac, is called the ecliptic. Caroline, But many of the stars in these constellations ap- pear beyond the zodiac. 31. What Jo the comets resemble, and what is remarkable in their orbits f 32. What is said of the number of comets ? 33. What is a constellation ? 34. How are the twelve constellations, or signs, called the zodiac, situated ? 35. Name tliem. 36. What is mfeant by the sun being in a sign ? 37. Whdt cause* the apparent change of the sun's place .'' m OF THE PLANETS. 87 Mrs. B. We have no means of ascertaining the distance of the fixed stars. When, therefore, they are said to be in the zodiac, it is merely implied that they are situated in that direc- tion, and that they shme upon us through that portion of the heavens, which we call the zodiac. Emily, But are not those large bright stars, which are called stars of the first magnitude, nearer to us, than those small ones which we can scarcely discern.^ Mrs. B, It may be so ; or the difference of size and brillian- cy of the starsCinay proceed from their difference of dimensions; tliis is a point which astronomers are not enabled to determine, ronsidering them as suns, I see no reason why different suns oidd not vary in dimensions, as well as the planets belonging them. * Emily, What a wonderful and beautiful system this is, and }iow astonishing to think that every fixed star may probably be attended by a similar train of planets! Caroline. You will accuse me of being very incredulous, but I cannot help still entertaining some doubts, and fearing that there is more beauty than truth in this system. It certainly may be so; but there does not appear to me to be sufficient evidence to prove it. It seems so plam and obvious that the earth is mo- tionless, and that the sun and stars revolve round it ; — your solar system, you must allow, is directly in opposition to the evidence qf our senses. « Mrs. B. Our senses so often mislead us, that we should not place implicit reliance upon them. Caroline. On what then can we rely, for do we not receive all our ideas through the medium of our senses ? Mrs. B. It is true that they are our primary source of knov- led^e; but the mind has the power of reflecting, judging, j;])' deciding upon the ideas received by the organs of sense. This faculty, which we call reason, has frequently proved to us, that our senses are liable to err. If 3^ou have ever sailed on the water, with a very steady breeze, you must have seen the houses, trees, and every object on the shore move, while you were sailing. Caroline. I remember tliinking so, when I was very young ; but I now know that their motion is only apparent. It is true that my reason, in this case, corrects the error of my sight. Mrs. B. It teaches you, that the apparent motion of the ob- jects on shore, proceeds from your bemg yourself moving, and that you are not sensible of your own motion, because you meet vvid^no resistance. It is only when some obstacle impedes our mOTron, that we are conscious of moving; and if you were to 38. The stars appear of different magnitudes, by what may this be caused ? 39. We are not sensible of the motion of the earth ; what fact is mentioned to illustrate this point ? 40, What does this teach usf I 38 OF THE PLANETS. close your eyes when you were sailing on calm water, with a steady wind, you would not perceive that you moved, for you could not feel it, and you could see it only by observing the • hange of place of the objects on shore. So it is with the motion of the earth : every thing on its surface, and the air that surrounds It, accompanies it in its revolution ; it meets with no resistance : therefore, like the crew of a vessel sailing with a fair wind, in a calm sea, we are insensible of our motion. Caroline. But the principal reason why the crew of a vessel in a calm sea do not perceive their motion, is, because they move, exceedingly slow, while the earth, you say, revolves with great velocity. Mrs. B. It is not because they move slowly, but because they move steadily, and meet with no irregular resistances, that the crew of a vessel do not perceive their motion ; foirlhey woukl be equally insensible to it, with the strongest winTl, provided it it were steady, that they sailed with it, and that it did not agi- tate the water"*^ but this last condition, you know, is not possible, for the wind Will always produce waves which offer more or less resistance to the vessel, and then the motion becomes sensible, because it is unequal. Caroline. But, granting this, the crew of a vessel have a f)roof of their motion, which the inhabitants of the earth cannot lave, — the apparent motion of the objects on shore, or their hav- ing passed from one place to another. Mrs. B. Have w^e not a similar proof of the earth's motion, On the apparent motion of the sun and stars ? Imagine the earth to be sailing round its axis, and successively passing by every star, which, like the objects on land, we suppose to be moving instead of ourselves. I have heard it observed by an aerial tra- veller in a balloon, that the earth appears to sink beneath the ^ i)alloon, instead of the balloon rising above the earth. It is a law which we discover throughout nature, and worthy of its great Author, that all its purposes are accomplished by the nlost simple means ; and what reason have we to suppose this law infringed, in order that we may remain at rest, while the sun and stars move round us; their regular motions, which are explained by the laws of attraction, on the first supposition, would be unintelligible on the last, and the order and harmony of the universe be destroyed. Think what an immense circuit the sun and stars would make daily, were their apparent motions, real. We know many of them, to be bodies more considerable than our earth; for our eyes vainly endeavour to persuade us, that^ey 41. Would the slowness, or the rapidity of the motion, if steady, produce any sensible difference ? 42. If we do not feel the motion of the earth, how may we be convinced of its reality ? OF THE PLANETS. 89 are little brilliants sparkling in the heavens; while science teaches us that they are immense spheres, whose apparent dimensions are diminished by distance. Why then should these enormous globes daily traverse such a prodigious space, merely to prevent the necessity of our earth's revolving on its axis ? Caroline, I think I must now be convinced. But you will, I hope, allow me a little time to familiarise to myself, an idea so different from that which I have been accustomed to entertain. And pray, at what rate do we move? Mrs. B. The motion produced by the revolution of the earth on its axis, is about seventeen miles a minute, to an inhabitant on the equator. Emily. But does not every part of the earth move with the same velocity ? 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 farthest from the axis of motion. But in the earth's revolution round the sun, every part must move with equal velocity } Mrs. B. Yes,;about a thousand 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 nof^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 the beginning of the sixteenth century it was generally discarded, and the solar system, such as I have shown you, was established by the celebrated astronomer 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, and who demonstrated its truth, Emily. It appears, indeed, far less difficult to trace by obser- vation the motion of the planets, than to divine by what power 43. Were we to deny the motion of the earth upon its axis, what must we admit respecting the heavenly bodies ? 44. What distance is an inhabitant on the equator carried in a minute by the diurnal motion of the earth ? 45. Why is not the velocity every where equally great? 46. What distance does the earth travel in a minute, in its revolution round the sun? 47. What was formerly supposed respecting the motion of all the heavenly bodies? 48. What do we mean by the Copernican system, and what is said respecting Coper- nicus and Newton? H2 90 OF THE PLANETS. they are impelled and guided. I wonder how the idea of gra- vitation could first have occurred to sir Isaac Newton ? Mrs. B. It is said to have been occasioned by a circumstance 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 an orchard, he observed an apple fall from a tree, and was led to consider what could be the cause which brought it to the ground.' 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 superior genius to find matter for wonder, observation, and research, in circumstances which, to the ordinary mind, appear trivial, because they are common; and with which they are satisfied, because they are natural; without reflecting that nature is our grand field of observation, that with- in it, is contained our whole store of knowledge ; in a word, that to study the works of nature, is to learn to appreciate and ad- mire the wisdom of God. Thus, it was the simple circumstance of the fall of an apple, which led to the discovery of the laws upon which the Copernican system is founded; and 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 ; tlie golden apples by which Atalanta won the race ; nay, even the apple winch William Tell shot from the head of his son, can- not be compared to this ! 49. What circumstance is said to have given rise to the specilations of Newton, on the subject of gravitation ? CONVERSATION VHI. ON THE EARTH. OF THE TERRESTRIAL GLOBE.— OF THE FIGURE OF THE EARTH. — OF THE PENDULUM. — OF THE VARIATION OF THE SEASONS, AND OF THE LENGTH OF DAYS AND NIGHTS. — OF THE CAUSES OF THE HEAT OF SUM- MER. — OF SOLAR, SIDERIAL, AND SaUAL OR MEAN TIME. MRS. B. As the earth is the planet in which we are the most particu- larly interested, it is mj intention this morning, to explain to jou the effects resulting from its annual, and dmrnal motions ; but for this purpose, it will be necessary to make you acquainted with the terrestrial globe: you have not either of you, I conclude, learnt the use of the globes P Caroline. No; I once indeed, learnt by heart, the names of the lines marked on the globe, but as I was informed they were only imaginary divisions, they did not appear to me worthy of much attention, and were soon forgotten. Mrs. B. You supposed, then, that astronomers had been at the trouble of inventmg 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 plate 8. fig. 2. you wUl find them all deline- ated : and you must learn them perfectly, if you wish to make any proficiency in astronomy. Caroline. I was taught them at so early an age, that I could not understand their meaning ; and I have often heard you say, that the only use of words, was to convey ideas. Mrs. B. A knowledge of these lines, would have conveyed some idea of the manner in which they were designed to divide the globe into parts; although the use of these divisions, might at that time, have been too difficult for you to understand. Child- hood is the season, when impressions on the memory are most strongly and most easily made : it is the period at which a large stock of terms should be treasured up, the precise 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 understand. Had you been early made acquainted witn the terms which relate to 92 ON THE EARTH. figure and motion^ how much it would have facilitated your pro- gress in natural philosophy. I have been obliged to confine myself to the most common and familiar expressions, in explain- ing the laws of nature; although I am convinced that approprir and scientific terms, might have conveyed more precise and t curate ideas, had you been prepared to understand them. Emily. You may depend upon our carefully learning th'' names of these lines, Mrs. B.; but before we commit the. memory, will you have the goodness to explain them to us ? Mrs. B. Most willingly. This figure of a 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, distinguished by the names of the north and the south pole. The circle C D, which divides the globe into two equal parts between the fioles, and equally distant from them, is called the equator, or equinoctial line; that part of the globe to the north of the equator, is the northern nemisphere ; that part to the south of the equator, the southern hemisphere. The small circle E F, which surrounds the north pole, is called the arctic circle; that G H, which surrounds the south pole, the antarctic circle; these are also called polar circles. There are two circles, intermediate between the polar circles and the equator; that to the north I K, called the tropic of Cancer; that to the south, L M, called the tropic of Capricorn. Lastly, this circle, L K, which divides the globe into two equdl parts, crossing the equator and extending northward as far as the tro- pic of Cancer, and southward as far as the tropic of Capricorn, is called the ecliptic. The delineation of the ecliptic on the ter- restrial globe is not without danger of conveying false ideas; for the ecliptic (as I have before said) is an imaginary circle in the heavens, passing through the middle of the zodiac, and situated in the plane of the eartli's orbit. Caroline. I do not understand the meaning of the plane of the earth's orbit. Mrs. B. A plane, is an even flat surface. Were you to bend a piece of wire, so as to form a hoop, you mi»ht then stretch a piece of cloth, or paper over it, like the head of a drum; this would form a flat surface, which might be called the plane of the hoop. Now the orbit of the «arth, is an imaginary circle, sur- rounding the sun, and you can readily imagine a plane extend - 1. What does the line A B, (fig. 2 plate 8.) represent, and what are its ex- tremities called ? 2. What is meant by the equator, and how is it situated ? 3. There are two hemispheres; how are they named and distinguished? 4. What are the circles near the poles called? 5. What do the lines I K, and L M, represent ? 6. What circle is in part represented by the line L K ? 7. Against what mistake must you guard respecting thialine? 8. What is meant by a plane, and how could one be represented ? Tlateh. ++ + + ON THE EARTH. ing from one side of this circle to the other, filling up its whole area : such a plane would pass through the centre of the sun, di- viding it into hemispheres. You may then imagine this plane extended beyond the limits of the earth's orbit, on every side, until it reached those fixed stars which form the signs of the zodiac^ passing through the middle of these signs, it would give you the place of that imaginary circle in the lieavens, call the ecliptic; which is the sun's apparent path. Let fig. 1. plate 9, represent Kuch a plane, S the sun, E the earth with its orbit, and ABC I) the ecliptic passing tlu-ough the middle of the zodiac. Emily. If the ecliptic relates only to the heavens, why is it described upon the terrestrial globe ? Mrs. B. (It is convenient for the demonstration of ^ 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 globeftand the obliquity of the eclip- tic sliows'.how much the earth's axis is inclined to the plane of its orbit.. But to return to fig. 2. plate 8. The spaces between the several parallel circles on the terres- trial globe are called zones: that which is comprehended between the tropics is distinguished by the name of the torrid zone; the spaces which extend from the tropics to the polar circles, the north and south temperate zones; and the spaces contained with- in the polar circles, the frigid zones. By the term zone is meant a belt, or girdle, the frigid zones, however, are not belts, but circles, extending 285 degrees from their centres, the poles. The several lines which, you observe to be drawn from one pole to the other, cutting the equator at right angles, are called meri- dians; the number of these is unlimited, as a line passing through any place, directly to the poles, is called the meridian of that place. When any one of these meridians is exactly opposite to the sun, it is mid -day, or twelve o'clock in the day, at all the places situated any where on that meridian; and, at the places situated on the opposite meridian, it is consequently midnight. Emily. To places situated equally distant from these two meridians, it must then be six o'clock. 9. Describe what is intended by the plane of the earth's orbit. 10. Ex- tending this plane to the fixed stars, what circle would it form, and among what particular stars would it be found? 11, What is fig. 1. plate 9, design- ed to represent ? 12. The ecliptic does not properly belong to the earth, for what purpose then is it described on the terrestrial globe ? 13. What does the obliquity of the ecliptic to the equator serve to show ? 14. Within what limits do you find the torrid zone? 15. What two zon^s are there between the torrid, and the two frigid zones ? 16. Where are the frigid zones situat- ed? 17. What is meant by the term zone ; and are the frigid zones properly so called? 18. How do meridian lines extend, and what is meant by the meridian of a place? 19. What is said of the meridian to which the sun is opposit*?, and where is it then midnight ? 94 ox THE EARTH. Mrs. B, Yes; if they are to the east of the sun's meridian it is six o'clock in the afternoon, because they will have previously passed the sun; if to the west, it is six o'clock in the morning, and that meridian will be proceeding towards the sun. Those circles which divide the globe into two equal parts, such as the equator and the ecliptic, are called greater circles; to distinguish them from those which divide it into two unequal parts, as the tropics, and polar circles, which are called lesser circles. All circles, you know, are imagined to be divided into 560 equal parts, called degrees, and degrees are again divided into 60 equal parts, called minutes. The diameter of a circle is a right line drawn across it, and passing through its centre; were you, for instance, to measure across this round table, that would give you its diameter; but were you to measure all round the edge of it, you would then obtain its circumference. 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 \ 80 degrees. Mrs. B. Very well; and what number of degrees are there from the equator to one of the poles? Caroline. The equator being equally distant from either pole, that distance must be half of a meridian, or a quarter of the cir- cumference of a circle, and contain 90 degrees. Mrs. B. Besides the usual division of circles into degrees, the ecliptic is divided into twelve equal parts, called signs, which bear the name of the constellations through which this circle passes in the heavens. The degrees measured on the meridians from the equator, either towards the north, or towards the souths are called degrees of latitude, of which there may be 90; those measured from east to west, either on the equator, or any of the lesser circles, are called degrees of longitude, of which there maybe 180; these lesser circles are also called parallels of lati- tude. Of these parallels there may be any number; a circle drawn from east to west, at any distance from the equator, will always be parallel to it, and is therefore called a parallel of latitude. 20. What hour is it then, at places exactly half T^-ay between these meri- dians? 21. How are greater and lesser circles distinguished? 22. What part of a circle is a degree, and how are these further divided ? 23. What is the diameter, and what the circumference of a circle, and what proportion do they bear to each other? 24. What part of a circle is a meridian ? 26. How many degrees are there between the equator and the poles? 26. Into what parts, besides degrees, is the ecliptic divided ? 27. How are degrees of latitude measured, and to what number do they extend? 28. On what circles are degrees of longitude measured, and to what number do they extend ? 29. What is a parallel of latitude' ON THE EARTH. 95 Emily, The degrees of longitude must then vary in length, according to the dunensions of the chcle on which they- are reck- oned ; those, for instance, at the polar circles, will be considerably smaller than those at the equator? Mrs, B, Certainly; 'since the degrees of circles of different dimensions do not vary in number, they must necessarily vary in leng-th., 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? Mrs, B, 1 Sixty geographical miles, which is equal to 69 1 English statute miles; or about one-sixth more than a common laile. Emily, The degrees of longitude at the equator, must then be of the same dimensions, with a degree of latitude. Mrs, B. They would, were the earth a perfect sphere; but it is not exactly such, f)eing somewhat protuberant .about the equator, and flattened towards the poles. This form proceeds from the superior action of the centrifugal power at the equator, and as this enlaro-es the circle, it must, in the same proportion, increase the length of the degrees of longitude measured on it. Caroline, I thought I had understood the centrifugal force perfectly, but I do not comprehend its effects in this instance. Mrs, B, You know that the revolution of the earth on its axis, must give to every particle a tendency to fly off from the cen- tre, that this tendency is stronger, or v/eaker, in proportion to the velocity widi which the particle moves; now a particle situated near to one of the poles, makes one rotation in the same space of time as a 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 equa- tor than in the polar regions: it gradually decreases as you leave the e(iuator and approach the poles, at which points the cen- trifugal force, entirely ceases. Supposing, therefore, the earth to have been originally in a fluid state, the particles in the torrid zone would recede much farther from the centre than those in the frigid zones; thus the polar regions v/ould become flattened, and those about the equator elevated. As a large portion of the earth is covered with water, the Creator gave to it the form, denominated an oblate spheroid, otherwise the polar regions would have been without water, ~ 30. Degrees of longitude vary in length; what is the cause of this? 31. What is the length of a degree of latitude, and why do not these vary ? 32. What causes the equator to be somewhat larger than a great circle passing throu;;h the poles, and what effect has this on degrees of longitude measured on the equator? 33. What is the cause of this form being given to the earth ? 96 ON THE EARTH. and those about the equator, would have been buried several ; miles below the surface of the ocean. | Caroline. I did not consider that the particles in the neigh- j bourhood of the equator, move with greater velocity than those \ about the poles; this was the reason I could not understand joii. ^ Mrs. B. You must be careful to remember, that those pa of a body which are farthest from the centre of motion, must m< with the greatest velocity: the axis of the eartli is the centre its diurnal motion, and the equatorial regions the parts most d. tant 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 Ci tre of motion than your feet; the mountain-top than the vallej . and the more distant any part of a body is from the centre of mo- \ tion, the larger is the circle it will describe, and the great - therefore must be its velocity. Emily. I have been reflecting, that if the earth is not a per- fect circle— Mrs. B. A spliere you mean, my dear: a circle is a round ; line, every part of which is equally distant from the centre; a \ spliere or globe is a round body, the surfiice of which is every ' where equally distant from the centre. Emily. If, then, the earth is not a perfect sphere, but pro- minent at the equator, and depressed at the poles, would not a body weigh heavier at tlie equator than at the poles .^ For the earth ])eing thicker at the equator, the attraction of gravity per- pendicularly downwards must be 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 attrac- tion, the more strongly it is attracted; because it is then nearest to the whole mass of attracting matter. In regard to its effects, you might consider the whole power of gravity, as placed at the centre of attraction. Emily. But were you to penetrate deep into the earth, would gravity increase as you approached the centre? Mrs. B. Certainly not ., I am referring only to any situation on the surface of the earth. Were you to penetrate into the inte- rior, the attraction of the parts above you, would counteract that of the parts beneath you, and consequently diminish the power of gravitj'^ in proportion as you approach the centre; and if you 34. What would have been a consequence of the centrifua^al force, had the earth been a perfect sphere ? 35. A body situated at the poles, is at- toacted more forcibly than if placed at the equator, what is the reason ? ON THE EARTH. 9/ reached that point, being equally attracted by the parts all around YOU, the eftects of gravity would cease, and you would be with- (jut weight. Emily. Bodies, then, should weigh less at the equator than at the poles, since they are more distant from the centre of gra- vity in the former than in the latter situation? Mrs. B. And this is really the case; but the difference of weight would be scarcely sensible, were it not augmented by ano- ther 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 conduc- ing more to the preservation than the destruction of order, — the centiifugal force. This we have just observed to be strongest at the equator; and as it 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 bodies weigh lightest at the equator, where the centrifugal force is greatest; and heaviest at the poles, where this power is least: the weight being diminished at the equator, by both the causes mentioned. Caroline. Has the experiment been made in these different situations ? Mrs. B. Louis XIV. of France, sent philosophers both to the equator, and to Lapland, for this purpose: the severity of the cli- mate, and obstruction from the ice, have hitherto rendered every attempt to reach the pole abortive; but the difference of gravity at the equator, and in Lapland is very perceptible. Caroline. Yet I do not comprehend how the difference of weight could be ascertained, for it the body under trial decreased in weight, the weight which was opposed to it in the opposite scale must have diminished in the same proportion. For in- stance, 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 ascertained by this means. Mrs. B. Your observation is perfectly just: the difference of gravity in bodies situated at the poles, and at the equator, can- not be ascertained by weighing them; a pendulum was therefore used for that purpose. 36. What effect would be produced upon the gravity of a body, were it placed beneath the surface of the earth, and what supposing it at its centre ? 37. What two circumstances combine, to lessen the weight of a body on the equator ? 38. Why could not this be proved by weighing a body at the pole», and at the equator ? I 98 eN THE EARTH. Caroline. What, the pendulum of a clock? 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 by the other end it is suspended to a fixed point, about which it is made to vibrate. When not in motion, a pendulum, obeying the general law of attraction, hanffs like a plumb line, perpendicular to the surface of the earth, but if you raise the pendulum, gravity will bring it back to its perpendicular position. It will, however, not remain station- ary there, for the momentum it has acquired during its descent, will impel it onwards, and if unobstructed, it will rise on the opposite side to an equal heightj from thence it is brought back by gravity, and is again forced upwards, by the impulse of its momentum. Caroline. If so, the motion of a pendulum would be perpetual, and I thought you said, that there was no perpetual motion on the earth. Mrs. B. The motion of a pendulum is opposed by the resist- ance of the air in which it vibrates, and by the friction of the part by which it is suspended: were it possible to remove these obsta- cles, the motion of a pendulum would be perpetual, and its vibra- tions perfectly regular j each being of equal distance, and per- formed 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 pendulum must conse- quently be uniform. Caroline. No, Emily, you are mistaken; the force is not every where the same, and therefore the eftect 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 vibrations of the pen- dulum will be slower at the former place than at the latter. Mrs. B. You are perfectly right, Caroline; it was by this means that the difference of gravity was discovered, 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 our 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 manner different from us. Mrs. B. The only alteration required is to lengthen the pen- 39. What is a pendulum? 40. What causes it to vibrate ? 41. Why are not it« vibrations perpetual ? 42. Two pendulums of the same length, will not, in different latitudes, perform their vibrations in equal times, what is the cause of this ? 43. To what use has this property of the pendulum been ap- j)Ued .' ON THE EARTH. 9.9 dulum 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 vibrates quicker at the pole than at tlie equator, it is supposed to be of the same length. A pendulum which vibrates seconds in this latitude is about 39-f inches long. In order to vibrate at the equator in the same space of time, it must be somewhat shorter; and at the poles, it must be proportionally lengthened. /The vibrations of a pendulum, resemble the descent of a bod} on an inclined plane, and are produced by the same cause; now you must recollect, that the greater the perpendicular height of such a plane, in proportion to its length, the more rapid will be the descent of the body; a short pendulum ascends to a greater height than a larger one, in vibrating a given distance, and of course its descent must be more rapid.^ I shall now, I think, be able to explain to you the cause of the variation of the seasons, and the difference in the length of the days and nights in those seasons; both effects resulting from the same cause. In moving round the sun, the axis of the earth is not perpen- dicular to the plane of its orbit. Supposing this round table to represent the plane of the earth's orbit, and this little globe, the earth ; through this I have passed a wire, representing its axis and poles. In moving round the table, I do not hold the wire perpendicular to it, but obliquely. Emily. Yes, I understand, the earth does not go round the sun in an upright position, its axis is slanting or oblique; and, it of course, forms an angle with a line drawn perpendicular to the plane of the earth's orbit. Mrs. B. All the lines, which you learnt in your last lesson, are delineated on this little globe; you must consider the ecliptic as representing the plane of the earth's orbit; and the equator, which crosses the ecliptic in two places, then shows the degree of obliquity of the axis of the earth; which amounts to SSj degrees, very nearly. The points in which the ecliptic intersects the equator, are called the equinoctial points. But I believe I shall render the effects of the obliquity of the earth's axis clearer to you, by the revolution of 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 44. What change must be made in pendulums situated at the equator and at the poles, to render their vibrations equal ? 45. What do the vibrations of a pendulum resemble, and why will it vibrate more rapidly if shortened ? 46. In the revolution of the earth round the sun, what is the position of its axis ? 47. How much is the axis of the earth inclined, and with what line does it form this angle f 48. What is represented by fig. 2, plate 9 ' 100 ON THE EARTH. in the midst of summer^ or what is called the summer solstice, which is on the 21st of June. Emily. You hold the wire awrj, I suppose, in order to show that the axis of the earth is not upright? Mrs. B. Yesj in summer, 'the north pole is inclined towards the sun. In this season, therefore, the northern hemisphere en- joys much more of his rajs than the southern. The sun, you see, now shines over the whole of the north frigid zone; and not- withstanding the earth's diurnal 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 darkness. 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, Avhile the last are involved in perpetual darkness. Mrs. B. You judge with too much precipitation; examine a little further, and you will find, that the two frigid zones share an equal fate. We shall now make the earth set off from its position in the summer solstice, and carry it round the sun; observe that the pole is always inclined in the same direction, 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 tiie ecliptic cuts, or crosses, the equator, and which is called the autumnal equinox. Emily. The sun now shines from one pole to the other, just as it would constantly do, if the axis of the earth were perpendicu- lar 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, 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 darkness, whilst it illumines that of the south; this change was gradually preparing as I moved the earth from sum- mer to autumn; the arctic circle, which was at first entirely illumined, began to have short nights, which increased as the earth approached the autumnal equinox; and the instant it pass- ed that point, ihe long night of the north pole commences, and 49. How ia the north pole inclined in the middle of our summer, and what effect has this on the north frigid zone? 50, In what direction does the north pole always point? 51. What is shown by the position , he earth at B, in the figure ? 52. How does the sun then shine at the poles, and what is the effect on the days and nights ? ON THE EARTH. lOX the south pole begins to enjoy the light of the sun. We shall now make the earth proceed 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 win- ter 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. Not so either: the inhabitants of the torrid zone have much more heat than we have, as the sun's rays fall per- pendicularly twice in the course of a year, on every place within tlie tropics, while they shine more or less obliquely on the rest of the world, and almost horizontally at the poles; for dur- ing their long day of six months, the sun moves round their ho- rizon without either rising or setting; the only observable dif- erence, is that it is more elevated by a few degrees at mid-day, than at midnight. 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 vertically as at the equator; but its rays will fall upon him more obliquely in au- tumn, and winter, than in summer. Caroline. And therefore, the inhabitants of the temperate 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 respec- tive poles incline towards, 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 exactly the same changes take place in the southern liemisphere, as tliose 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 ecliptic again cuts the equator, on the 21st of March, she is situated, with respect to the sun, exactly in the same position, as in the autumnal equinox; and the only difference 53. When the earth has passed the autumnal equinox, what changes take place at the poles, and also in the whole northern and southern hemispheres ? 54. Why is the heat greatest within the torrid zone ? 55. How does the sua appear at the poles, during the period of day there ? 56. In what will the days and nights differ in the temperate zone, from those at the poles, and at the equator ? I 2 102 ON THE EARTH. with respect io 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. Mrs. B. Yes, for the half of the globe which is enlightened, extends exactly from one pole to the other, the sun has just risen to the north pole, and is just setting to the south pole 5 but in every other part of the globe, the day and night is of twelve hours length; hence the word equinox, which is derived from the Latin, meaning equal night. As our summer advances, the days lengthen in the northern hemisphere, and shorten in the southern, till the earth reaches the summer solstice, when the north frigid zone is entirely iliumined, 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 explanation of the cause of the diiferent lengths of our days and nights, and of the variation of the seasons; and the more I learn, the more I admire the simplicity of means by which such wonderful eftects are produced. Mrs. B. I Cnow not which is most worthy of our admiration, the causes, or the effects of the earth's revolution round the sun. The mind can find no object of contemplation more sublime, than the course of this magnificent globe, impelled by the com- bined powers of projection and attraction, to roll in one invaria- ble course, around the source of light and heat: and what can be more delightful than the beneficent effects of this vivifjing power on its attendant planet. It is at once the gra.nd pnnci- ple which animates and fecundates nature. Emily. There is one circumstance in which this little ivory globe appeai-s 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 case with the earth when neither moon nor stars are visible. Mrs. B. This is owing to the light of the pandle, being re- jflected by the walls of the room, on every part of the globe, con- sequently 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 vour pardon, Mrs. B., I think that the moon, and stars, answer the purpose of walls in reflecting the sun's light to us in the night. 57. Trace the earth from the winter solstice to the vernal equiuox, and in- form me what changes take place. 58. What takes place at the time of the yernal equinox, and what is meant by the term ? 59. In proceeding from the rexTwd equinox to the summer solstice, what changes take place .^ m ON THE EARTH. 103 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 superior heat of the equatorial parts of the earth, arises' from the rays falling perpendicularly on those regions^ whilst they fall, obliquely on these more north- ern regions; now I do not understand why perpendicular rays should afford more heat than oblique rays. Caroline, You need only hold your hand pei*pendicularly over the candle, and then hold it sideways obliquely, to be sen- sible of the difference. Emily. I do not doubt the fact, but I wish to have it explained . Airs. B. You are quite right; if Caroline had not been satis- fied with ascertaining the fact, without understanding it, she would not have brought forward the candle as an illustration; the reason why you feel so much more heat if you hold your hand perpendicularly over the candle, than if you liold it side- ways, is because a stream of heated vapour constantly ascends from the candle, or any otlier burning body, which bein^ lighter than the air of the room, does not spread laterally but rises per,' pendicularly, and this led you to suppose that the rays were notter in tlie latter direction. Had you reflected, you would liave discovered that rays issuing from the candle sideways, 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 because fewer of them fall upon an equal portion of the earth; this will be under- stood better by referring to plate 10. 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 mucli stronger in the former than in the latter; A B, you see, represents the equatorial regions, 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, buit for the greater heat of our summers, as ^he sun shines less obliquely in summer than in winter. Mrs, B. This you will see exemplified in figure 2, in which the earth is represented, as it is situated on the 21st of June, and England receives less oblique, and consequently a greater 60. From what cause arises the superior heat of the equatorialTegions ? 61. Why should oblique rays afford less heat than those which are perpendi- cular? 62. How is this explained by fig. 1. plate 10? 63. How do you ac- count for tlie superior heat of summer, and how is this exemplified in fig. 2 and 3, plate 10 ? 104 ON THE EARTH. number of rays, than at any other season; and figure 3, shows the situation of England on the 21st of 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 perpendi- cular rays; which is, that (they have a greater portion of the at- mosphere to traverse; and tkough it is true, that the atmosphere is itself a transparent body, freely admitting the passage of the sun's rays, yet it is alw ays loaded more or less with dense and foggy vapour, which the rays of the sun cannot easily penetrate^ therefore, the greater the quantity of atmosphere the sun's rays have to pass through in their way to the eartn, the less heat they will retain when they reach it. This will be better understood, by referring to fig. 4. The dotted line round the earth, de- scribes the extent of the atmosphere, and the lines which pro- ceed 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 great- er extent of atmosphere. Caroline. And this, no doubt, is the reason why the sun, in the morning and in tlie evening, gives so much less heat, than at mid -day. Mrs. B. The diminution of heat, morning and evening, is certainly owing to the greater obliquity of the sun's rays; and they are also affected by i\\^ other, both the cause, which 1 have just explained to you; the difficulty of passing through a foggy atmosphere is perlmps more particulai'ly applicable to them, as mist and vapours are prevalent about the time of sunrise and sunset. 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 hori- zon, tlie more heat he will communicate to the earth. Caroline. Both the longest days, and the most perpendicular rays, are on the 21st of June; and yet the greatest heat prevails in July and August. Mrs. B. Those parts of the earth which are once heated, retain the heat for some length of time, and the additional heat they receive, occasions an elevation of temperature,> although the days begin to shorten, and the sun's rays to fall more ob- liquely. 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. 64. What other cause lessens the intensity of oblique rays ? 65. How is this explained by fig. 4.'* 66. What causes conspire to lessen the solar heat in the morning and evening ? 67. The greatest heat of summer is after the solstice, and the greatest heat of the day, after 12 o'clock, although the sun's rays are then most direct, how is this accounted for ^ ON THE EARTH. 105 Emily. And pray, have the other planets the same vicissi- tudes ot seasons, as the earth? Mrs, B, Some of them more, some less, according as their axes deviate more or less from the perpendicuiar, to the plane of their orbits. The axis of Jupiter, is nearly perpendicular to the plane of his orbit; the axes of Mars, and of Saturn, are each, in- clined at angles of about sixty degrees; whilst the axis of Venus is believed to be elevated only fifteen or twenty degrees above her orbit; the vicissitudes of her seasons must therefore be con- siderably greater than ours. For further particulars respecting the planets, I shall refer you to Bonnycastle's Introduction to Astronomy. I have but one more observation to make to you, relative to the earth's motion; which is, that although we have but 365 days and nights in the year, she performs 366 complete revolutions on her axis, during that time. Caroline. How is that possible? for every convplete revolu- tion must bring the same place back to the sun. It is now just twelve o'clock, the sun is, therefore, on our meridian; in twen- ty-four hours will it not have returned to our meridian again, and will not the earth have- made a complete rotation on its axis? Mrs. B. If the earth had no progressive motion in its orbit whilst it revolves on its axis, this would be the case; but as it advances almost a degree westward in its orbit, in the same time that it completes a revolution eastward on its axis, it must re- volve nearly one degree more in order to bring the same meri- dian back to the sun. Caroline. Oh, yes ! it will require as much more of a second revolution to bring the same meridian back to the sun, as is- equal to the space the earth has advanced in her orbit; that is, nearly a degree; this difference is, however, very little. Mrs. B. These small daily portions of rotation, are each equal to the three hundred and. sixty -fifth part of a circle, which at the end of the year amounts to one complete rotation. Emily. That is extremely curious. If the earth then, had no other than its diurnal motion, we should have 366 days in the year. Mrs. B. We should have 366 days in the same period of time that we now have 365; but if we did not revolve round the sun, we should have no natural means of computing years. You will be surprised to hear, that if time is calculated by the 68. Is there any change of seasons in the other planets ? 69. What is said respecting the axes of Jupiter, of Mars, and of Saturn ? 70. In 365 days, how many times does the earth revolve on its axis? 71. How is this ac- counted for ? 72. Do the fixed stars require the same time as the sun, to re- turn to the same meridian .' 106 ON THE EARTH. stars instead of the sun, the irregularity which we have just,no- ticed does not occur, and that one complete rotation of the earth on its axis, brings the same meridian back to any fixed star. Emily. That seems quite unaccountable; for the earth ad- vances in her orbit with regard to the fixed stars, the same as with regard to the sun. Mrs, B. True, but then:%e distance of the fixed stars is so immense, that our solar system is in comparison to it but a spot, and the whole extent of the earth's orbit but a pointy, therefore, whether the earth remain stationary, or whether it revolved in its orbit during its rotation on its axis, no sensible difference would be produced with regard to the fixed stars. One com- plete revolution brings the same meridian back to the same fixed star; hence the fixed stars appear to go round the earth in a shorter time than the sun by three minutes fifty-six seconds of time. Caroline, These three minutes fifty-six seconds is the time which the earth takes to perform the additional three hundred and sixty-fifth part of the circle, in order to bring the same me- ridian back to the sun. Mrs. B. Precisely. Hence the stars gain every day three minutes fifty-six seconds on the sun, which makes them rise that portion of time earlier every day. When time is calculated by the stars it is called sidereal time; when by the sun, solar, or apparent time. Caroline. Then a sidereal day is three minutes fifty-six se- conds shorter, than a solar day oi twenty -four hours. Mrs. B. I must also explain to you what is meant by a side- real year. The common year, called the solar or tropical year, contain- ing 365 days, five hours, forty-eight minutes and fifty -two se- conds, is measured from the time the sun sets out from one of the equinoxes, or solstices, till it returns to the same again; but this year is completed, before the earth has finished one entire revolu- tion in its orbit. Emily. I thought that the earth performed one complete re- volution in its orbit, every year; what is the reason of this varia- tion? Mrs. B. It is owing to the spheroidal figure of the earth. The elevation about the equator produces much the same effect as if a similar mass of matter, collected in the form of a moon, revolved round the equator. When this moon acted on the earth, in conjunction with, or in opposition to the sun, variations 73. How is this accounted for ? 74. What is meant by the solar and the sidereal day? 75. What is the diflference in time between them ? To. What is the length of the tropical year '' "^L^ % 'Hi^ Pl.iA.TEJg. I ON THE EARTH. 107 in the earth's motion would be occasioned, and these variations produce what is called the precession of the equinoxes. Emily, What does that mean? I thought the equinoctial points, were fixed points in the heavens, in which the equator cuts tlie ecliptic. Mrs, B, These points are not quite fixed, but have an appa- rently retrograde motion, among the signs of the zodiac; that is to say, instead of being at every revolution in the same place, they move backwards. Thus if the. vernal equinox is at A, (fig. 1. plate XI.) the autumnal one, will be at B, instead of C^and the following vernal equinox, at D, instead of at A, as would be the case if tlie equinoxes were stationary, at opposite points of the earth's orbit. Caroline. So that when the earth moves from one equinox to the other, though it takes half a year to perform the journey, it has not travelled through half its orbit. Mrs. B. And, consequently, when it returns again to tlie first equinox, it has not completed the whole of its orbit. In order to ascertain when the earth has performed an entire revo- lution in its orbit, we must observe when the sun returns in con- junction witli any fixed star; and this is called a sidereal year. Supposing a fixed star situated at E, (fig. 1. plate XI.) the sun would not appear in conjunction with it, till the earth had return- ed to A, when it would have completed its orbit. Emily. And how much longer is the sidereal, than the solar year? 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 render them sensible. In regard to time, I must further add, that^the earth's diurnal motion on an inclined axis, together with its annual revolution in an elliptic orbit, occasions so much complication in its motion, as to produce many irregularities) therefore the true time can- not be measured by the apparent place of the sun. A peifectly correct clock, 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 15th of April, the l6tJi of June, the 2Sd of August, and the 24th of De- cember. 77. The solar year is completed before the earth has made a complete revo- lution in its orbit, by what is this caused ? 78. What is this called, and what is represented respecting it by fig. 1, plate 11 ? 79. By what means can we ascertain the period of a complete revolution of the earth in its orbit, as illus- trated by the fixed star E, in fig. 1 ? 80. What difference is there in the leng:th of the solar and sidereal year? 81. Why can we not always -ascertain the true time by the apparent place of the sun ? 108 ON THE MOON. Emily. And is there any considerable difference between solar time, and true time? Mrs. B. The greatest difference amounts to between fifteen and sixteen minutes. Tables of equation are constructed for the purpose of pointing out, and correcting these differences between solar time and equal or mean time, which is the denomination given by astronomers, to true time. 82. What would be the greatest difference between solar, and true time, as indicated by a perfect clock ? CONVERSATION IX, ON THE MOON. •F THE moon's motion. — PHASES OF THE MOON. — ECLIPSES OF THE MOON. — ECLIPSES OF JUPITER's MOONS. — OF LATITUDE AND LONGI- TUDE. — OF THE TRANSITS OF THE INFERIOR PLANETS. — OF THE TIDES. MRS. B. We shall, to-day, confine our attention to the moon, which offers many interesting phenomena. The moon revolves round the earth in the space of about twenty -nine days and a half; in an orbit, the plane of which is inclined upwards of five degrees to that of the earth; she ac- companies us in our revolution round the sun." Emily. Her motion then must be of a complicated nature; for as the earth is not stationary, but advances in her orbit, whilst the moon goes round her, the moon, in passing round the sun, must proceed in a sort of scolloped circle. Mrs. B. That is true; and there are also other circumstances which interfere with the simplicity, and regularity of the moon's motion, but which are too intricate for you to understand at present. The moon always presents the same face to us, by which it is evident that she turns but once upon her axis, while she per- 1. In what time does the moon revolve round the earth? what is the incli- oation of her orbit .'' and how does she accompany the earth i* 2. As the moon revolves round the earth, and also accompanies it in its annual revo- lution, in what form would you draw the moon's orbit ? ON THE MOON. 109 forms a revolution round the earth; so that the inhabitants of the moon have but one day, and one night, in the course of a lunar month. Caroline, We afforrl them, however, the advantage of a magnificent moon to enligliten their lon^ nights. Mrs. B, That advantage is put partial; for since we always see the same hemisphere of tlie moon, the inhabitants of that hemisphere alone, can perceive us. Caroline. One half of the moon then enjoys our light, while the other half has constantly nights of darkness. If there are any astronomers in those regions, they would doubtless be' tempted to visit the other hemisphere, in order to behold so grand a luminary as we must appear to them. But, pray, do they see the earth under all the changes, which the moon exhi- bits to us.*^ Mrs.B. Exactly so. These changes are called^ the phases of the moon, and require some explanation. In fig. 2, plate 11, let us say, that S represents the sun, E the earth, and A B C D E F G H, the moon, in different parts of her orbit. When the moon is at A, her dark side being turned towards the earth, we shall not see her as at «; but her disappearance is of very short, duration, and as she advances in her orbit, we perceive her un- der the form of a new moon: when she has gone throu£;h one eighth of her orbit at B, one quarter of her enlightened nemis- phere will be turned towards the earth, and she will then appear horned as at h; when she has performed one quarter of her orbit, she shows us one half of her enlightened side, as at c, and this is called her first quarter; at d she is said to be gibbous, and at e the whole of the enlightened side appears to us, and the moon is at full. As she proceeds in her orbit, she becomes again gib- bous, and her enlightened hemisphere turns gradually away from us, until vshe arrives at G, which is her third quarter; proceeding thence she completes her orbit and disappears, and then again resumes her form of a new moon, and passes successively, through the same changes. When the moon is new, she is said to be in conjunction with the sun, as they are then both in the same direction from the earth; at the time of full moon, she is said to be in opposition, because she and the sun, are at opposite sides of the eartn; at the time of her first and third quarters, she is said to be in her quad- 3. What causes the moon always to present the same face to the earth, and what must be the len^h of a day and night to its inhabitants ? 4. Can the earth be seen from every part of Uie moon, and will it always exhibit the same appearance ? 5. What are the changes of the moon called ? 6. How are these changes explained by fig. 2. plate 11? 7. What is meant by her first quarter? 8. What by her being horned, and her being gibbous? 9. What J)y her being full? 10. What by her third quarter ? 110 ON THE MOON. ratures, because she is then one-fourth of a circle, or 90°, from her conjunction, or the period of new moon. Umiiy. Are not the eclipses of the sun produced bj the moon passing between the sun and the earth ? Mrs. B. Yes; when the moon passes between the sun and the earth, she intercepts his rajs, oi', in other words, casts a shadow on the earth, then tlie sun is eclipsed, and daylight gives place to darkness, while the moon's shadow is passing over us. When, on the contrary, the earth is between the sun and the moon, it is we who intercept the sun's rays, and cast a shadow im the moon; she is then said to be eclipsed, and disappears from our view. Emily, But as the moon goes round the earth every month, she must be, once during that time, between the earth and the sun; and the earth must likewise be once between the sun and the moon, and yet we have not a solar and a lunar eclipse every month? • Mrs, B. I have already informed you, that the orbits of the earth and moon are not in the same plane, but cross or intersect each other; and the moon generally passes either above or below that of the earth, when she is in conjunction with the sun, and does not therefore intercept its rays, and produce an eclipse; for this can take place only when the moon is in, or near her nodes, which is the name given to'those two points in which her orbit crosses that of the earth?- eclipses cannot happen at any other time, because it i;! then only, that they are both in a right line with the sun. Emily. And a partial eclipse of the moon takes place, I sup- pose, when, in passing by the earth, she is not sufficiently above or below the shadow, to escape it entirely? Mrs. B. Yes, one edge oi her disk then dips into the shadow, and is eclipsed; but as the earth is larger than the moon, when eclipses happen precisely at the nodes, they are not only total, but last for upwards of three hours. A total eclipse of the sun rarely occurs, and when it happens, the total darkness is confined to one particular part of the earth, the diameter of the shadow not exceeding 180 miles; evidently showing that the moon is smaller tlian the sun, since she cannot; 11. What is meant by her conjunction ? — what by her being in opposition ? — ^what by her quadratures? 12. By what are eclipses of the sun caused? 13. What causes eclipses of the moon ? 14. What is meant by the moon's nodes? 15. Why do not eclipses happen at every new and full moon? 16. What causes partial eclipses of the moon? 17. When the moon is ex- actly in one of her nodes, what length of time will she be eclipsed? 18. Are total eclipses of the sun frequent, and when tliey happen what is their extent. ■* PUATKXll. I ^ C ^ ►' '^^^^. ^ ON THE MOON. Ill entirely hide it from the earth. In fig. 1, plate 12, you will find a solar eclipse described; S is the sun, M the moon, and E the earth; and the moon's shadow, you see, is not lar;»e enough to cover the earth.^ The lunar eclipses, on the contrary, are visible from every part of the earth, where the moon is above the horizon; and we discover, by the length of time wliich the moon is pas^in^ through the earth's shatlow, that it would be sufficient to eclipse her totally, were she many times her actual sizej^it follows, therefore, that the earth is much larger than the moon. .In fig. 2, S represents the sun, which pours forth rays of light in straight lines, in everi^direction. E is the earth, and IM the moon. Now a ray of light coming from one extremity of the sun's disk, in the direction A B, will meet another, coming from the opposite extremity, in the direction C B; the shadow of the earth cannot therefore extend beyond B; as the sun is larger than the earth, the shadow of the latter is conical, or in the figure of a suaar loaf, it gradually diminishes, and is much smaller than the earth where the moon passes through it, and yet we find the moon to be, not only totally eclipsed, but to remain for a considerable length of time in darkness, and hence we are enabled to ascertain its real dimensions. Emily. When the moon eclipses the sun to us, we must be eclipsed to the moon.^ Mrs. B. Certainly; for if the moon intercepts tlie sun's ra] s, and casts a shadow on us, we must necessarily disappeai to the moon, but only partially, as in fig. 1. Caroline. There must be a great number of eclipses in tb« distant planets, which liave so many moons.^ Mrs, B. Yes, few days pass without an eclipse taking place; for among the number of satellites, one or the other of them are continually passing either between their primary and the sun; or between the planet, and each othcci Astronomers are so well acquainted with the motion of the planets, and their satellites, that they have calculated not only tne eclipses of our moon, but those of Jupiter, with such perfect accuracy, that it has aftbrded a means of ascertaining the longitude. Caroline. But is it not very easy to find both the latitude and longitude of any place by a map or globe? Mrs. B. If you know where you are situated, there is no difficulty in ascertaining the latitude or longitude of the place, by referring to a map; but supposing that you had been a length 19. What does this prove respecting the size of the moon? 20. What is shown in lig. 1, plate 12? 21. How are lunar eclipses visible, and what is proved by their duration? 22. What is illustrated by fig. 2, plate 12? 23. What remark is made respecting those planets which have several moons ? 24. What use is made of the eclipses of the satellites of Jupiter.'* lis ON THE MOON. of time at sea, interrupted in your course by storms, a map would aftbrd you very little assistance in discovering where you were. Caroline. Under such circumstances, I confess I should be equally at a loss to discover either latitude, or longitude. Mrs. B. The latitude is usually found by taking the alti- tude of the sun at mid -day; that is to say, the number of degrees that it is elevated above the horizon, for the sun appears more elevated as we approach the equator, and less as we recede ft'om it. Caroline. But unless you can see the sun, how can you take its altitude.-^ ^ Mrs. B. When it is to^o cloudy to see the sun, the latitude is sometimes found at nigh t,i^by the polar star; the north pole of the eartli, points constantly towards one particular part of the hea- vens, in which a star is situated, called the Polar star: this star is visible on clear niojits, from every part of the northern hemis- phere; the altitude of the polar star, is therefore the same number of degrees, as that of the pole; the latitude may also be deter- mined by observations made on any of the fixed stars if- the situa- tion therefore of a vessel at sea, with regard to nortli'and south, is easily ascertained. The difficulty is, respecting east and west, that is to say, its longitude. As we have no eastern poles from which we can reckon our distance, some particular spot, or line, must be fixed upon for that purpose. The ^English, reckon from the meridian of Greenwich, where the royal observatory is situ- ated; in French maps, you will find that the longitude is reckon- ed from the meridian of Paris. The rotation of the earth on its axis in 24 hours from west to east, occasions, you know, an apparent motion of the sun and stars in a contrary direction, and the sun appears to go round the earth in the space of 24 hours, passing over fifteen degrees, or a twenty -fourth part of the earth's circumference every hour; therefore, when it is twelve o'clock in London, it is one o'clock in any place situated fifteen degrees to the east of London, as the sun must have passed the meridian of that place, an hour be- fore he reaches that of London. For the same reason it is eleven o'clock in any place situated fifteen degrees to the west of London, as the sun will not come to that meridian till an hour later. If then the captain of a vessel at sea, could know precisely what was the hour at London, he could, by looking at his watch, 25. How is the latitude of a place usually found ? 26. By what other means may latitude be found ? 27. From what is longitude reckoned? 28. How does the rotation of the earth upon its axis, govern the time at different places ? ON THE MOON. 117 habitants 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 vertical, but to the inhabitants of the torrid zone. . Caroline. In the torrid zone, then, I hope you will giant 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 naturally partaking of the earth's motion, in its rotation from west to east, the moon, in forming a tide, has to contend against the eastern motion of the waves. AH matter, you know, by its inertia, makes some resistance to a change of state 5 the waters, there- fore, do not readily yield to the attraction of the moon, and the eftect of her influence is not complete, till three hours after she has passed the meridian, where it is full tide. When a body is impelled by any force, its motion may con- tinue, after the impelling force ceases to act: this is the case with all projectiles. A stone thrown from the hand, continues its motion for a length of time, proportioned to the force given to it: there is a perfect analogy between this effect, and me con- tinued rise of the water, after the moon has passed the meridian at any particular place. Emily. Pray what is the reason that the tide is three-quar- ters 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. The earth revolves on its axis in about twenty -four hours; if the moon were stationary, therefore, the same part of our globe would, every twenty-four hours, return beneath the moon; but as during our daily revolution, the moon advances in her orbit, the 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, therefore, 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 I had to make to you on the subject of astronomy; at our next interview, I shall attempt to explain to you the elements of hydrostatics. 45. Why in the open ocean, is it high water, some hours after the moon has passed the meridian? 46. Why are the tides three-quarters of an hour later every day ? CONVERSATION X. ON THE MECHANICAL PROPERTIES OF FLUIDS. DEFINITION OF A FLUID. — DISTINCTION BETWEEN FLUIDS AND LlftUIDS. — OP NON-ELASTIC FLUIDS. — SCARCELY SUSCEPTIBLE OF COMPRESSION. — OF THE COHESION OP FLUIDS. — OF THEIR GRAVITATION. — OF THEIR EaUILIBRIUM. — OF THEIR PRESSURE. OF SPECIFIC GRAVITY. OF THE SPECIFIC GRAVITY OF BODIES HEAVIER THAN WATER. OF THOSE OF THE SAME WEIGHT AS WATER. — OF THOSE LIGHTER THAN WA- TER.— OF THE SPECIFIC GRAVITY OF FLUIDS. MRS. B. We have hitherto confined our. attention to the mechanical properties of solid bodies, which have been illustrated, and, I nope, thoroughly impressed upon your memory, by the conver- sations 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, when applied to liquids, is divided into two parts, hydrostatics and hydraulics. Hydro- statics, treats of the weight and pressure of fluids; and 4iydrau- lics, of the motion of fluids, and the effects produced by this motion. A fluid isr a substance which 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 solids. From tlie sliglit cohesion, of the particles of fluids, and the facility with which they slide over each other, it is inferred, that they have but a slight attraction for each other, and that this attraction is equal, in every position of their § articles, and therefore produces no resistance to a perfect free- om of motion among themselves. Caroline. Pray what is the distinction between a fluid and a liquid ? , Mrs. B. Liquids comprehend only one class of fluids. There . What ai-e the two divisions of the science which treats of the mechanical properties of liquids ? 2. Of what do hydrostatics and hydraulics treat ? Ll. What is a fluid defined to he ? 4. From what is fluidity supposed to ar! 5, Into what two classes are fluids divided ? MECHANICAL PROPERTIES OF FLUIDS. 119 is another class, distinguished bj the name of elastic fluids, or gases, which compreliends the air of the atmosphere, and all the various kinds of air with which you will become ac- quainted, when you study chemistry. Their mechanical pro- perties we shall examine hereafter, and confine our attention this morning, to those of liquids, or non-elastic fluids. Water, and liquids in general, are scarcely susceptible of being compressed, or squeezed into a snialler space, than that which they naturally occupy. Such, however, is the extreme minuteness of their particles, that by strong compression, they sometimes force their way through the pores of the substance which confines them. This was shown by a celebrated experi- ment, 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 tine dew. Many pnilosophers, however, think that this experiment is too much relied upon, as it does not appear that it has ever been repeated 5 it is possible, therefore, that there may have been some source of error, which was not discovered by the experimenters. Fluids, appear to gravitate more freely, than solia bodies; for the strong cohesive attraction of the particles of the latter, in some measure coun- teracts the eff*ect 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, m other words, the wood converted into a fluid, no suonort could be afforded by the legs, for the particles no longer cofier- ing together, each would press separately and independent! v, and would be brought to a level with the surface of the earth. "^ Einily, This want of cohesion is then tlie reason why fluids can never be formed into figures, or maintained in heaps; for though it is true the wind raises water into waves, they are im- mediately afterwards destroyed by gravity, and water always finds its level. J\lrs. B. Do you understand what is meant by the level, or equilibrium of fluids? Emily. I believe I do, though I feel rather at a loss to ex plain it. Is not a fluid level when its surface is smooth and flat, as is the case with all fluids, when in a state of rest? Mrs, B. Smooth, if you please, but not flat; for the defini- tion of the equilibrium of a fluid is, that every part of the sur- face is equally distant from tlie point to which they gravitate, that is to say, from the centre of the earth; hence the surface 6. What is said of the incompressibility of liquids, jand what experiment is related ? 7. Ought this experiment to be considered as conclusiye ? 8. Whj do fluids appear to gravitate more freely than solids ? 120 MECHANICAL PROPERTIES OF FLUIDS. of all fluids must be spherical, not flat, since ihej will partake of the spherical form of the globe. This is very evident m 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 result of their particles gravitating independently of each others for when any particle of a fluid, accidentally finds itself elevated above the rest, it is attracted down to the level of tlie 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 I have seen a drop of oil, float on the surface ©f water, without mixing v/ith it. Mrs. B. They do not mix, because their particles repel each ©ther, and the oil rises to the surface, because oil is a lighter liquid than water. If you were to pour water over it, the oil would still rise, being forced up by the superior gravity of the ] water. Here is an instrument called.^ spirit-level, (fig. 1 , plate 13.) which is constructed upon the principle of the equilibrium of fluids. It consists of a snort tube A B, closed at both ends, and containing a little water, or more commonly some spirits: it is so nearly filled, as to leave only a small bubble of air; when tlie tube is perfectly horizontal, this bubble will occupy the middle of it, but when not perfectly horizontal, the water runs to the lower, and the bubble of air or spirit rises to the upper end; by this instrument, tlie level of any situation, to which we apply it, may be ascertained. From the strong cohesion of their particles, you may there- fore consider solid bodies as gravitating in masses, while every particle of a fluid may be considered as separate, and gravi- tating independently of each other. Hence the resistance of a fluid, is considerably less, than that of a solid body; for the resistance of the particles, acting separately, is more easily overcome. Emily. A body of water, in falling, does certainly less injurj- than a solid body of the same weight. Mrs. B. The particles of fluids, acting thus independent- ly, press against each other in every direction, not only down- wards, but upwards, and laterally or sideways; and in conse- quence of this equality of pressure, every particle remains at rest, in the fluid. If you agitate the fluid, you disturb this 9. "When is a fluid said to be in equilibrium ? 10. What is there in the nature of a fluid, which causes it to seek this level ? 11. What circumstance! occasion oil to float upon water ? 12. What is the nature and use of the in- strument represented in fig. 1, plate 13 ? 13. What difference is there in the gnr&ritatiou of aolid maseee, and of fluids i Jt*UATEiin. MECHANICAL PROPERTIES OF FLUIDS. 121 equality of. pressure, and the fluid will not rest, till its equili- brium IS restored. Caroline. Tl\e 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 up- wards, 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 continue to run out of sucli an opening, be- cause 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? Mrs. B. If the particles of fluids were arranged in regular columns, thus, (fi|^. 2.) there would be no lateral pressure, for when one particle is perpendicularly above the other, it can only press downwards; but as it must continually happen, that a par- ticle presses between two particles beneath, (fig. 3.) these last, must suffer a lateral pressure. 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, therefore, en- tirely from the pressure downwards, or the weight of the liquid above; and consequently, the lower the orifice 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 difterent heights; we shall open them, and you will see with what different degrees of velocity, the water issues from them. Do you understand this, Carolinv? Caroline. Oh yes. The water from the upper spout, receiv- ing but a slight pressure, on account of its vicinity to the sur- face, 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. S. Very well; and you must observe, that as the late- ral pressure, is entirely owine to the pressure downwards, it is not affected by the horizont^ 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. 14. What results as regards the pressure of fluids? 15. How is this illus- trated by %. 2, 3, plate 13? 16. From what does the lateral pre£«iu:e pro- ceed ? and to what is it proportioned, as exemplified in fig. 5, plate 13 ? Li 122 MECHANICAL PROPERTIES OF FLUIDS. Emily. The breadtli and widtli of the vessel then, can be of HO consequence in this respect. The lateral pressure on one •ide, in a cubical vessel, is, I suppose, not so great as the pres- sure downwards upon the Ijottoni. « Mrs, B. *No; m a cubical vessel, the pressure downwards 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 whole 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 unaccountable, as it is in direct opposition to gravity. Mrs. n. And yet it is in consequence of their pressure downwards. When, for example, you pour water into a tea- pot, the water rises in the spout, to a level M'ith the water in the pot. The particles of water at the bottom of the pot, are press- ed upon by the particles above themj to this pressure they will yield, if there is any mode of making way for the superior par- ticles, and as they cannot descend, they will change tlieir di- rection, and rise in the spout. Suppose the tea-pot to be filled with columns of particles of water, similar to that described in fig. 4., the particle 1, at the bottom, will be pressed laterally by the particle 2, and by this pressure be forced into the spout, where, meeting with the par- ticle 3, it presses it upwards, and this pressure will be continued from 3 to 4, from 4 to 5, and so on, till the water in the spout, has risen to a level with that in the pot. Emily. If it were not for this pressure upwards, forcing the water to rise in the spout, the equilibrium of the fluid weuld be destroyed. Caroline. True; but then a tea-pot is wide and 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 quantity, as will fill the spout. But would the same effect be produced, if the spout and tlie pot, were of equal dimensions? Mrs. B. Undoubtedly it would. You may even reverse the experiment, by pouring water into the spout, and you will find that the water will rise in the pot, to a level with that in the spout; for the pressure of the small quantity of water in the spot t, will force up and support, the larger quantity in the pot. V. Has the extent of the surface of a fluid, any effect upon its pressure downwards? 18. What will be the difference between the pressure upon the bottom, and upon one side of a cubical vessel ? 19. What oco^sions th« upward pressure, and how is it explained by fig. 4, plate 13/ MECHANICAL PROPERTIES OF FLUIDS. 123 In the pressure upwards, as well as that laterally, jou 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 experiment by filling this large glass goblet, by means of this narrow tube, (fig. 6.) Caroline. Look, Emily, as Mrs. B.^fills it, how the water rises in the goblet, to maintain an equilibriuJI with that in the tube. Now, Mrs. B., will you let me fill the tube, by pouring water into the goblet r Mrs. B. That is impossible. However, you may try the experiment, and I doubt not that you will be able to account for its failure. Caroline. It is very singular, that if so small a column 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 re- quired to fill the tube:— oh, I see now the reason, the water in the goblet, cannot force that in the lube 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 its level in the tube. I shall now explain to you the meaning of the specific gravity of bodies. Caroline. What! is there another species of gravity, with which we are not yet acquainted.^ Mrs. B. No: the specific gra^dty 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; that is to say, that the first are heavy, and the latter light, in comparison with the generality of sub- stances in nature. Would you call wood, and chalk, light or heavy bodies? Caroline. Some kinds of wood are heavy, certainly, as oak and mahogany; others are light, as cedar and poplar. Emily. I think I should call wood in general, a heavy Kody; for cedar and poplar, are light, only in comparison to wood of a heavier description. I am at a loss to determine whether chalk 20. How could the equilibrium of fluids be exemplified by pouring water in at the spout of a-tea-pot ? 21. How by the apparatus represented at fig. 6» plate 13? 22. What is meant by the specific gravity of a body? 23. What do we in common mean by calling a body heavy, or light ? 124 MECHANICAL PROPERTIES OF FLUIDS. * should be ranked as a heavy, or a light body; I should be inclin- ed to say the former, if it was 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 compari- son, to which we could refer the weight of all other bodies. Mrs. B. The necessity of such a standard, has been so much felt, that a body has been fixed upon for this purpose. What substance do youi^hink would be best calculated to answer this end? Caroline, It must be one generally known, and easily obtain- ed; lead or iron, for instance. Mrs. B. The metals, would not answer the purpose well, for several reasons; they are not always equally Compact, and they are rarely quite pure; two pieces of iron, for instance, although of the same size, might not, from the causes mentioned, weigh exactly alike. Caroline. But, Mrs. B., if you compare the weight, of equal quantities of dift'erent 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. When therefore we compare the weight of different kinds of bodies, it would be absurd to take quantities of equal weighty we must take quantities of equal hulk; 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 or- der 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 specific gravity of bodies, there- r'ure, we must compare equal bulks, and we shall find that their specific gravity, will be proportional to their weights. The body which has been adopted as a standard of reference, is distilled, or rain water. Emily. I am surprised that a fluid should have been chosen tor this purpose, as it must necessarily be contained in some ves- sel, and the weight of the vessel, will require to be deducted. Mrs. B. You will find that the comparison will be more easily made with a fluid, than with a solid; and water you know can be every where obtained. In order to learn the specific gra- vity 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, first in air, and 21. Why would not the metals answer to compare other bodies with? 25. What must be supposed equal in estimating the specific gravity of a body ? 26. What has been adopted as a standard for comparison ? MEeHANICAL PROPERTIES OF FLUIDS. , 125 then in water. If jou wei^h a piece of gold, in a »las9 of water, will not the gold displace just as much water, as is equal to its own bulk? Caroline. Certainly, where one body is, another cannot be at the same time; so that a sufficient quantity of water must be re- moved, in order to make way for the gold. Mrs. B. Yes, a cubic inch of water, to make room for a cu- bic inch of gold; remember that the bulk, alone, is to be consider- ed; the weight, has nothing to do with the quantity of water dis- placed, for an inch of gold, does not occupy more space, and therefore will not displace more water, than an inch ot ivory, or any other substance, that \\A\\ 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? Mrs. B. On account of the upward pressure of the particles of water, which in some measure supports the gold, and by so do- ing, diminishes its weight, j 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 whicli 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 watec will offer some resistance to its descent. Caroline. And the resistance which water offers to the de- scent 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 : the resistance of the fluid, is proportioned to the bulk, and not to the weight, of the body immersed in it; all bodies of the same size, therefore, lose the same quantity of their weight in water. Can you form any idea what this loss will be? Emily. I should think it would be equal to the weight of the water displaced; for, since that portion of the water was sup- ported 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 suspended, it would restore the balance. You must observe, that when you weigh a body in water, in order to ascertain its specific gravity, you must not sink the dish of tlie balance in the water; but either suspend the body to a 27. What is the first step in ascertaining the specific gravity of a solid ? 28. What quantity of water will the solid displace ? 29. Why will a solid weigh less ia water than in air, and to what will the loss of weight be equal ? L 2 126 MECHANICAL PROPERTIES OF FLUIDS. hook at the bottom of the dish, or else take off the dish, and sus- pend to the arm of the balance a weight to counterbalance the other dish, and to this attach the solid to be weighed, (fig. 7.) Now suppose that a cubic inch of gold, weighed 19 ounces out of wa- ter, and lost one ounce of its weight bj being weighed in water, what would be its specific gravity? Caroline. The cubic inch of water it displaced, must weigh that one ounces and as a cubic inch of gold, weighs 19 ounces, gold is 19 times, as heavy as water. Emily. I recollect having seen a table of the comparative weights of bodies, in which gold appeared to me to be estimated at 19 thousand times, the weiglit 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 Sie 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. Carolitie. We may call the weight of water, for example, on6, 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, specific gravity, means how many times more a body weighs, than an equal bulk of water. Mrs. B. It is rather the weight of a body compared with a portion of water equal to it in bulk; for the specific gravity of hiany substances, is less than that of water. Caroline. Then you cannot ascertain the specific gravity of audi substances, in the same manner as that of gold; \ov a body that is lighter than water, will float on its surface, without dis- placing any of it. Mrs. B. If a body were absolutely without weight, 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, there- fore, displace some quantity. If the body be lighter than wa- ter, it will not sink to a level with its surface, and therefore it will not displace so much water as is equal to its bulk; but only If you will take the taouble of following it through these various changes, you will un- derstand 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 earth, the heat, by separating the particles of water, rendered 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 va- pour; in consequence of which, it ascended into the atmosphere^ where it formed clouds. Mrs. B. We have then alreadjr 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. B. Because the atmosphere diminishes in density, 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 specific gravity; and there, you know, it will remain stationary. By the frequent accession of fresh vapour, it gradually accumulates, so as to forni those large bo- dies of vapour, which we call clouds: and the particles, at length unitinff, become too heavy for the air to support, and fall to the ground. Caroline. They do fall to the ground, certainly, when it rains; but, accord.ing to your theory, I should have imagined, 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 can- not be so heavy as the lowest regions of the atmosphere, other- wise 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 1 . Why do not the frequent rains, fill the eeirth with water ? S. Why will vapour rise ? to what height will it ascend, and what will it form ? 3. How may drops of rain be formed ? 136 OF SPRINGS, FOUNTAINS, &C. watery particles come within the sphere of each other's attrac- tion, 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 consequently descends to the earth. Caroline, How v/onderfully curious! Mrs. B. It is impossible to consider any part of nature at- tentively, without being struck with admiration at the wisdom it displays; and I hope you will never contemplate 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 de- stroyed by the excess of moisture; if, on the other hand, tlie plants were not nourished and refreshed by occasional showers, the drought would be equally fatal to them. If the clouds con- stantly remained in a state of vapour, they might, as you re- marked, 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. Water then ascends in the form of vapour, and descends in that of rain, snow, or hail, all of which ultimately become water. Some of this falls into the various bodies of water on the sur- face of the globe, the remainder upon the land. Of the latter, part reascends in the form of vapour, part is absorbed by the roots of vegetables, and part descends into the earth, where it forms springs. Entity. Is there then no difference between rain water, and spring water ? Mrs, B. They are originally the same; but that portion of rain water which goes to supply springs, dissolves a number of foreign particles, which it meets with m its passage through the various soils it traverses. Caroline. Yet spring water is more pleasant to the taste, appears more transparent, and, I should have supposed, would have been more pure than rain water. Mrs. B. No; excepting distilled water, rain water is the most pure we can obtain; it is its purity which renders it insipia; whilst the various salts and diiferent ingredients, dis- solved in spring water, give it a species of flavour, which habit renders agreeable; these salts do not, in any degree, aft'ect its transparency; and the filtration it undergoes, through gravel and 4. What becomes of the water after it has fallen to the earth ? 5. What is the difference between rain water, and that from springs ? 6. Why is rain more pure than spring water ? 7. Why i« spring water more agreeable to the palate ? OF SPRINGS, FOUNTAINS, &,C. 151 sand, cleanses it from all foreign matter, which it has not the power of dissolving. Emily, How is it that the rain water does not continue to descend by its gravity, instead of collecting together, and form- ing springs? Mrs, B. When rain falls on the surface of the earth, it continues making its way downwards through tlie pores and crevices in the ground. When several drops meet in their sub- terraneous passage, they unite and form a little rivulet; this, in its progress, meets with otlier rivulets of a similar description, and they pursue their course together within the eartli, till they are stopped by some substance, such as rock, or clay, which they cannot penetrate. Caroline. But you say that there is some reason to believe that water can penetrate even the pores of gold, and it cannot meet with a substance more dense? Mrs. B. But if water penetrate the pores of gold, it is only when under a strong compressive force, as in the Florentine experiment; now in its passage towards the centre of tlie earth, it IS acted upon by no other power than gravity, which is not sufficient to make it force its way, even through a stratum of clay. This species of earth, though not remarkably dense, be- ing of great tenacity, will not admit the particles of water to pass. When water encounters any substance of this nature, therefore, its progress is stopped, and it is diffused through the porous earth, and sometimes the pressure of the accumulating waters, forms a bed, or reservoir. This will be more clearly explained by fio-. 9, plate 13, which represents a section, of the interior 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 con- tinual accession of water it receives from the ducts or rivulets «, «, «, a,) finds a passage 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 downwards towards the centre of the earth; and the spiing in this figure runs in an horizontal direction. Mrs. B. Not entirely. There is some declivity from the reservoir, to the spot where the water issues out of the ground; and gravity, you know, will bring bodies down an inclined plane, as well as in a perpendicular direction. Caroline. But though the spring may descend, on first issu- ing, it must afterwards rise to reach tne surface of the earth; and that is in direct opposition to gravity. 8. What causes the water to collect and form springs ? 9. Why cannot water penetrate through clay ? 10. What is represented by fig. 9, plate 13? 1S£ OF SPRINGS, FOUNTAINS, &C. Mrs. B. A spring can never rise above the level of the re- servoir whence it issues; it must, therefore, find a passage to some part of the surface of the earth, that is lower, or nearer the centre, tlian the reservoir. It is true that, in this figure, the spring rises in its passage from B to C; but this, I think, witli a little reflection, jou will be able to account for. Emily, Oh, jes; it is owing to the pressure of fluids up- wards; and the water ri<«es in the duct, upon the same principle as it rises in the spout of a tea-pot; that is to say, in order to preserve an equilibrium with the water in tlie reservoir. Now 1 think I understand the nature of springs: the water will flow through a duct, whether ascending 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 houses, if it is originally brought from a height, superior to any to which it is conveyed. Have you never observed, when the pavements of the streets have been mending, the pipes which serve as ducts for the con- veyance of the water through the town? Emily. Yes, frequently; and I l\ave remarked that when any of these pipes have been opened, the water rushes upwards from them, with great velocity; which, I suppose, proceecls from the pressure of the water in the reservoir, which forces it out. Caroline. I recollect having once seen a very curious glass, called Tantalus's cup; it consists of a goblet, containing a small figure of a man, and whatever quantity of water you pour into the goblet, it never rises higlier than the breast of the figure. Do you know how that is contrived? Mrs. B. It is by means of a s^'phon, or bent tube, which is concealed in the body of the figure. This tube rises through one of the less, 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. (fi^. 1, plate 14.) When you pour water into the glass A, it must rise in the syphon B, in pro- portion 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 running 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 scar- city of water, and my father has been at considerable expense to dig a well; after penetrating to a great depth, before water could 11. How can you account for its rising upwards, as represented at C ? 12. In conveying water by means of pipes, how must the reservoir be situat- ed? 13. What is the instrument called, which is represented in Dlate 14, %. 1, — and how does it operate ? m n Oy SPRINGS, FOUNTAINS, &C. 13S be found, a spring was at length discovered, but the water rose only a few feet above the bottom of the well 5 and sometimes it is quite dry. Mrs. B, This has, however, no analogy to Tantalus's cupj but is owing to the very elevated situation of your country- house. Emily. I believe I guess the reason. There cannot be a re- servoir 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 witliout 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 considerable depth below the summit of the hill. Caroline. Your explanation appears very clear and satisfac- tory; but I can contradict it from experience. At the very top of a hill, near our country-house, there is a large pond, and, ac- cording 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 the Alps, and I can assure you, that tliere is a fine lake on the summit of Mount Cenis, 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 the- ory 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 neighbourhood, 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 occa- sionally be dry. Mrs. B. Because the reservoir from which it flows, being i* an elevated situation, is but scantily supplied with water; after a long drought, therefore, it may be drained, and tlie spring dry, till the reservoir be replenished by fresh rains. It is not un- common to see springs flow with great violence in wet seasons, which at other times, are perfectly dry. Caroline. But tliere is a spring in our grounds, which more 14. Why are wells rarely well supplied with water, in elevated situations ? 15. When water is found in elevated situations, whence is it supplied? 16. Wells and springs, at some periods well supplied, fail at otherB ; how is thii ac- counted for? M 1S4 OF SPRINGS, FOUNTAINS, &C. frequently flows in dry, than in wet w^atherj how is that to be accounted for? Mrs. B. The spring, probabl}^, comes from a reservoir at a great distance, and situated very deep in the ground: it is, therefore, some length of time before tlie rain reaches the reser^ Toirj and another considerable portion must elapse, whilst the water is making its way, from the reservoir, to the surface of the earthj so that the dry Aveather may probably have succeeded the rains, before the spring begins to flow; and the reservoir may be exhausted, by the time the wet weatlier sets in again. Caroline. 1 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 tlie reservoir being imper- fectly supplied. Independently of the situation, this is always the case, when the duct, or ducts, which convey the water into the reservoir, are smaller than those which carry it off*. Caroline. If it runs out, faster than it runs in, it will of course sometimes be empty. Do not rivers also, derive their source from springs.^ Mrs. B. Yes, they generally take their source in mountain- ous countries, where springs are most abundant. Caroline. I understood you that springs were more rare, in elevated situations. Mrs. B. You do not consider that mountainous countries, abound equally witli high, and low situations. Reservoirs of wa- ter, wliich are formed in the bosoms of mountains, generally find a vent, eitlier on their declivity, or in the valley beneath; while subterraneous reservoirs, formed in a plain, can seldom find a passage to tlie surface of the earth, but remain concealed, unless discovered by digging a well. When a spring once issues at the surface of the earth, it continues its course externally, seeking always a lower ground, for it can no longer rise. Emily. Then what is the consequence, if the spring, or, as 1 should now ratlier call itf the rivulet, runs into a situation, which is surrounded by higlier ground.^ Mrs. B. Its course is stopped; the water accumulates, and it forms a pool, pond, or lake, according to the dimensions of the body of water. The lake of Geneva, in all probability, owes its origin to the Rhone, which passes through it: if, when this river first entered the valley, which now forms the bed of the 17. Some springs flow abundantly in dry weather, which occasionally fail in wet weather, how may this be explained? 18. What is meant by inter- niittin* springs ? 19. Whence do rivers, in general, derive their water ? 20. Why do sjH-iiigs abound more in mountainous, than in level countries OF SPRINGS, FOUNTAINS, &C. 135 Lake, it found itself surrounded by higher grounds, its waters would there accumulate, till they rose to a level with that part of the valley, wliere the Rhone now continues its course beyond the Lake, and from whence it flows through valleys, occasionally forming other small lakes, till it reaches the sea. Emily. And are not fountains, of the nature of springs? Mrs. B. Exactly* A fountain is conducted perpendicularly upwards, by the spout or adjutage A, through which it flows; and it will rise nearly as high as the reservoir B, from whence it proceeds. (Plate 14. fig. 2.) Caroline. Why not quite as hij^h? Mrs. B. Because it meets with resistance from the air, in its ascent; and its motion is impeded by friction against the !»pout, where it rushes out. Emily. But if the tube through which the water rises be smooth, can there be any friction? especially with a fluid, whose particles yield to the slightest impression. Mrs. B. Friction, (as we observed in a former lesson,) may be diminished by polishing, but can never be entirely destroyed; and though fluicls, are less susceptible of friction, than solid bo- dies, they are still affected by it. Another reason why a foun- tain will not rise so high as its reservoir, is, that as all the water which spouts up, has to descend again, it in doing so, presses, or strikes against the under parts, and forces them sideways, spread- ing the column into a head, and rendering it botli wider, and shorter, than it otherwise would be. At our next meeting, we shall examine the mechanical pro- perties of the au', which being an elastic fluid, differs in many respects, from liquids. 21. How are lakes formed? 22. What causes water to rise in fountain?, and how is this explained by figure 2, plate 14 ? 23, Why will not the foun- tain rise to the height of the water in the reservoir ? CONVERSATION XIL ON THE MECHANICAL PROPERTIES OF AIR. *F THE SPRING OR ELASTICITY OF THE AIR. — OF THE WEIGHT OF THE AIR. — EXPERIMENTS WITH THE AIR PUMP. — OF THE BAROMETER. — MODE OF WEIGHING AIR. — SPECIFIC GRAVITY OF AIR. OF PUMPS. DESCRIPTION OF THE SUCKING PUMP. — DESCRIPTION OF THE FORCING PUMP. MRS. B. At our last meeting we examined the properties of fluids in *^neral, and more particularly of such as are called non-elastic fluids, or liquids. There is another class of fluids, distinguished by the name of aeriform, or elastic fluids, the principal of which is the air we breathe, which surrounds the earth, and is called the atmo- sphere. Emily, There are then other kinds of air, besides the atmo- sphere? Mrs. B. Yes; a great variety; butf^they differ only in their chemical, and not in their mechanical properties; and as it is the latter we are to examine, w^e shall not at present inquire into their composition, but confine our attention to the mechanical properties of elastic fluids in general. Caroline. And from whence arises this difference, between elastic, and non-elastic fluids? Mrs. B. There is no attraction of cohesion, between the par- ticles of elastic fluids; so that the expansive power of heat, has no adversary to contend v.'ith, but gravity; any increase of tempera- ture, therefore, expands elastic fluids considerably, and a dimi- nution, proportionally condenses them. The most essential point, in which air, differs from other fluids, is in its spring or elasticity; that is to say, its power of increas- ing, or diminishing in bulk, accordingly as it is more, or less, com- pressed: a power of which I have informed you, liquids are al- most wholly deprived. 1. Into what two kinds are fluids divided.'' 2. There are different kinds of elastic fluids, in what properties are they alike, and in what do they dif- fer ? 3. In what particular do elastic, differ from non-elastic, fluids .' 4. "What is meant by Uie elasticity of air ^ M^ECHANICAL PROPERTIES OF AIR. 137 Emily. I think I understand the elasticity of the air very well from what you formerly said of it; but what perplexes me is, its having gravity; if it is heavy, and we are surrounded by it, why do we not feel its weight? Caroline. It must be impossible to be sensible of the weight of such infinitely small particles, as those of wliich the air is composed: particles which are too small to be seen, must be too light to be felt. Mrs. B. You are mistaken, my dear; the air is mucli heavier than you imagine; it is true, that the particles which compose it, are small; but then, reflect on their quantity: the atmosphere extends in height, a great number of miles from the earth, and its gravity is such, that a man of middling stature, is computed (when the air is heaviest) to sustain the weight of about 14 tons. Caroline. Is it possible! I should have thought such a weight would have crushed any one to atoms. Mrs. B. That would, indeed, be the case, if it were not for the equality of the pressure, on every part of the body; but when thus diftused, we can bear even a much greater weight, without any considerable inconvenience. In bathing we support the weight and pressure of the water, in addition to that of tlie atmo- sphere; but because this pressure is equally distributed over the body, we are scarcely sensible of it; whilst if your shoulders, your head, or any particular part of your frame, were loaded with the additional weight of a hundred pounds, you would soon sink under the fatigue. Besides this, our bodies contain air, the spring of which, counterbalances the weight of tlie external air, and ren- ders us insensible of its pressure. Caroline. But if it were possible to relieve me from the weight of the atmosphere, should I not feel more light and agile? Mrs. B. On the contrary, the air within you, meeting with no external pressure to restrain its elasticity, would distend your body, and at length bursting some of tlie parts which con- fined it, put a period to your existence. Caroline. This weight of the atmosphere, tlien, which I was so apprehensive would crush me, is, in reality, essential to my preservation. Emily. I once saw a person cupped, and was told that thft swelling of the part under the cup, was produced by taking away from that part, the pressure of the atmosphere; but I could not understand how this pressure produced such an effect. Mrs. B. The air pump aftbrds us the means of making a great variety of interesting experiments, on the weight, and pressure of 5. What is said respecting the weight of the atmosphere ? 6. Why do we not feel the pressure of the air ? 7 What would be the eflfect of relieving us from atmospheric pressure ? M ^ 138 MECHANICAL PROPERTIES OF AIR. the air: some of them you have already seen. Do you not recol- lect, that in a vacuum produced within the air pump, substances of various v^^eights, fell to the bottom in the same time^ why does not this happen in the atmosphere ? Caroline. I remember you told us it was owing to the resist- ance which light bodies meet with, from the air, during their fall. Mrs. B. Or, in other words, to the support which they re- ceived from the air, and which prolonged the time of their fall. Now, if the air were destitute of weight, how could it support other bodies, or retard their fall ? I shall now show you some other experiments, which illustrate, in a striking manner, both tlie weight, and elasticity of air. I shall tie a piece of bladder over this glass receiver, which, you will observe, is open at the top as well as below. Caroline. Why do you wet the bladder lirst.^ Mrs. B. It expands by wetting, and contracts in drying; it is also more soft and pliable when wet, so that I can make it fit better, and when dry, it will be tighter. We must hold it to the fire in order to dry it; but not too near, lest it should burst by sudden contraction. Let us now fix it on the air pump, and ex- haust the air from underneath it — ^you will not be alarmed if you hear a noise ? Emily. It was as loud as the report of a gun, and the blad- der is burst I Pray explain how the air is concerned in this ex- periment. Mrs. B. It is the eftect of the weight of the atmosphere, on the upper surface of the bladder, when I had taken away the air from the under surface, so that there was no longer any reaction to counterbalance the pressure of the atmosphere, on the receiver. You observed how the bladder was pressed inwards, by the weight of the external air, in proportion as I exhausted the receiver; and before a complete vacuum was formed, the bladder, unable to sustain the violence of the pressure, burst with the explosion you have just heard. I shall now show you an experiment, which proves the expan- sion of the air, contained within a body, when it is relieved irora the pressure of the external air. You would not imagine that there was any air contained within this shrivelled apple, by its appearance; but take notice of it when placed within a receiver, from which I shall exhaust the air. Caroline. How strange ! it grows quite plump, and looks like a fresh -gathered apple. Mrs. B. But as soon as I let the air again into the receiver, 8. How may the weight of the air be shown by the aid of the air pump, and a piece of bladder ? 9. How is this explained ? 10. How jnay its eias- ticfty be exhibited, by an apple, <4nd by a bladcter? MECHANICAL PROPERTl£S O* AIR. 159 the apple, you see, returns to its shrivelled state. When I took away the pressure of the atmosphere, the air within the apple, ex- panded, and swelled it out; but the instant the atmospherical air was restored, the expansion of the internal air, was checked and repressed, and the apple shrunk to its former dimensions. You may make a similar experiment with this little bladder, which you see is perfectly flaccid, and appears to contain no air: in this state I shall tie up the neck of the bladder, so that what- ever air remains within it, may not escape, and then place it un- der the receiver. Now observe, as I exhaust the receiver, how the bladder distends; this proceeds from the great dilatation of the small quantity of air, which was enclosed within the bladder, when I tied it up; but as soon as I let the air into the receiver, that which the bladder contains, -condenses and shrinks into its small compass, within the folds of the bladder. Emily. These experiments are extremely amusing, and they afford clear proofs, both of the weight, and elasticity of the airj but I should like to know, exactly, how much the air weighs. Mrs, B. A column of air reaching to the top of the atmo- sphere, and whose base is a square inch, weighs about 15lbs. therefore, every square inch of our bodies, sustains a weight of lolbs. : and if you wish to know the weight of the whole of the atmosphere, you must reckon how many square inches there are on the surface of the globe, and multiply them by 15. Emily. But can we not ascertain tne weight of a small quan- tity of air? Mrs. B, With perfect ease. I shall exhaust the air from this little bottle, by means of the air pump: and having emptied tlie bottle of air, or, in other words, produced a vacuum within it, I secure it by turning this screw adapted to its neck: we may now find the exact weight of this bottle, by putting it into one of the scales of a balance. It weighs, you see, just two ounces; but when I turn the screw, so as to admit the air into the bottle, the scale which contains it, preponderates. Caroline. No doubt the bottle filled with air, is heavier than the bottle void of air; and the additional weight required to bring the scales a^ain to a balance, must be exactly that of the air which the bottle now contains. Mrs, B. That weight, you see, is almost two grains. The dimensions of this bottle, are six cubic inches. Six cubic inches of air, therefore, at the temperature of this room, weighs nearly 2 grains. 1 1. What is the absolute weight of a given column of atmospheric air, and how could its whole pressure upon the earth be ascertained ? 12. How can the weight of a small bulk of air be found f 140 MECHANICAL PROPERTIES OF AIR. Caroline. Why do you observe the temperature of the rooiU;^ in estimating the weight of the air? Mrs. B. Because heat rarities air, and renders it ligliter; tlierefore the warmer the air is, which you weigh, the lighter it will be. If you should now be desirous of knowing the specific gravity of this air, we need only fill the same bottle, with water, and thus obtain the weight of an equal quantity of water — which you see is 1515 grs.; now by comparing the weight of water, to that of air, we find it to be in the proportion of about 800 to 1. As you are acquainted with decimal arithmetic, you will un- derstand what I mean, when I tell you, that water being called 1000, the specific gravity of air, will be 1.2. I will show you another instance, of the weight of the atmo- sphere, wliich I think will please you: you know what a barome- ter is.^* Caroline. It is an instrument which indicates the state of the weather, by means of a tube of quicksilver; but how, I can- not exactly say. Mrs. B. It is by showing the weight of the atmosphere, which has great influence on the weather. The barometer, is an instrument extremely simple in its construction. In order that you may understand it, I will show you how it is made. I first fill with mercury, a glass tube A fe, (fig. 3, plate 14.) about three feet in length, and open only at one end; then stopping the open end, with my finger, I immerse it in a cup C, contain- ing a little mercury. Emily. I^rt of the mercury which was in the tube, I ob- serve, runs down into the cup; but why does not the whole of it subside, for it is contrary to the law of the equilibrium of fluids, that the mercury in the tube, should not descend to a level with tliat in the cup? Mrs. B. The mercury that has fallen from the tube, into the cuj), has left a vacant space in the upper part of the tube, to which the air cannot gain access; this space is therefore a per- fect vacuum; the mercury in the tube, is relieved from the pres- sure of the atmosphere, whilst that in the cup, remains exposed to it. Caroline. Oh, now I understand it; the pressure of the air en the mercury in the cup, forces it to rise in the tube, where there is not any air to counteract the external pressure. 13. In ascertaining the weight of air, we take account of its temperature — Why ? 14. How could you ascertain the specific gravity of air, and wha*. would it be ? 15. What are the essential parts of a barometer, as representaJ plate 14,%, 3? R MECHANICAL PROPERTIES OF AIR, 141 ^mily. Or rather supports the mercury in the tube, and vents it from falling. Mrs. B. That comes to the same thing; for the power that I can support mercury in a vacuum, would also make it ascend, when it met with a vacuum. Thus you see, that the equilibrium of the mercury is destroy- ed, only to preserve the general equilibrium of fluids. Caroline. But this simple apparatus is, in appearance, very unlike a barometer. Mrs. B. It is all that is essential to a barometer. The tube and the cup, or a cistern of mercury, are fixed on a board, for the convenience of suspending it; the brass plate on the upper part of the board, is graduated into inches, and tenths of incnes, for the purpose of ascertaining the height at which the mer^^^iry stands in the tube; and the small moveable metal plate, serves to show that height, with greater accuracy. Emily. And at what height, will the weight of the atmo- sphere sustain the mercury? Mrs. B. About 28 or 29 inches, as you will see by this barometer; but it depends upon the weight of the atmosphere, which varies much, in different states of the weather. The great- er the pressure of the air on the mercury in the cup, the higher it vn[\ ascend in the tube. Now can you tell me whether the air is heavier, in wet, or in dry weather? Caroline. Without a moment's reflection, the air must be heaviest in wet weather. It is so depressing, and makes one feel so heavy, while in fine weather, I feel as light as a feather, . and as brisk as a bee. Mrs, B. Would it not have been better to have answered with a moment's reflection, Caroline? It would have convinced you, that the air must be heaviest in dry weather; for it is then, that the mercury is found to rise in the tube, and consequently, the mercury in the cup, must be most pressed by the air. Caroline. Why then does the air feel so heavy, in bad weather? Mrs. B. Because it is less salubrious, when impregnated with damp. The lungs, under these circumstances, do not play so freely, nor does the blood circulate so well; thus obstructions are frequently occasioned in the smaller vessels, from which arise colds, asthmas, agues, fevers, &c. Emily. Since the atmosphere diminishes in density, in the upper regions, is not the air more rare, upon a hill, than in a plain; and does the barometer indicate this difference? 16. What sustains the mercury in the tube? 17. Of what use are the divisions in the upper part of the instrument? 18. To what height will the mercury rise, and what occasions this height to vary? 19. VVhen is the mercury highest, in wet, or in dry weather ? 20. What occasions the sensa- tion of oppression, in damp weather ? 14£ MECHANICAL PROPERTIES OF AIR. Mrs. B. Certainly. This instrument, is so exact in its in- dications, that it is used for the purpose of measuring the height of mountains, and of estimating the elevation of balloons; the mercury descending in the tube, as you ascend to a greater height. Jtlmily. And is no inconvenience experienced, from the thin- ness of the air, in such elevated situations? Mrs. B. Oh, yes; frequently. It is sometimes oppressive, from being insufficient for respiration; and the expansion which takes place, in the more dense air contained within the body, is often painful : it occasions distention, and sometimes causes the bursting of the smaller blood-vessels, in the nose, and ears. Be- sides in such situations, you are more exposed, both to heat, and cold; for though the atmosphere is itself transparent, its lower regions, abound with vapours, and exhalations, from the earth, which float in it, and act in some deoree as a covering, which preserves us equally from the intensity of the sun's rays, and from the severity of the cold. Caroline. Pray, Mrs. B., is not the thermometer constructed on the same principles as the barometer? Mrs. B. Not at all. The rise and fall of the fluid in the thermometer, is occasioned by the expansive power of heat, and the condensation produced by cold: the air has no access to it. An explanation of it would, therefore, be irrelevant to our pre- sent subject. Emily. I have been reflecting, that since it is the weight of the atmosphere, which supports the meixury, in the tube of a barometer, it would support a column of any other fluid, in the same manner. Mrs. B. Certainly; but as mercury, is heavier than all other fluids, it will support a higher column, of any other fluid; for two fluids are in equilibrium, when their height varies, inversely as their densities. We find the weight of the atmosphere, is equal to sustaining a column of water, for instance, of no less than 32 feet above its level. Caroline. The weight of the atmosphere, is then, as great as that of a body of water of 32 feet in height. Mrs. B. Precisely; for a column of air, of the height of the atmosphere, is equal to a column of water of about '32 feet, or one of mercury, of from 28 to 29 inches. The common pump, is dependent on this principle. By the 21. Why will the barometer indicate the height of mountains, or of bal- loons ? 22. Is any inconvenience experienced by persons ascending to great heights, and from what cause ? 23. Wliat occasions the rise and fall of the mercury, in a thermometer ? 24. To what height will the pressure of the atmosphere raise a column of water ? 25. What governs the difference be- tween the height of the mercury, and of the water ? MECHANICAL PROPERTIES OF ALR. 143 act of pumpin», the pressure of the atmosphere is taken off the water, which, in consequence, rises. The body of a pump, consists of a large tube or pipe, whose lower end is immersed in the water which it is designed to raise. A kind of stopper, called a piston, is fitted to this tube, and is made to slide up and down it, by means of a metallic rod, fastened to the centre of the piston. Emily. Is it not similar to the syringe, or squirt, with which you first draw in, and then force out water? Mrs. B. It is5 but you know that we do not wish to force the water out of the pump, at the same end of the pipe, at which we draw it in. The intention of a pump, is to raise water from a spring, or well; the pipe is, therefore, placed perpendicularly over the water, which enters it at the lower extremity, and it issues at ^horizontal spout, towards the upper part of the pump; to effect this, there are, besides the piston, two contrivances called valves. The pump, tlierefore, is rather a more compli- cated piece of machinery, than the syringe. Caroline. Pray, Mrs. B. , is not the leather, which covers the opening, in the lower board of a pair of bellows, a kind of valve? Mrs. B. It is, valves are made in various forms; any con- trivance, which allows a fluid to pass in one direction, and pre- vents its return, is called a valve; that of the bellows, and of the common pump, resemble each other, exactly. You can now, I think, understand the structure of the pump. Its various parts, are delineated in this figure: (fig. 4. plate 14.) A B is the pipe, or body of the pump, P the piston, V a valve, or little door in the piston, which, opening upwards, admits the water to rise through it, but prevents its returning, and Y, is a similar valve, placed lower down in the body of the pump; H is the handle, wnich in this model, serves to work the piston. When the pump is in a state of inaction, the two valves are closed by their own weight; but when, by working the handle of the pump, the piston ascends; it raises a column of air which rested upon it, and produces a vacuum, between the piston, and the lower valve Y; the au' beneath this valve, which is immedi- ately over the surface of the water, consequently expands, and forces its way through it; the water, then, relieved from the pressure of the air, ascends into the pump. A few strokes of the handle, totally excludes the afr from the body of the pump, and fills it witli water, which, having passed through both the valves, runs out at the spout. Caroline. I understand this perfectly. When the piston is 26. How does the common pump, raise water from a well ? 27. What is meant by a piston ? 28. Describe the construction, and use, of a valve. 29. What are the parts of the pump, as represented, %. 4, plate 14 ? 144 MECHANICAL PROPERTIES OF AIR. elevated, the air, and the water, successively rise in the pump, for the same reason as the mercury, rises in the barometer. Emily, I thought that water was drawn up into a pump, by suction, in the same manner as water may be sucked through a •traw. Mrs, B, It is so, into the body of the pump; for the power of suction, is no other than that of producing a vacuum over one part of the liquid, into which vacuum the liquid is forced, by the pressure of the atmosphere, on another part. The action of sucking through a straw, consists Jn drawmg in, and confining the breath, so as to produce a vacuum in the mouth; in conse- quence of which, the air within the straw, nishes into the mouth, and is followed by the liquid, into which, the lower end of the straw, is immersed. The principle, you see, is the same, and the only difference consists in the mode of producing a vacuum. In suction, the muscular powers answer the purpose of the pis- ton and valve. Emily, Water cannot, then, be raised by a pump, above 32 feet; for the pressure of the atmosphere will not sustain a column of water, above that height. Mrs. B, I beg your pardon. It is true that there must never be so great a distance as 32 feet, from the level of the water in the well, to the valve in the piston, otherwise the water would not rise through that valve; but when once the water has passed that opening, it is no longer the pressure of air on the reservoir, which makes it ascend; it is raised by lift- ing it up, as you would raise it in a bucket, of which the piston formed the bottom. This common pump is, therefore, called the sucking, or liftinff pump, as it is constructed on both these principles. The rod to which the piston is attached, must be made sufficiently long, to allow the piston to be within 32 feet of the surface oi the water in the well, however deep it may be. There is another sort of pump, called the forcing pump: it con- sists of a forcing power, added to the sucking part of the pump. This additional power, is exactly on the principle of the syringe: by raising the piston, you draw the water into the pump, and by causing it to descend, you force the water out. Caroline, But the water must be forced out at the upper part of the pump; and I cannot conceive how that can be done hy the descent of the piston. Mrs, B, Figure 5, plate 14, will explain the difficulty. The large pipe, A B, represents the sucking part of the pump, wifiich differs from the lifting pump, only in its piston P, being unfurn- 30. How do these parts act, in raising the water ? 31. In what doea that whichis commonly called auction, consist ? 32, How must the piston be litu- ated in the pump ? 33. What other kind of pump is described ' MECHANICAL PROPERTIES OF AIR. 145 ished with a valve, in consequence of which the water cannot rise above it. When, therefore, the piston descends, it shuts the valve Y, and forces the water (which has no other vent) into the pipe D: this is likewise furnished with a valve V, which, opening upwards, admits the water to pass, but prevents its return. The water, is thus first raised in the pump, and then forced into the pipe, by the alternate ascending, and descending motion of the piston, after a few strokes of the handle to fill the pipe, from whence the water issues at the spout. Emily. Does not the air pump, which you used in the experi- ments, oti pneumatics, operate upon the same principles as the sucking piimp? Mrs. JB. Exactly. The air pumpwhicli I used (plate 1, fig. 2,) has two hollow, brass cylinders, called barrels, which are made perfectly true. In each of those barrels, there is a piston^ these are worked up, and down, by the same handle; the pistons, are furnished with valves, opening upwards, like those of the common pump: there are valves also, placed at the lower part of each barrel, wliich open upwards; there are therefore two pumps, united to produce the same effect: two tubes, connect these barrels with the plate, upon which I placed the receivers, which were to be exhausted. Emily. I now understand how the air pump acts; the re- ceiver contains air, which is exhausted, just as it is by the com- mon pump, before the water begins to rise. Mrs. B. Having explained the mechanical properties of air, I think it is now time to conclude our lesson. When next we meet, I shall give you some account of \vind, and of sound, which will terminate our observations on elastic fluids. Caroline. And I shall run into the garden, to have the plea sure of pumping, now that I understand the construction of a pump. Mrs. B. And, to-morrow, I liope you will be able to tell me, whether it is a forcing, or a common lifting pump. 34. How is the forcing pump constructed, as shown in plate 14, fig. 5 ? 35. Describe the conBtruction and operation of the air pump, (fig. 2, plat© !•) N CONVERSATION XIH. ON WIND AND SOUND- 9V WIND IN GENERAL. — OF THE TRADE-WIND. — OF THE PERIODICAL TRADE-WINDS. — OF THE AERIAL TIDES. — OF SOUNDS IN GENERAL. — OF SONOROUS BODIES. — OF MUSICAL SOUNDS. OF CONCORD OR HARMONY, AND MELODY. MRS. B. Well, Caroline, have you ascertained what kind of pump you have in your garden? Caroline. I think it must be merely a lifting pump, because no more force is required to raise the handle than is necessary to lift its vi^eight J and as in a forcing pump, by raising the handle, you force the water into the smaller pipe, the resistance the water offers, must require an exertion of strength, to overcome it. , Mrs. B. I make no doubt you are rights for lifting pumps, being simple in their construction, are by far the most common. I have promised to day to give you some account of the nature of wind. Wind is nothing more than the motion of a stream, or current of air, generally produced by a partial change of temper- ature in the atmosphere; for when any one part is more heated than the rest, that part is rarefied, the air in consequence rises, and the equilibrium is destroyed. W^hen this happens, there necessarily follows a motion ot the surrounding air towards that part, in order to restore it; this spot, therefore, receives winds from every quarter. Those who live to the north of it, experi- ence a north wind; those to the south, a south wind: — do you comprehend this? Caroline. Perfectly. But what sort of weather must those people have, who live on the spot, where these winds meet and mterfere? Mrs. B. They have most commonly turbulent and boister- ous weather, whirlwinds, hurricanes, rain, lightning, thunder, &c. This stormy weather occurs most frequently in the torrid zone, where the heat is greatest: the air being more rarefied 1, What is wind, and how is it generally produced ? 2. How do the winds blow, around the place where the air becomes rarefied ? 3. What effect is likely to be produced where the ^vinds meet .•• ON WIND ANDT SOUND. 147 there, than in any other part of the globe, is lighter, and conse- quently, ascends; whilst the air from the north and south, is continually flowing in, to restore the equilibrium. Caroline. This motion of the air, would produce a regular and constant north wind, to the inhabitants of the northern hemisphere; and a south wind, to those of the southern hemi- sphere, and continual storms at the equator, where these two ad- verse winds would meet. Mrs. B. These winds do not meet, for they each change their direction before they reach the equator. The sun, in mov- ing over the equatorial regions from east to west, rarefies the air as it passes, and causes the denser eastern air to flow west- wards, in order to restore the equilibrium, thus producing a re- gular east wind, about the equator. Caroline. The air from the west, then, constantly goes to meet the sun, and repair the disturbance which his beams have produced in the equilibrium of the atmosphere. But I wonder now you will reconcile these various winds, Mrs. B.; you first led me to suppose there was a constant struggle between oppo- site winds at the equator, producing storm and tempest; but now I hear of one regular invariable wind, which must naturally be attended by calm weather. Emily, I think I comprehend it: do not these winds from the north aiid south, combine with the easterly v/ind a^out the equator, and form, what are called, the trade- winds? 3Ir8. B. 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 and east, produces a regular south- east wind; these winds extend to about thirty degrees on each side of the equator, the regions further distant from it, expe- riencing only their respective northerly and southerly winds Caroline. But, Mrs. B., if the air is constantly flowing fj the poles, to the torrid zone, there must be a deficiency of air, in the polar regions? Mrs. B.^ The light air about the equator, which expands, 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 polar, atmospheric regions, would soon be exhausted by the stream of air, which, in the lower strata of the atmosphere, they are constantly sending to- wards the equator. Caroline. There is then a sort of circulation of air in the at- 4. In what part of the globe is the air most rarefied, and what is the con- sequence ? 5. How do these winds change their direction as they approach the equator ? 6. How are the trade-winds produced, and how far do they expend ? 7. How is the equilibrium in the air restored J, from t48 OJf WIND AND SOUND. mosphere; the air in the lower strata, flowing from the poles to- wards the equator, and in the upper strata, flowing back from the equator, towards the poles. Mrs. B. Exactly 5 I can show you an example of this circu- lation, on a smaller scale. The air of this room, being more rarefied, than the external air, a wind or current of air is pour- ing in from the crevices of the windows and doors, to restore the equilibrium j but the light air, with which the room is filled, must find some vent, in order to make way for the heavy air that en- ters, ''if you set the door a-jar, and hold a candle near the up- per part of it, you will find that the flame will be blown out- wards, 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 inclination 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. Mrs. B. Besides the general, or trade-winds, there aie others, which are called periodical, because they blow in con- rary directions, at particular periods-. Emily. I have heard, Mrs. B., that the periodical winds, .ailed, in the torrid zone, the sea and land breezes, blow to- Avards the land, in the day time, and towards the sea, at night: 'A'hat is the reason of that? Mrs. B. The land reflects into the atmosphere, a much j^reater quantity of the sun's rays, than the water; therefore, liat part of the atmosphere which is over the land, is more Ilea ted and rarefied, than that which is over the sea: this occa- sions the wind to set in upon tiie land, as we find that it regu- larly does on the coast of Guinea, and other countries in the torrid zone. There, they have only the sea breeze, but on the islands, they have, in general, both a land and sea breeze, the latter being produced in the way described; whilst at night, during the absence of the sun, the earth cools, and the air is consequently condensed, and flows from the land, towards the Tl(iS. body, is projected forward in strj^ight 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 radiates light in every di- rection. (Fig. 1, plate 15.) 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, are so extreme- ly minute, that thev 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, pro- ceeding from any one point of a luminous body, as fig. 2» Caroline. Is light then a substance composed of particles, like other bodies.^ Mrs. B. That is a disputed point, upon which I cannot pre- tend to decide. In some respects, light is obedient 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 motion, it does not seem to be influenced by those of gravity. It has never been discovered to have weight, though a variety of inte- resting experiments 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 error; we shall, therefore, confine our at- tention to those properties of light, which are well ascertained. Let us return to the examination of the effects of the radia- tion of light, from a luminous ])ody. Since the rays of light are projected in straight lines, when they meet with an opaque body through which they are unable to pass, they are stopped short in their course; for they cannot 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 opposite side of it; and if^ this darkness fall upon a wall, a sheet of paper, or any object whatever, it forms a shadows Emily. A shadow, then, is nothing more than darkness pro- duced by the intervention of an opaque body, which prevents the rays of light from reaching an object behind it. 6. How is light projected from lumiuous bodies, and how, from every point of such bodies, (fi^. 1, plate 15 ?) 7. Why do not the rays of light from dif- ferent points, stop each other's progress ? 8. What is a ray, and what a pen- cil of rays? fig. 2, plate 15. 9. Do we know whether light is a substance, similar to bodies in general ? 10. When a ray of light falls upon an opaque body, wUat is the result .' ON OPTICS. 159 Caroline. Why then are shadowg of different degrees of darkness^ for I should have supposed, from your definition of a shadow, that it would have been perfectly black? Mrs, B. It frequently happens that a shadow is produced by an opaque body, interrupting the course of the rays from ohe luminous body, while light from another, reaches the space where the shadow is formed; in which case, the shadow is proportion- ally fainter. This happens when the opaque body is lighted by two candles: if you extinguish 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 illuminated by light reflected from the walls of the room, and other surroundmg objects.": You must observe, also, that when a shadow is produced by the interruption of rays from a single luminous body, the dark- ness is proportioned to the intensity of the light. Emily. I should have supposed the contrary; for as the light reflected from surrounding objects on the shadow, must be in proportion to the intensity of the light, the stronger the light, the more the shadow will be illumined. Mrs. B. Your remark is perfectly just; but as we' have no means of estimating the degrees of light, and of darkness, but by comparison, the strongest light will appear to produce the deep- est shadow. Hence a total eclipse of the sun, occasions a more sensible darkness than midnight, as it is immediately contrast- ed with the strong light of noonday. Caroline. The re-appearance of the sun, after an eclipse, must, by the same contrast, appear 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 diminisli in size, till it terminates 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 teriestrial objects.^ Their shadows, far from diminishing, are alwa^ larger than the object, and increase with the distance from it. Mrs. B. In estimating the effect of shadows, we must con- sider the dimensions of the luminous body; when the luminous body is less, than tlie opaque body, the shadow will increase 11. In what does shadow consist? 12. Why are they, in general, but partially dark? 13. Upon what does the intensity of a shadow depend? 14. How are shadows affected by the size of thfe luminous body, as represent- ed in plate 15, fig. 3 ? 15. When is the shadow larger than the intercepting- body ? "160 On optics. with the distance. This will be best exemplified, by observini^- 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, be- ing much smaller than myself. Mrs. B, Yes. The shadow of a figure as A, (fig. 4.) varies in size, according to the distance of the several surfaces BCD K, on which it is described) CaroHne. I have observed, that two candles, produce two ivhadows from the same object; wiiilst it would appear, from what you said, that they should ratlier produce only half a sha- llow, that is to say, a very faint one. Mrs. B. The nftniber of lights (in diflerent directions) while it decreases the intensity of the shadows, increases their number, whicli always corresponds with that of the lights; for each lights makes the opaque body cast a different shadow, as illustrated by iio'. 5. whicli represents a ball' A, lighted by three candles, B, O, D; and you ol)serve the light B, produces the shadow b, the ''ii;ht C, the shadow c, and the light D, the shadow dj, but nei- hor of these shadows will be very dark, because the light of one iiudle only, is intercepted by the ball; and the spot is still illu- ninated by the other two.,' Emily. I think we now underetand the nature of shadowy very well; but pray, what becomes of the rays of light, which opaque bodies arrest in tlieir course, and tlie interruption of which, is the occasion of sliadows? Mrs. B. Your question leads to a very important propert}^ <»f light. Reflection. When rays of light encounter an opaque body,; tliey cannot pass through it, and part of them are absorbed by it, and part are reflected, and rebound; just as an elastic ball rebounds, when struck against a wall. By reflection, we mean that the light is turned back again, rhrouiiii the same medium which it had traversed in its first course. Emily. And is light, in its reflection, governed by the same laws, as solid, elastic bodies? « Mrs. B. Exactly. If a ray of light fall perpendicularly on an opaque body, it is reflected baok in the same line, towards the point whence it proceeded. If it fall obliquely, it is reflect- ed obliquely, but in the opposite direction; the ray which falls u])on the reflecting surface, is called the incident ray, and that ^vhich leaves it, the reflected ray; the angle of incidence, is al- 16. What is explained by fig. 4, plate 15 .'* 17. What will be the effect of :>everal lights, as in fig. 5, plate 15 ? 18. Why will nffither of these shadows be very dark? 19. What becomes of the Ijght which falls upon an opaque body ? 20. What is meant by reflection ? ON OPTICS. 161 ^rays^qual to the angie of reflection. You recollect that law in mechanics? Emily. Oh yes, perfectly. , Mrs. B. If you will shut the shutters, we will admit a ray of the sun's light, through a very small aperture, and I can show you how it is reflected. I now hold this mirror, so that the ray shall fall perpendicularly upon it. Caroline. I see the ray which falls upon the mirror, but not that which is reflected hj it. Mrs. B. Because it is turned directly back again; and the ray of incidence, and that of reflection, are confounded together, both being in the same line, though in opposite directions. Emily. The ray then, which appears to us single, is really double, and is composed of the incident ray, proceeding to the mirror, and of the reflected ray, returning from the mirror. Mrs. B. Exactly so. We will now separate them, by hold- ing the mirror M, (ng. 6,) in such a manner, that the incident ray, A B, shall fall obliquely upon it— you see the reflected ray, B C, is marching oft' in another direction. If we draw a line from the point of incidejice B, perpendicularly, to the mir- ror, it will divide the angle of incidence, from the angle of re- flection, and you will see that they are equal. Emily. Exactly; and now, that you hold the mirror, so that the ray falls more obliquely upon it, it is also reflected more obliquely, preserving the equality of the angles of incidence, and of reflection. Mrs. B. It is by reflected rays only, that we see opaque ob jects. Luminous bodies, send rays of light immediately to our eyes, but the rays which they send to other bodies, are invisible to us, and are seen, only when they are reflected by those bo- dies, to our eyes. Emily. But have we not just seen the rjiy of light, in its pass- age from the sun to the mirror, and its reflections.'^ yet, in nei- ther case, were those rays in a direction to enter our eyes. Mrs. B. What you saw, was the light reflected to youi eyes, by small particles of dust floating in the air, and on which tne ray shone, m its passage to, and from, the mirror. Caroline. Yet I see the sun, shining on that house yonder, as clearly as possible. Mrs. B, Indeed you cannot see a single ray, which passes 21. What is meant by the incident, and reflected rays? 22. What is the result, when the incident ray falls perpendicularly, and what, when it falls obliquely ? 23. What two angles are always equal in this case ? 24. To what law in mechanics, is this analogous, as represented in %. 4, plate 2 ? 25. What is represented by fig. 6, plate 15 ? 26. By what light are we ena- bled to see opaque, and by what, luminous bodies ? 27. What enables us to see a ray of light in its passage, through a darljened room i* 162 ON OPTICS. from the sun to the house; jou see, by the aid of those rays, wliich enter your eyes; therefore, (it is the rays which are re- flected by the house, to you, and not those which proceed di- rectly from the sun, to the house, that render the building visi- ble to you. > Caroline. Why then does one side of the house appear to be in sunshine, and the other in 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, receives, and reflects more light, and therefore, appears more lumi- nous and vivid,' than tlie 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^ sight; and as light is more, or less, absorbed by the bodies it strikes upon, every time a ray is reflected, its intensity is dimin- ished. Caroline. Still I cannot reconcile to myself, the idea that we do not see the sun's rays shining on objects, but only those which such 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 ap- pears to shine on one part of it only.'^ 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 particularly by moon- light. I have frequently observed a vivia streak of moonshine oil the se^, while the rest of the water remained in deep obscu- rity, and yet there was no apparent obstacle to prevent the moon from shining equally on every part of the water. Mrs. B. By moonlight 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 observ^e it so distinctly. 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? * Mrs. B. The reflected rays, ai-e not attracted out of their natural course, by your eyes. The direction of a reflected ray, you know, depends on that of the incident ray; the sun's ray's, therefore, which fall with various degrees of obliquity upon the water, are reflected in directions equally various; some of these 28. By what reasoning would you prove that an object, such, for exaniple, as a house, is seen by reflected light ? 29. Why may one side of such object appear more bright than another side? 30. How is the fact exemplified by the sun, or moon, shining upon water f 31 . Why is this best evinced by moon- lisht.? ON OPTICS. will meet your eyes, and you will see tUem, but those which fall '''^«;"'^'^retlk rs;a.hi.e, then wM upo^rU^Twate., is composed of those rays which by then- reflec- tion, happen to fall upon my eyes? tot^u ;hadow, really illuminated by the sun, and its rays re- ""'X T^N^Sat is a different case, from the sheet of wa- ter Tim side of the house,is really in shadow; it is the west si, wMch the sun cannot shine upon, tdllhe at™-„^^^^^ £„%. Those objects, tl.en, w neh are '"""""^^ t-J '^^^^^^^^ x/htrr^Shri^s^i^wU:^^^^^^^^ 3^..^^r'^Lf^r:he^-^|r:ti^- trees cast a shadow, by what light do you see itr Emilv Since it is not by the sun's direct rays, it must De oy thofe reflected on it from other objects, and which it again re- ^'fJoZ' But if we see all terrestrial objects by reflected lio-ht ?a we do the moon,) why do they appear so tnght and Slious? I should have''s«pposed that reflected rays, would have been dull and faint, like those of the moon. Mn B. The moon reflects the sun's ight, with as much vividness as any terrestrial object If you look at it on a clear nlht It will appear as bright as a sheet of water, the walls of a house, or any object seen by daylight, and on whicji tlie sun sZes. The rays of tlie moon are Soubtless feeble, when com- pared with those of the sun; but that would not be a fair com- parfsonTfor the former are incident, the latter, reflected rays ^ Mine True; and when we see terrestiial objects by mLu?5!theu|ht has been twice reflected, and is consequent- ^^'ifi-rs.'^iS^'Tn^ravtrsing thl a.ft.r ^osphere, the rays, both of the sun, and moon, lose some of their light. ^ For though the pure air, is a transparent medium, which transmits flie . -^ys oi iignt freelv, we have observed, that near the surface of the ea. .. " n, it i» loaded with vapours and exhalations, by which some portion t ' them 'are absorbed. , . , x. Caroline. I have often noticed, that an object on the summit 32. By what light do we see the moon, and why is it comparatively fee- ^ ble ? 33. What eiroumstance, renders objects seen by moonlight, still less vivid ? 164 ON OPTICS. Mrs. B. That may have some sensible effprf. hn+ u Mrs. B I shall hereafter describe thi strucW „f tl.« very particularly, but will „„w observrihat the smll rn?^*l spot, which IS generally called the sio-ht of X „f ■ "',"' .lenominated th'e ;,,,;.-/and thtt the X^ t ex^L" „Wj optic nerve on the back part of tlie ball of thepl? l\^ ra\rf".Hr7 ^ IP ^^'!' "h-t'Lte*v^:';q/''^h^; will be rendered very distinct j? i" ^ locus, it about by the wind. The landscape, would be perfectTit weTe i^?^. i/. It IS not enough to admire, you must understand eSil Thf' "'''^"',^ of darkening the room/fn order to exhibit It. The camera obscura, sometimes consists of a sm.ll box, properly fitted up, to represent external objects! refl^c;:crtor^^^^^^ ^^ ti. ....^.^^^^^^ admitted t^^^^^^^^^^^ and which are alcove A'^;./''?'^J>^^ ft« glittering weathercock, at the top of the cock >• ''' ^^l^P^ate 16.) represent it in this spot, a; for the weather- .-. tlore, \ieing much higher than the aperture in the shutter, only water tew of the rays, which are reflected by it, in an obliquely de- cending direction, can find entrance there. The rays of light, ^^ ^ o.>^tv^^^* " *"®^* ^y *^® P"P^^ °^ ^^^ ®y®-^ 35. What by the retina r app' 36. How do the rays of light operate on the eye in producing vision? 37. the How may this be exemplified, in a darkened room ? 38. What is meant by iigr a camera obscura? 39. How is it explained in plate IG ' ON OPTICS* 165 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 lower 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. t^mily. And the rays of light, from the steps, (B) r^' ^ne. alcove, in entering the aperture, ascend, and will descm6e those steps in the highest, instead of the lowest, part of th« landscape. Mrs. B, Observe, too, that the rays comi?A- iiom the alcove, which is to our left-, d-t^crfbe it on the wall, to the right; while those, whi^-t- are reflected by the walnut tree, C D, to our right, t'l'Jiineate 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 aper- ture; those which come from 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 situa- tion, the very reverse of that which it does in nature. Caroline. Excepting the flower-pot, E F, which, though its position is reversed, has not changed its situation in the land- scape. Mrs. B. The flower-pot, is directly in front of the aperture; so that its rays, fall perpendicularly upon it, 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 re- presentation of the object, which is painted on the retina, and not the object itself? Mrs. B. If, by sight, you understand that sense, by which tJie presence of objects is perceived by the mind, through the means of the eyes, we certainly see only the image of those ob- jects, painted on the retina. Caroline. This appears to me quite incredible. Mrs. B. ^he nerves, are the only part of our frame, capable of sensation: they appear, therefore, to be the instrumeMts, which the mind employs in its perceptions; for a sensation, al- 40. Why are the objects inverted and reversed? 41. What analogy is there between the camera ob§cura, and the eye ? 42. Is it the object, or its pictiu-e on tlie retina, which presents to tlie mind an idea of the object seen i* 166 ON OPTICS. ways 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 smelling only particles which are actually in contact with the nerves ot the nose. We have already observed, that the odour of a flower consists in effluvia, composed of very mi- nute particles, which penetrate the nostrils, and strike upon the olfactory nerves, which instantly convey the idea of odour to the mind. Emily. And sound, though it is said to be heard at a dis- tance, is, in fact, heard only when the vibrations of the air, which convey it to our ears, strike upon the auditory norve. Caroline, There is no explanation required, to prove that the senses of feeling and of tasting, are excited only by contact. Mrs, 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 real objects cannot be brought to touch the optic nerve, the image of them is conveyed thither by the rays of light, proceeding from real ob- jects, which actually strike upon the optic nerve, and form that image which the mind perceives. Caroline. While I listen to your reasoning, I feel convinced; 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 cannot reconcile to myself, the idea, that I do not really see this book which I hold in my hand, nor the words which I read in it. Mrs. B, Did it ever occur to you as extraordinary, that you never beheld your own face.^ Caroline. No; because I so frequently see an exact repre- sentation 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 perfect mirror, than any made by art. Emily. But is it possible, that the extensive landscape, which I now behold from the window, should be represented on so small a space, as the retina of the eye? Mrs. B. 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 alone, which forms the feathers of the butterfly, and the organs of the minutest insect, can 43. By what organs is sensation produced, and how must these organs be affected? 44. How will the idea of contact, apply to objects not torching the eye ? ON OPTICS. 167 pourtray so admirable and perfect a miniature, as that which is it presented on the retina of the eye. Caroline. But, Mrs. B., if we see only the image of objects, 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 ob- scura; as the rays, from the difterent parts of the landscape, in- tersect each other on entering the pupil, in the same manner as they do, on entering the camera obscura. The scene, however, does not excite the idea of being inverted, 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 object, 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 con- sider 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 tiie retina; 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, ac- cording as the object was higher or lower, than myself. Mrs. B. 1 be^ your pardon. When you look up, to an ele- vated 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, convinces 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. When 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. But I have a further proof in favour of what I have advanced, which, I hope, will remove your remaining doubts: I shall, how- ever, defer it till our next meeting, as the lesson has been suffi- ciently long to-day. 45. Why do not objects appear reversed to the eye, at in the camera ^ scura ? CONVERSATION XV. OYTlCS—eontinued, ON THK ANGLE OF VISION, AND THE REFLECTION OF MIRRORS. -INGLE 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 further proofs yoii have to oifer, in support of your theory. You must allow, that it was ratlier provoking to dismiss us as you did at our last meeting. 3Irs. B. You press so hard upon me with your objections, that you must give me time to recruit my forces. Can you tell me, Caroline, why objects at a distance, appear smaller than they really are? Caroline, I know no otlier reason than their distance. Mrs. B, It is a fact, that distance causes objects to appear smaller, but to state the fact, is not to give the reason. We must refer again to the camera obscura, to account for this circum- stance; and you will find, that the difterent apparent dimen- sions of objects at difterent distances, jTToceed from our seeing, not the objects themselves, but merely their image on the re- tina. Fig. 1, plate 17, represents a row of trees, as viewed in the camera obscura. I have expressed the direction of the rays, from the objects to the image, by lines. Now, observe, the ray which comes from' the top of the nearest tree, and that which comes from the foot of the same tree, meet at the aperture, form- ing an angle of about twenty-five degrees; the angle under which we see any object, is called, the visual angle, or, angle of vision. 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 consider- ably smaller than those of the object, but the proportions are perfectly preserved. 1. What is meant by the angle of vision, or the risujil angle f Plate 3SVM. ^^^ ^ \ %k^"^i^sr^v~-i ON THK ANGLE OF VISION. 16l> Now, let US notice the upper and lower ray, from the most distant tree; they form an angle of not more tiian twelve or fif- teen degrees, and an image of proportional dimensions. Thus, two objects of the same size, as the two trees of the avenue, form figures of diff'erent sizes in the camera obscura, according to their distance; or, in other words, according to tlie angle of vision under which they are seen. Do you understand this? Caroline. Perfectly. Mrs. Ri Then you have only to suppose, that the represen- tation in the camera obscura, is similar to that on the retina. Now, since objects of the same magnitudes, appear to be of ditterent dimensions, when at different distances from us, let me ask you which it is, that you 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. 1 must confess, that reason is in favour of the lat- ter. 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. Our senses are imperfect, but the experience we acquire by the sense of touch, corrects the illusions of our siglit, 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 the 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. B. 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 the window through which you see it. It is your knowledge of the real mag- nitude of the house which prevents your attending to its appa- rent size. If you were accustomed 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 separates the two rows, forms a smaller visual an^le, in proportion as it is more distant from us; therefore, the width of the road gradu- ally diminishes, as well as the size of the trees, till at length the 2. Why do objects of the same size appear smaller when distant, than when near? 3. Why do not two object?, known to be equal in size, appear to differ, when at different distances f'rou^ tU eye ? 4. How is this exemplified, by a house seen thiough a window: P 170 i)N THE ANGLE OF VISION. road apparently terminates in a point, at which the trees seem to meet. Emily. I am very glad to understand this, for I have lately begun to learn perspective, which appeared to me a very dry study; but now tnat I am acquainted with some of the principles on which it is founded, I shall find it much more interesting. Caroline. In drawing a view from nature, it seems that we do not copy the real objects, but the image they form on the retina of our eyes? Mrs. B, Certainly. In sculpture, we copy nature as she really exists; in paintmg, we represent her, as she appears to us. We must now conclude the observations that remain to be made, on the angle of vision. If the rays, proceeding from the exti-emities of an object, with an ordinary degree of illumination, do not enter the eye under an angle of more than two seconds, which is the 1- 1800th part of a degree, it is invisible. There are, consequently, two cases in which objects may be invisible; if they are either so small, or so distant, as to form an angle of less than two seconds of a ilegree. In like manner, if the velocity of a body does not exceed 20 degrees in an hour, its motion is imperceptible. Caroline. A very rapid motion may then be imperceptible, provided the distance of the moving body, is sufficiently great. Mrs. B. Undoubtedly; for the greater it& distance, the smaller will be the an^le, under which its motion will appear to the eye. It is for this reason, that the motion of the celestial bodies is invisible, although inconceivably rapid. Emily. I am surprised, that so great a velocity as 20 degrees an hour, should be invisible. Mrs. B. The real velocity depends upon the space compre- hended in each degree, and upon the time, in which the moving body, passes over that space. But we can only know the ex- tent of this space, by knowing the distance of the moving body, from its centre of motion; 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 respective lines, C and D; if they perform their walk in 5. Why do rows of trees, forming an avenue, appear to approach as they recede from the eye, until they eventually seem to meet? 6. In drawing a view from nature, what do we copy ? 7. What is the diiference in sculpture, in this respect? 8. Excepting the rays from an object enter the eye, under a certain angle, they caimot be seen ; what must this angle exceed ? 9. What two circumstances may cause the angle to be so small, as not to produce vi- sion? 10. Motion may be so slow as to become imperceptible, what is said on this point? 11. Under what circumstances may a body, moving with great rapidity, appear to be at rest? 12. Upon what does the real velocity of a body, depend? ON THE ANGLE OF VISION. 171 the same space of time, they must have proceeded at a very dif- ferent rate; and yet to an eye situated at E, they will appear to have moved witn equal velocity, because they will botli have gone through an equal number of degrees, though over a very unequal length of ground. The number of de^ees over whicli a body moves in a given time, is called its angular velocity; two bodies, you see, may have the same angular, or apparent velo- city, whilst their real velocities may differ almost infinitely. Sight is an extremely useful sense, no doubt, but it cannot al- ways 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 error, 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 purposes of life. To convince you how requisite experience is, to correct the errors of sight, I shall relate to you, the case of a young man, who was blind from his infancy, and who recovered his sight at the age of fourteen, by the operation 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 another, that he acquired an idea of their respective dimensions, their relative situations, and their dis- tances. Caroline. The idea that objects touched his eyes, is, how- ever, 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 rays, on the optic nerve of each eye, is so perfectly similar, that they produce but a single sen- sation; the mind, therefore, receives the same idea, from the retina of both ej es, and conceives the object 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 13. What must be known, to enable us to ascertain the real space contain- ed in a degree? 14. What is explained by %. 2, plate 17? 15. What is said respecting the evidence afforded by our senses, and how do we correct th€ errors into which they would lead us ? 16. An image of a visible object is formed upon the retina of each eye, why, therefore, are not objects seers double ? irS REFLECTION OF MIRRCRS. through the window, with botli ejes open, at some object exactly opposite to one of the upright bars of the sash. Caroline. I now see a tree, the body of which, appears to be in a line exactly opposite to one of the bars. Mrs. B. If you now shut your right eye, and look with the left, it will appear to the left of the bar; then liy closing the left eye, and looking with the other, it will appear to the right of tne bar.' Caroline. That is true, indeed! Mrs. B. There are, evidently, two representations of the tree in different situations, which must be owing to an image of ^ it being formed (m each eye; if the action of the rays, there- Tore, on each retina, were not so perfectly similar as to produce but one sensation, we should see double; and we find that to be the case with some persons, who are afflicted with a disease in one eye, which prevents the rays of light from affecting it in the same manner 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 ob- scura, and on the retina of the eye.^ Mrs. B. Because tlie rays do not enter the mirror by a small aperture, and cross each other, 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 situ- ated in tiie same manner as the object 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 A\'liole of my figure in it.^ 3Irs. B. It is not necessary that the mirror should be more than half your height, in order that you may see the whole of your person in it, (tig. 3.) The ray of light A B, from your eye, which falls perpendicularly on the mirror B I), will be reflected back, in the same line; but the ray from your feet, will fall ob- . liquely on the mirror, for it must ascend in order to reach it; it will, therefore, be reflected in the line A D: and since we view objects in the direction of the reflected rays, which reach the eye, and since 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. 17. By what experiment can you prove that a separate image of an object is formed in each eye? 18. Under what circumstances are objects seen dou- ble ? 19. Why is not tha image of an object inverted in the common mirror : 20. Your whole figure -may be seen in a looking-glass, which is not more than half your height; how is this shown in fig. 3. plate 17? REFLECTION OF MIRRORS. 17i Emily, Then I do not understand why I should not see the wliole of my person in a much smaller mirror, for a ray of light fiom 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; the ray would, therefore, be reflected above your head, and you could not see it. 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 direction; the angles of incidence, and reflection, being equal. Caroline, place your- self 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 direc*' tion 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, not straight before me, however, but before her; and it appears at the same distance behind the glass, that she is in front of it. Mrs. B. You must recollect, that we always see objects in the direction of tlie 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 reflects the rays of light; for glass is a transparent body, which should transmit them I Mrs. B. It is not the glass that reflects the rays which form the image you beliold, but the silvering behind it; this silvering is a compound of mercury and tin, which forms a brilliant me- tallic coating. The glass acts chiefly as a transparent case, through whicli the rays find an easy passage, to, and from, the quicksilver. Caroline. Why then should not mirrors be made simply of mercury? Mrs. B. Because mercury is a fluid. By amalgamating it with tinfoil, it becomes of the consistence of paste, attaches itself to the glass, and forms, in fact, a metallic mirror, which 21. Why is the image invisible to the person, when not standing directly before the glass ? 22. In what situation may a second person see the image reflected? 23. In what direction will an object always appear to the eye? 24. How is this explained by fig. 4, plate 17 i* 25. What is it that reflects the rays in a looking-glass ? 174 REFLECTION OF MIRRORS. would be much more perfect without its glass cover, for the purest glass is never perfectly transparent; some of the yrjs, therefore, are lost during their passage through it, by being either absorbed, or irregularly reflected. This imperfection of glass mirrors, has introduced the use of metallic mirrors, for optical purposes. Emily. But since all opaque bodies reflect the rays of light, I do not understand why they are not all mirrors. Caroline. A curious idea indeed, sister; it would be very gratifying to see oneself in every object at which one looked. Mrs. B. It is very true that all o])aque objects reflect light; but the surface of bodies, in general, is so rough and uneven, that the reflection from them is extremely irregular, and prevents the rays from forming an ima^e on the retina. This, you will be able to understand better, when I shall explain to you the na- ture of vision, and the structure of the eye. You may easilv conceive the variety of directions in which rays would be reflected by a nutmeg-grater, on account of the inequality of its surface, and the number of holes w ith which it is pierced. All solid bodies more or less resemble the nutmeg- grater, in these respects; and it is only those which are suscepti- ble of receiving a polish, tliat 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 con- fined to this class of bodies; for I have often seen myself, in a liighly polished mahogany table. Mrs. B. Certainly; but as that substance is less durable, 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 optics; the plain^ or flat-, which are the common mirroi's we have just mentioned; convex mirrors, and concave mirrors. The reflection of the two latter, is very diiFerent from that of the former. The plain mir- ror, we have seen, does not alter the direction of the reflected rays, and forms an image behind the glass, exactly similar to the object before it. A convex mirror has the peculiar proper- ty of making the reflected rays diverge, by which means it diminishes the image; and a concave mirror makes the rays converge, and under certain circumstances, magnifies the image. Emily. We have a convex mirror in the drawing-room, 26. All opaque bodies reflect some light, why do they not all act as mir- rors ? 27. What substances form the most perfect mirrors, and for what rea- son ? 28. What are the three kinds of mirrors usually employed for optical purposes ? 20. How are the rays of light affected by them ? 1*LATE REFLECTION OF MIRRORS. 175 which forms a beautiful mininture picture of tlie objects in the room; and I have often amused myself witli looking at my magnified face in a concave mirror. Kut I hope you will ex- plain to us, why the one enlarges, while the other diminishes the objects it reflects. Mrs. B. Let us begin by examining tlie reflection of a con- vex mirror. Tliis is formed of a portion of the exterior surface of a sphere. When several parallel rays fall upon it, that ray only which, if prolonged, would pass through the centre or axis t)f the mirror, is perpendicular to it. In order to avoid confu- sion, I have, in fig. 1, plate 18, drawn only three parallel lines, A B, C I), 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 \Vould be so to a flat mirror; but as 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 falls perpendicularly to the earth, when gravity attracts it towards the centre. Mrs. B. In order, therefore, that rays may fall perpendicu- larly to the mirror at B and F, the rays must be in tlie direction of the dotted lines, which, you may observe, 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, wiirbe reflected? Emilij. Yes, I think so: the middle ray, falling perpendicu- larly on the mirror, will be reflected in the same line: the two" outer rays falling obliquely, will be reflected obliquely to Gand H; for the dotted lines you have drawn are perpendiculars, which divide the angles of incidence and reflection, of those two rays. Mrs. B. Extremely well> Emily: and since we see objects in the direction of tlie reflected ray, we shall see the ima^^e 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 tlie surface and centre of the sphere, and is called the imaginary focus of the mirror. Caroline. Pray, what is the meaning of focus? Mrs. B. A point at which converging rays, unite. And it is 30. What is the form of a convex mirror, and how do parallel rays fall upon it, as represented in fig. 1, plate 18 ? 31. What is represented by the dotted line in the same figure ? 32. Explain by the figure, how the parallel rays will be reflected. 33. At what distance behind such a mirror, would an image, produced by parallel rays, be formed? 34. What is that point d^omi- nated ? 176 REFLECTION OF CONVEX MIRRORS. in this case, called an imaginary focus^ because the rays- do not really unite at that point, but only appear to do so: for the rays do not pass through the mirror, since they are reflected by it. Emily. I do not yet understand why an object appears smaller, when viewed in a convex mirror. Mrs. B. It is owing to the divergence of i\\Q^ reflected rays. You have seen that a convex mirror, by reflection, converts parallel rays into divergent rays; rays that fall upon the mirror divergent, are rendered still more so by reflection, and conver- gent 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 extremi- ties, falling convergent on the mirror, will be reflected less con- vergent, 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 h. Caroline. But the reflected rays, do not appear to me to con- verge less than the incident rays. 1 should have supposed that, on the contrary, they converged more, since they meet in a point. Mrs. B. They would unite sooner than they actually do, if tliey were not less convergent than the incident rays: for ob- serve, that if the incident rays, instead of being reflected 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; and the more distant the latter is from the mirror, tlie smaller is the image reflected by it. You will now easily understand the nature of the reflection of concave mirrors. These are formed of a portion of the internal surface of a hollow sphere, and their peculiar property is to con- verge the rays of light. Can you discover, Caroline, in what direction the three parallel rays, X B, C D, E F, are reflected, which fall on the concave mirror, M N, (fig. 3.) ? Caroline. I believe I can. The middle ray is sent back in the same line, in which it arrives, that being 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 35. What is meant by a focus ? 36. Why is the point behind the mirror, called the imaginary focus ? 37. Why does an object appear to be lessened by a conyex mirror, (fig. 2.) ? 38. What is a concave mirror, and what its peculiar property ? 39. How are parallel rays reflected by a coueave mirror, as explained by fig. 3, plate 18.* aEFLECTION OF CONCAVE MIRRORS. 177 order that those angles may be equal, the two oblique rays must be reflected to L, where they will unite with the middle ray. Mrs. B. Very well explained. Thus you see, that when any number of parallel rays fall on a concave mirror, they are all reflected to a focus: for in proportion as the rays are more distant from the axis of tiie mirror, they fall more obliquely upon it, and are more obliquely reflected; in consequence of which they come to a focus in the direction of the axis of the mirror, at a point equally distant from the centre, and the surface, of the sfMiere; 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.^ 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 mirror M N. Divergent rays are brought to a more distant focus than parallel rays, as in figure 5, where the focus is at L; but what is called the true focus of mirrors, either convex or concave, is that of parallel rays, and is equally distant from the centre, and the surface of the spherical mirror. I shall now show you the real reflection of rays of light, bj^ a metallic 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. 1 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: bu<> it is not yet exactly in the focus; as I ap- proach the paper to that point, observe how tlie brightness of the spot of light increases, while its size diminishes. Caroline. That must be occasioned by the rays approaching closer together. I think you hold the paper just in the focus now, the li^ht is so small and dazzling—Oh, Mrs. B., the paper has taken hre! 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 mir- rors. Emily. I have often heard of the surprising effects of burn- ing mirrors, and I am quite delighted to understand their na- ture. Caroline. It cannot be the true focus of the mirror, at which 40. Where is the focus of parallel rays, in a concave mirror? 41. If rays fall on it convergent, how are they reflected ? 42. How if divergent ? 43. How, and why, may concave, become burning mirrors ? 178 THE REFLECTION OF MIRRORS. the rays of the sun unite, for as they proceed from so large a body, they cannot fall upon the mirror parallel to each other. Mrs, B. Strictly speaking, they certainly do not. But when rays, come from such an immense distance as the sun, 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 re- presented. 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.^ (Fig. 6.) Caroline. That, I 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 concave mirror fall divergent upon it, and are reflected, parallel. 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 re- flected rays converge to a focus; when, on the contrary, the in- cident rays proceed from the focus, they are reflected parallel. This is an important law of optics, and since you are now ac- quainted with the principles on which it is founded, I hope that you will not forget it. Caroline. I am sure that we shall not. But, Mrs. B., you said that the image was formed in the focus of a concave mirror; yet I have frequently seen glass concave mirrors, where the ob- ject 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 magni- fied behind the mirror, or, as you would say, within it. Caroline. I do not understand why the image should be larger than the object. Mrs. B. This results from the convergent property of the concave mirror. If an object, A B, (fio-. 7.) be placed between the mirror and its focus, the rays from its extremities fall diver- gent on the mirror, and on being reflected, become less divergent, as if they proceeded from C : to an eye placed in that situation, 44. Why may rays of light coming; from the sun, be viewed as parallel to each other? 45. If a luminous body, as a burning taper, be placed in the focus of a concave mirror, how will the rays from it, be reflected ? (fig. 6.) 46. What feet is explained by fig. 7, plate 18 ? SBFimage will ON REFRACTION AND COLOURS. 179 image will appear magnified 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 pro- perty of light, no less interesting, and which is called refraction. CONVERSATION XVI. ON REFRACTION AND COLOURS. TRANSMISSION OF LIGHT BY TRANSPARENT BODIES. — REFRACTION. — RE- FRACTION BY THE ATMOSPHERE. — REFRACTION BY A LENS. — REFRAC- TION BY THE PRISM. OF COLOUR FROM THE RAYS OF LIGHT. — OF THE COLOURS OF BODIES. The refraction of light will furnish the subject of to-day's lesson. Caroline, That is a property of which I have not the faintest idea. 3Irs, 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 passing from one medium, into another oj different density, fall obliquely, it is turned out of its course. The ray of light is then said to be refracted. 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 sup- pose the two mediums to be air, and water; if a ray of light J)asses from air, into water, it is more strongly attracted by the atter, on account of its superior density. Emily, In what direction does the water attract the ray ? 1. What is meant by the refraction of light ? 2. What is believed to be the cause of refraction ? 180 THE REFRACTION OF LIGHT. Mrs. B. The ray is attracted perpendicularly towards the water^ in the same manner in which bodies are acted upon by If then a ray, A B, (fig. 1, plate 19.) fall perpendicularly 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 oblique- ly, 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 den- ser medium, and that it there begins to be affected by its attrac- tion; this attraction, if not counteracted by some other power, would draw it perpendicularly to the water, at B; but it is also impelled by its projectile force, which the attraction of the den- ser medium cannot overcome; die ray, therefore, acted m by both these powers, moves in a direction between them, and in- stead 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 the water? Mrs. B. It is owing to the refraction of the rays, reflected 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 refraction takes place, when a ray passes from a dense into a rare medium; 1 should suppose that it would be less, attracted by the latter, than by the former. Mrs. B. And it is precisely on that accoiint that the ray is refracted. Let the upper half of fig. 2,'i'epresent glass, and the lower half water, let C B represent a ray, passing obliquely from the glass, into water: glass, being the denser medium, the ray will be more strongly attracted by that which it leaves than by that which it enters. The attraction of the glass 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. Mrs. B. The rule upon this subject is this; when a ray of Hghf passes from a rare into a dense medium, it is refracted to- wards the perpendicular; when from a dense into a rare medium^ it is refracted from the perpendicular. By the perpendicular is meant a line, at right angle with the refracting surface. This 3. How is a ray refracted in passing obliquely from air into water ? 4. How is this refraction explained in fig. 1, plate 19? 5. What is fig. 2 in- tended to explain ? 6. What is the rule respecting refraction, by different mediums ^ VA Tlj\te XIX, Jxc, J. Fu,.2 T 1) iy.4. ♦ \ V THE REFRACTION OF LIGHT. 181 may be seen in fig. 1, and fig. 2, where the lines A E, are the perpendiculars. Caroline, But does not the attraction of the denser medium aftect 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 insensiblej it ap- pears, therefore, to be 'refracted only at the point at which it passes from one medium into the other. Now that you understand the principle of refraction, I will show 3^ou the real refraction of a ray of li^ht. Do you see the flower painted at the bottom of the inside of this tea-cup? (Fi^. 3.) Emily. Yes. — But now you have moved it just out of sight j 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 flower, the rays re- flected 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, and bent downwards when passing out of the water, into the air, so as again to enter mj eyes. Mrs. B. You have explained it perfectly: fig. 3. will help to imprint it on your memory. You must observe that when the flower becomes visible by the refraction of the ray, you do not see it in the situation which it really occupies, but the image of the flower appears higher in the cup; for as objects always ajp- pear to be situated in the direction of the rays which enter the eye, the flower will be seen at B, in the direction of the refracted ray. Emily. Then, when we see the bottom of a clear stream of water, the rays which it reflects, being refracted in their pass- age 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 shal- low. Accidents have frequently been occasioned by this cir- cumstance; and boys, who are in the habit of bathing, should be cautioned not to trust to the apparent shallowness of water, as it will always prove deeper than it appears. The refraction of light prevents our seeing the heavenly 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 described in fig. 4, plate 19., 7. What is meant by the perpendicular? 8. How does fig. 3, plate 19, elucidate the law of refraction ? 9. What will be the effect on the apparent situation of the flower ? 10. What effect has refraction upon the apparent depth of a stream of water f Q 182 THE REFRACTION OF LIGHT. the dotted 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 obliquely on it, at A, and is refracted to B; then, since we see the object in the direction of the refracted ray, a specta- tor at B, will see an image of the sun at C, instead of its real situation, at 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, therefore, are al- ways refracted, in this latitude. There is also another obstacle to our seeing the heavenly bodies in their real situations: light, though it moves with extreme velocity, is about eight minutes and a quarter, in its passage from the sun to the earth; tlierefore, when the rays reach us, the sun must have /{uitted 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 quarter, 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. Certainly; the effect being the same, whether it is our earth, or the heavenly bodies, which move: it is more easy to represent things as they appear to be, than as they really are. Caroline. During the morning, then, when tlie sun is rising towards the meridian, we must (from the length of time the li^ht is in reaching us) see an image of tlie sun below that spot which it really occupies. Emily. But the refraction of the atmosphere, counteracting this effect, we may, perhaps, between the two, see the sun in its real situation. Caroline. And in the afternoon, w^hen the sun is sinking 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 man it really is. Mrs. B. The refraction of tlie sun's rays, by the atmosphere, prolongs our days, as it occasions our seeing an image of the sun, both before he rises, and after he sets; when below our horizon, he still shines upon the atmosphere, and his rays are thence refracted to the earth: so likewise we see an image of the sun, 11. How does the atmosphere refract the rays of the sun, as represented, fig. 4? 12. Why have we the rays of the sun always refracted? 13. What length of time is required for light to travel from the sun, to the earth ? 14. What effect has this upon his apparent place ? 15. How is the length of the day affected by refractiwi.^' THE REFRACTION OF LIGHT. 183 previously to his rising, the rays that fall upon the atmosphere being refracted to the earth. Caroline. On the other hand, we must recollect that light is eight minutes and a quarter on its journey; so that, by the time it reaches the earth, the sun may, perhaps, have risen above the horizon. Emily. Pray, do not glass windows, refract the light? Mrs. B. They do; but this refraction would not be percep- tible, were the surfaces of the glass, perfectly flat and parallel; because, in passing through a pane of glass, the rays sutfer two refractions, which, being in contrary directions, produce nearly the same eft'ect as if no refraction had taken place. Emily. I do not understand that. Mrs. B. Fi^. 5, plate 19, 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 in- stead of continuing its course in the same direction, as the dot- ted 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 imVo suffered any refraction : the apparent, differing so little from the true place of any object, when seen through glass of ordinary thickness. Emily. So that the effect which takes place on the ray en- tering 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 sensi- ble effect is produced. Caroline, I think the effect is very sensible, for, in looking through the glass of the window, I see objects very much dis- torted; articles which I know to be straight, appear bent and broken, and sometimes the parts seem to be separated to a dis- tance from each other. Mrs. B. That is because common window glass is not flat, its whole surface being uneven. Rays from any object, falling upon it under different angles, are, consequently, refracted in various ways, and thus produce the distortion you have observed. Emily. Is it not in consequence of refraction, that the glasses in common spectacles, magnify objects seen through them? Mrs. B, Yes. Glasses of this description are called lenses^ 16. How are rays refracted, which fall obliquely upon a flat pane of glass, (fig. 5, plate 19 ?) 17. What is the reason that objects are distorted, when seen through common window glass.' 184 ON REFRACTION AND COLOURS. of these, there are several kinds, the names of which it will be necessary for you to learn. Every lens is formed of glass, ground so as to form a segment of a sphere, on one, or both sides. They are all represented at fig. 1, plate 20. The most common, is the double convex lens, D. This is thick in the middle, and thin at the edges, like common spectacles, or reading glasses. A B, is a plano-convex lens, being flat on one side, and convex on the other. E is a double concave^ being, in all respects, the reverse of D. C is a plano-concave, flat on one side, and concave on the other. F is called a meniscus, or concavo-convex, being concave on one, and convex on the other side. A line passing through the centre of a lens, is called its axis. Caroline. I should like to understand how the rays of light are refracted, by means of a lens. Mrs. B. When parallel rays (fig. 6.) fall on a double con- vex lens, that only, which falls in the direction of the axis of the lens, is perpendicular to the surface^ the other rays, falling obliquely, are refracted towards the axis, and will meet at a point beyond the lens, called \i% focus. . Of the three rays, ABC, 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 second 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 sur- face of the lens? Mrs. B. The focal distance depends both upon the form of the lens, and on the refracting power of the substance of which it is made; in a glass lens, both sides of which are equally con- vex, 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 dis- tance, therefore, of the radius of the sphere. The property of those lenses 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 them. For the rays A and C, falling on the concave lens X Y, (fig. 7, plate 19.) instead of converging towards the ray B, in the axis of the lens, will each be attracted towards the thick edges of the lens, both on entering and quitting it, and will, thererore, by the first re- fraction, be made to diverge to a, c, and by the second, to d, e. Caroline. And lenses which have one side flat, and the other 18. What is meant by a lens ? 19. What are the five kinds called, repre- sented at fig. 1 , plate 20 ? 20. What is meant by the axis of a lens ? 21 . How are parallel rays, refracted by the double convex lens, fig. 6, plate 19? 22. What is meant by the focus of a lens ? 23. What is the focal distance of parallel rays, from a double convex lens ? 24. How are the rays refracted by a coacave lens, fig. 7, plate 19 r* Oir REFRACTION AND COLOURS. 185 convex, or concave, as A and B, (fig. 1, plate 20.) are, I suppose, less powerful in their refractions? Mrs. B. Yes,* the focus of the plano-convex, is at the dis- tance of the diameter of a sphere, of which the convex surface of the lens, forms a portion; as represented in figure 2, plate 20. The three parallel rajs, A B C, are brought to a focus by the plano-convex lens, X Y,at F. Emily. You have not explained to us, Mrs. B., how the lens serves to magnify objects. Mrs. B. By turning again to fig. 6, plate 19. you will readi- ly understand this. Let A C, be an object placed before the lens, and suppose it to be seen by an eye at F; the ray from the point A, will be seen in the direction F G, that from C, in the direc- tion F H; the visual angle, therefore, will be greatly increased, and the object must appear larger, in proportion. I must now explain to you the refraction of a ray of light, by a triangular piece of glass, called a prism. (Fig. 3.) Emilij. The three sides of this glass are fiat; it cannot, there- fore, brmg the rays to a focus; nor do I suppose that its refrac- tion 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 by a prism, can produce. Mrs. B. The refractions of the ray, both on entering and on quitting the prism, are in the same direction, (Fig. 3.) On en- tering the prism P, the ray A is refracted from B to C, and on quittmg it from C to D. In the first instance it is refracted to- wards, and in the last, from the perpendicular; each causing it to deviate in the same way, from its original course, A B. I will show you this by experiment; but for this purpose it will be advisable to close the window-shutters, and admit, through the small aperture, a ray of light, which I shall refract, by means of this prism. Caroline. Oli, what beautiful colours are represented on the opposite wall! There are all the colours of the rainbow, and with a brightness, I never saw equalled. (Fig. 4, plate 20.) Emily. I have seen an effect, in some respects similar to this, produced by the rays of the sun shining upon glass lustres; but now is it possible that a piece of white glass can produce such a variety of brilliant colours? Mrs. B. The colours are not formed by the prism, but ex- isted in the ray previously to its refraction. 25. What is the effect of one plane side in a lens ? 26. How is the focus of the plano-convex lens situated, fig. 2, plate 20 ? 27. How does a convex lens magnify objects, fig. 6, plate 19 ? 28. What is the article denominated which is represented at fig. 3, plate 20? 29. How will a ray be refracted, which enters on one side of the prism, in the direction A B .'' 30. VV'hat e^ feet is produced by this refraction, as represented in fie. 4, plate 20 '' Q2 186 ON REFRACTION AND COI^OURS. Caroline, Yet, before its refraction, it appeared perfectly •white. Mrs. B. The white rajs of the sun, are composed of rays, which, when separated, produce all these colours, although whea blended together, they appear colourless or white. Sir Isaac Newton, to whom we are indebted for the most im- portant discoveries respecting light and colours, 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 exhibited, (fig. 4.) in which are dis- played the following series of colours: red, orange, yellow, green, blue, indigo, and violet Emily. But how does a prism separate these coloured rays? Mrs. B. By refraction. It appears that the coloured rays have diiferent degrees of refrangibilit^; in passing through the prism, therefore, they take diiferent directions according to their susceptibility of refraction. The violet rays deviate most from tlieir original course; they appear at one of the ends of the spec- trum, A B: contiguous to the violet, are the blue rays, being those which have somewhat less refrangibility; then follow, in succession, 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 to- gether, can become white? Mrs. B. 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 circular piece of card, in compartments, with these seven colours, as nearly as possible in the proportion, and of the shade exhibited in the spectrum, and whirl it rapidly on a pin, it will appear white; as the velocity of the motion, wdl have the effect of blending the colours, in the impression which they make upon the eye. But a more decisive proof of the composition of a white ray is afforded, by reuniting these coloured rays, and forming with them, a ray of white light. Caroline. If you can take a ray of white light to pieces, and put it together again, I shall be quite satisfied. Mrs. B. 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 reunited, we find that they appear white as they did before refraction, I hope you wlW be convinced that the white rays, are a compound '' 31. Of what are the rays of white light said to be composed ? 32. What colours are produced? 33. By what property, in light, does refiraction enable w* to separate these different rays ? ON REFRACTION AND COLOURS. 187 of the several coloured rays. The prism P, you see, (fig. 5.) separates a ray of white liglit, 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., 1 cannot help thinking, that there may, perhaps, be but three distinct colours in the spectrum, red, yellow, and blue; and that the four otliers may consist of two of these colours blended together; for, in painting, we find, tliat by mixing red and yellow, we produce orange; with different proportions of red and blue, we make violet or any shade of pur- ple; 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 separate 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 intermediate colours, orange, green, violet, and indigo. Mrs. B. Your observation is, I believe, neither quite wrong, nor quite right. Dr. Wollaston, who has performed many ex- periments on the refraction of light, in a more accurate manner than had been previously done, by receiving a very narrow line of light on a prism, found that it formed a spectrum, consisting of rays of four colours only; but they were not exactly those you have named as primitive colours, for they consisted of red, green, Ijlue, 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. Caroline. But red and green mixed together, do not produce yellow? Mrs. B. Not in painting; but it may be so in the primitive rays of the spectrum. Dr. Wollaston observed, that, by increas- ing the breadth of the aperture, by which the line of light was admitted, the space occupied by each coloured ray in the spec- trum, was augmented, in proportion as each portion encroached on the neighbouring colour, and mixed with it; so that the in- tervention of orange and yellow, between the red and green, is o^t'ing, he supposes, to the mixture of these two colours; and the blue is blended on the one side with the green, and on the other with the violet, forming the spectrum, as it was originally observed by Sir Isaac Newton, and which I have just shown you. The rainbow, which exhibits a series of colours, so analogous 34. What experiment may be performed with a piece of card, so as to exemplify the compound nature of light ? 35. How can the same be shown by a lens, fig. 5. plate 20.' 36. Is it certakt tixat there are seven primitive colours in the spectrum.' 188 ON REFRACTION AND COLOURS. to those of the spectrum/is formed by the refraction of the sun's rays, in their passage through a shower of rain;" every drop of which acts as a prism, in separating tlie coloured rays as they pass through it; the combined etFect of innumerable drops, pro- duces the bow; which jou know can be seen, only when there are both rain, and sunshine. ) Emily. Pray, Mrs. B., cannot the sun's rays be collected to a focus by a lens, in the same manner as they are by a concave mirror? Mrs. B. The same effect in concentrating the rays, is pro- duced by the refraction with a lens, as by the reflection from a concave mirror: in tlie first,(the rays pass through the glass and converge to a focus, behind it; jn the latter, they are reflected from the mirror, and brought to a focus, before it. (A lens, when used for the purpose of collecting the sun's rays,^is called a burning glass. I have before explained to you, the manner in which a convex lens, refracts the rays, and brings them to a focus; (fig. 6, plate 19.) as these rays contain both light and heat, the latter, as well as the former, is refracted; and intense heat, as well as light, will be found in the focal point. The sun now shines very bright; if we let the rays fall on this lens, you will perceive the focus. Emily. Oh yes: the point of union of the rays, is very lumi- nous. I will hold a piece of paper in the focus, and see if it will take fire. The spot of light is extremely brilliant, but the paper does not burn? Mrs. B. Try a piece of brown ^aper; — that, you see, takes fire almost immediately. Caroline. This is surprising; for the light appeared to shine more intensely, on the white,, than on the brown paper. Mrs. B. The lens collects an equal number of rays to a focus, whether you hold the white or the brown paper, there; but the white paper appears more luminous in the focus, because most of the rays, instead of entering into the paper, are reflected by it; and this is the reason that the paper does not readily take fire: whilst, on the contrary, the brown paper, which absorbs more light and heat than it reflects, soon becomes heated and Caroline. This is extremely curious; but why should brown paper, absorb more rays, than white paper? Mrs. B. I am far from being able to give a satisfactory answer to that question. We can form but mere conjecture on this point; it is supposed that the tendency to absorb, or reflect 37. How is the rainbow produced, and what is necessary to its production? 38. How are the solar rays aflfected by a convex lens? 39. Why is such a lens, called a burning glass ? 40. Why are bodies of a dark colour, more readily inflamed, than those which are white ^ ON REFRACTION AND COLOURS. 189 rays, .^depends on the arrangement of the minute particles of the body, and that this diversity of arrangement renders some bodies susceptible of reflecting one coloured ray, and absorbing the others; whilst other bodies, have a tendency to reflect all the colours, and others again, to absorb them alL.' Emily. And how do you know which colours bodies have a tendency to reflect; or which to absorb? Mrs. B, { Because a body always appears to be of the colour which it reflects; for, as we see only by reflected rays, it can appear of the colour of those rays, only? Caroline. But we see all bodies of their own natural colour, Mrs. B.; the grass and trees, green; the sky, blue; the flowers of various hues. Mrs. B. ('True; but why is the grass green?'— because it ab- sorbs all, except the green rays; it is, therefore, these only which the gi-ass and trees reflect to our eyes, and this makes them appear green. The flowers, in the same manner, reflect the various colours of which they appear to us; the rose, the red rays; the violet, the blue; the jonquil, the yellow, &c.\ Caroline. But these are the permanent colours of the grass and flowers, whether the sun's rays sliine on them or not. Mrs. B. /Whenever you see those colours, the flowers must be illumined by some light; and light, from whatever source it proceeds, is of the same nature; composed of the various coloured rays which paint the grass, the flowers, and every coloured ob- ject in nature. Caroline. But, Mrs. B., the grass is green, and the flowers are coloured, whether in the dark, or exposed to the light? Mrs. B. Why should you think so? Caroline. It cannot be otherwise. i Mrs. B. A most philosophical reason indeed! But, as I never saw them in the dark, you will allow me to dissent from your opinion. Caroline. What colour do you suppose them to be, then, in the dark? Mrs. B. None at all; or black, which is the same thing. You can never see objects, without light. White light is compounded of rays, from which all the colours in nature are produced; there, therefore, can be no colour without light; and though a substance is black, or without colour, in the dark, it may become co loured, as soon as it becomes visible. It is visible, indeed, only by the coloured rays which it reflects; therefore, we can see it only when coloured. 41. What is believed to be the reason, why some bodies absorb more ray3 than others ? 42. What determines the colour of any particular body ? 43. What exemplifications are given? 44. By what reasoning is it proved, that bodies do not retain their colours in the dark ? 190 ON REFRACTION AND COLOURS. Caroline, All you say seems very true, and I know not what to object to it; yet it appears at the same time incredible ! What, Mrs. B., are we all as black as negroes in the dark? you make me shudder at the thought. Mrs. B. Your vanity need not be alarmed at the idea, as you are certain of never being seen, in that state. Caroline. That is some consolation, undoubtedly; but what a melancholy reflection it is, that all nature which appears so beautifully diversified with colours, is really one uniform mass of blackness! Mrs. B. Is nature less pleasing for being coloured, as well as illumined, by the rays of light? and are colours less beautiful, for being accidental, rather than essential properties of bo- dies ? Providence seems to have decorated nature with the enchant- ing diversity of colours, which we so much admire, for the sole purpose of beautifying the scene, and rendering it a source of sensible aratification : it is an ornament which embellishes nature, whenever we behold her. What reason is there to re- gret, that she does not wear it when she is invisible? Emily. I confess, Mrs. B., that I have had my doubts, as well as Caroline, though she has spared me the pains of express- ing them: but I have just thought of an experiment, which, if it succeed, will, I am sure, satisfy us both. It is certain, that we cannot see bodies in the dark, to know whether they have then any colour. Buv M^e may place a coloured body in a ray of light, which has been refracted by a prism; and if your theory is true, the body, of whatever colour it naturally is, must appear of the colour of the ray in which it is placed; for since it receives no other coloured rays, it can reflect no others. Caroline. Oh ! that is an excellent thought, Emily; will you stand the test, Mrs. B. ? Mrs. B. I consent: but we must darken the room, and ad- mit only the ray which is to be refracted; otherwise, the white rays will be reflected on the body under trial, from various parts of the room. With what do you choose to make the experi- ment? Caroline. This rose: look at it, Mrs. B., and tell me whe- ther it is possible to deprive it of its beautiful colour? Mrs. B. We shall see. — I expose it first to the red rays, and the flower appears of a more brilliant hue; but observe the green leaves — Caroline. They appear neither red nor green; but of a dingy brown with a reddish glow? 45. What proof of the truth of this theory of colours, may be aflforded by the prism ? ON REFRACTION AND COLOURS. 191 Mrs. B. They cannot appear green, because they have no green rays to reflect; neither are they red, because green bodies ab- sorb most of the red raysp( But though bodies, from the arrange- ment of their particles, have a tendency to absorb some rays, and reflect others, yet it is not natural to suppose, that bodies are so perfectly uniform in their arrangement, as to reflect only pure rays of one colour, and perfectly to absorb the others; it is found, i)\i the contrary, that a body reflects, in great abundance, the rays which determine its colour, and the others in a greater or less de«;ree, in proportion a,s they are nearer to or furtlier from its own colour, in the order of refrangibiiity. The green leaves of the rose, therefore, will reflect a few of the red rays, which, blended with their natural blackness, give them that brown tinge: if they reflected none of the red rays, they would appear perfectly black. Now I shall hold the rose in the blue rays — Caroline. Oh, Emily, Mrs. B. is right 1 look at the rose: it is no longer red, but of a dingy blue colour. Emily. This is the most wonderful, of any thing we have yet learnt. But, Mrs. B.,what is the reason that the green leaves, are of a brighter blue than the rose? Mrs. B. The green leaves reflect both blue and yellow rays, which produce a green colour. They are now in a coloured ray, which they have a tendency to reflect; they, therefore, re- flect more of the blue rays than the rose, (which naturally ab- sorbs that colour,) and will, of course, appear of a brighter blue. Emily. Yet, in passing the rose through the diflferent colours of the spectrum, the flower takes them more readily than the leaves. Mrs. B. Because the flower is of a paler hue. Bodies which reflect all the rays, are white; those which absorb them all, are black: between these extremes, bodies appear lighter or darker, in proportion to the quantity of rays they reflect or ab- sorb. This rose is of a pale red; it approaches nearer to white than to black, and therefore, reflects rays, more abundantly than it absorbs them. Emily. But if a rose has so strong a tendency to reflect rays, I should imagine that it would be of a deep red colour. Mrs. B. I mean to say, that it has a general tendency to reflect rays. Pale coloured bodies, reflect all the coloured rays to a certain degree, their paleness, being an approach towards whiteness: but they reflect one colour more than the rest: 46. Why will gieen leaves, when exposed to the red ray, appear of a dingy brown ? 47. Bodies, in general, when placed in a ray differing in colour irom their own, appear of a mixed hue, what causes this ? 48. Why will bodies of a pale, or light hue, most perfectly, assume the different colours of the spectrum ? 192 ON REFRACTION AND COLOURS. this predominates over the white, and determines the colour of the body. Since, then, bodies of a pale colour, in some degree reflect all the rajs of liglit, in passing through the various co- lours of the spectrum, they will reflect them all, with tolerable brilliancy; but will appear most vivid, in the ray of their natural colour. 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 absorb, 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 co- lours of the spectrum. Caroline. They must, however, reflect great quantities of the green rays, to produce so deep a colour. Mrs. B. f Deepness or darkness of colour, proceeds rather from a deficiency, than an abundance of reflected rays.^Remem- ber, that if bodies reflected none of the rays, they would be black; and if a body reflects only a few green rays, it will ap- pear of a dark green; it is the brightness, and intensity of the colour, which show that a great quantity of rays are reflected. Emily. A white body, then, which reflects all the rays, will appear equally bright in all the colours of the spectrum. 3Irs. B. Certainly. And this is easily proved by passing a sheet of white paper, through the rays of the spectrum. ( White, you perceive, results from a body reflecting all the rays which fall upon it; black, is produced, when they are all ab- sorbed ;;and colour, arises from a body possessing the power to decompose the solar ray, by absorbing some parts, and reflect- ing others. Caroline. What is the reason that articles which are blue, often appear green, by candle-light? Mrs. B. The light of a candle, is not of so pure a white as that of the sun: it has a yellowish tinge, and when refracted by the prism, the yellow rays predominate; and blue bodies reflect some of the yellow rays, from their being next to the blue, in the order of refrangibility; the superabundance of yellow ra^s, which is supplied by the candle, gives to blue bodies, a greenish huep Caroline. Candle-light must then give to all bodies^ a yel- lowish tinge, from the excess of yellow rays; and yet 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 strik- ing, when every surrounding object has a yellow tinge. Emily. Pray, why does the sun appear red, through a fog? 49. Upon what property in a body, does the darkness of its colour depend? 50. Why do some bodies appear -white, others black, and others of different colours ? 51 . From what cause do blue articles appear green, by candle-light ? ON REFRACTION AND COLOURS. 193 Mrs. B. (it is supposed to be owing to the rays, which are most refrangible, being also the most easily reflected :;in pass- ing through an atmosphere, loaded with moisture, as in foggy weather, and also in the morning and evening, when mists pre- vail, the vioUt^ indigOy blue, and green rays, are reflected back by the particles which load the air; whilst the yelloiv, orange, and red rays, being less susceptible of reflection, pass on, and reach the eye. ~j Caroline, And, pray, why is the sky of a blue colour? Mrs, B, You should rather say, the atmosphere; for the sky is a very vague term, the meaning of which, it would be diflTi- cult to define, philosophically. Caroline. But the colour of the atmosphere should be white^ since all the rays traverse it, in their passage to the garth. Mrs, B. Do not forget that the direct rays of light which pass from the sun to the earth, do not meet our eyes, excepting when we are looking at that luminary, and thus intercept them; in which case, you know, that the sun appears white. The atmosphere is a transparent medium, through whicli the sun's rays pass freely to the earth; (but the particles of which it is composed, also reflect the rays of lignt, and it appears tha\ they possess the property of reflecting the blue rays, the most copiously: the light, therefore, which is reflected back into the atmosphere, from the surface of the earth, falls upon these particles of air, and the blue rays are returned by reflection: 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 surface of the earth, would be illuminated, (the sky would ap- pear perfectly black.) Caroline, Oh, how melancholy would that be; and how per- nicious to the sight, to be constantly viewing 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, B, It arises from some chemical change, which takes place in the arrangement of the component 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 ten- dency to reflect several rays, which produce a dingy brown colour. 52. What is believed to be the cause, of the red appearance of the sun, through a fog-, or misty atmosphere ? 53. From what is the blue colour of the iky, thought to arise ? 54. What would be the colour of the sky, did not the atmosphere retlect light? 55 From what cause do some bodies change their colour, as leaves formerly green, become brown, and ink, veliow f R 194 ON REFRACTION AND COLOURS. An ink spot on linen, at first absorbs all the rays; but, from the action of soap, or of some other agent, it undergoes a chemi- cal 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 to which they have the greatest aversion, with which they will not incorporate, but re- ject, and drive from them. Mrs. B. It certainly is so; though I scarcely dare venture to advance such an opinion, whilst Caroline is contemplating her beautiful rose. Caroline. My poor rose ! you 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 found, 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 ri^the black is heated by absorbing the rays, the white is dazzling, by reflecting them. J 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, 1 shall give you a description of the eye. 56. Why is a black dress, warmer in the sunshine, than a white one of the same texture ? CONVERSATION XVH. ON THE STRUCTURE OF THE EYE, AND OPTICAL INSTRUMENTS. DESCRIPTION OF THE EYE. — OF THE IMAGE ON THE RETINA. — REFRAC- TION BY THE HUMOURS OF THE EYE. — OF THE USE OF SPECTACLES. — OF THE SINGLE MICROSCOPE. — OF THE DOUBLE MICROSCOPE. — OF THE SOLAR MICROSCOPE. — MAGIC LANTHORN.— REFRACTING TELESCOPE. — REFLECTING TELESCOPE, MRS. B. The body of the eye, is of a spherical form: (fig. 1. plate 21.) it has two membranous coats, or coverings^ the external one, a a a, is called the sclerotica, this is commonly known under the name of the white of the eye; it has a projection in that part of the eye which is exposed to view, b b, which is called(the trans- parent corneajbecause, when dried, it has nearly the consistence of very fine Korn, and is sufficiently transparent for the light to obtain free passage through it. The second membrane which lines the cornea, and envelops the eye, is called the choroid j c c c; tliis has an opening in front, just beneath the cornea, which forms the pupil, or sight of the eye, (/ (/, through which the rays of light pass into the eye. The pupil is surrounded by a coloured border called the iris: e e, which, by its muscular motion, always preserves the pupil of a circular form, whether it is expanded in the dark, or con- tracted by a strong light. This you will understand better by examining fig. 2. Emily. I did not know that the pupil was susceptible of va- rying 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 wnich it is placed. In a faint light, the pupil dilates so as to receive an additional quantity of rays, and in a strong light, it 1. What is the form of the body of the eye? fig. 1, plate 21. 2. What is its external coat called? 3. What is the transparent part of this coat denomi- nated ? 4. What is the second coat named ? 5. What opening is there in this? 6. What is the coloured part which surrounds the pupil? 7. The pupils iilate and contract, what purpose does this answer? 196 OPTICS. contracte, in order to prevent the intensity of the light from in- juring the optic nerve.,' pbserve Emily's eyes, as she sits look- ing towards the windows: the pupils 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 ftw 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 suifer pain, when from darkness, they suddenly come into a strong light; for the pupil being dilated, a quantity of rays must rush in, before it has time to contract. Emily. And when we go from a strong light, into obscurity, 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 objects: but in a few minutes it dilates, and we clearly perceive objects which were before in- visible. , Mrs. B. It is just so. The choroid c c, is embued with a black liquor, which serves to absorb all the rays that are irregu- larly reflected, and to convert the body of the eye, into a more perfect camera obscura. When the pupil is expanded to its ut- most extent, it is capable of admitting ten times the quantity ot lio-ht, that it does when most contracted, iln cats, and animals wtiich are said to«ee in the dark, the power of dilatation and con- traction of the pupil, is still greater; it is computed that the pupils of Iheir eyes may admit one hundred times more light at one time than at anotlier. . Within these coverings of the eye-ball, are contained, three transparent substances, called humours. * The first occupies the space immediately behind the cornea, and is called the aqueous humour, //, from its liquidity and its resemblance to water. Be- yond this, is situated the crystalline humour, g g, so called from its clearness and transparency: it has the form of a lens, and refracts the rays of light in a greater degree of perfection, than any that have been constructed by art: it is attached by two muscles, m m, to each side of the choroid. The back part of the eye, between the crystalline humour and the retina, is tilled by • the vitreous humour, h h, which derives its name from a resem- blance it is supposed to bear, to glass, or vitrified substances. The membranous coverings of the eye are intended chielly tor the preservation of the retina, i i, which is by far the most im- portant part of the eye, as it is that which receives the impres- 8 How could you observe the dilatation and contraction of the pupils ? 9. What purpose is the choroid said to answer? 10. In what animals is the chan-e in the iris greatest? 11. What are th« three humours denommated, and how are they situated? -if Plate 3X1. ^'M OPTICS. 197 sion of the objects of sight, and conveys it to the mind. The retina is formed by the expansion of the optic nerve, and is of a most perfect whiteness f this nerve proceeds from the brain, en- ters the eye, at zi, on the side next the nose, and is finely spread over the interior surface of the choroid. The rays of light which enter the e^e, by the pupil, are re- fracted 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 refracting humours: the image of objects was represented in the camera obscura, without any such assistance. Mrs. B. That is true; but the representation became much more strong and distinct, when we enlarged the opening 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 ob- ject which sends rays to our eyes, as points from which the rays diverge, as from a centre. Emily. These divergent rays, issuing from a single point, I believe you told us, were called a pencil of rays.^* Mrs. B. Yes. Now, divergent rays, on entering the pupil, do not cross each other; the pupil, however, is sufficiently large to admit a small pencil of them; and these„if not refracted to a focus, by the humours, would continue diverging after they had passed the pupil, would fall dispersed upon the retina, and thus the image of a single point, would be expanded over a large por- tion of the retina. The divergent rays from every other point of the object, would be spread over a similar extent of space, and would interfere and -be confounded with the first; so that no distinct image could be formed, and the representation on the re- tina would be confused, both in figure and colour.' Fig. 3. repre- sents two pencils of rays, issuing from two points of the tree, A B, and entering the pupil C, refracted by the crystalline humour D, and forming on tlie retina, at a b, distinct images of the spot they proceed from. Fi^. 4. ditfers from the preceding, merely from not being supplied with a lens; in consequence of which, the pencils of rays are not refracted to a focus, and no distinct im- age is formed on the retina. I have delineated only the rays issuing from two points of an object, and distinguished the two pencils in fig. 4. by describing one of them with dotted lines: the interference of these two pencils of rays on the retina, will enable you to form an idea of the confusion which would arke, 12. What is the part represented at i i, and of what does it consist? 13. What are the respective uses of the humours, and of the retina? 14 Why is it necessary the rays should be refracted? 15. How is this illustrat ed by fig. 3 and 4, plate 21 ? R 2 198 OPTICS. from thousands and millions of points, at tlie same instant pour- ing their divergent rays upon the retina. Emily. True; but I do not yet well understand, how the refracting humours, remedy this imperfection. Mrs. B. The refraction of these several humours, unites the whole of a pencil of rays, proceeding from any one point of an object, to a corresponding point on the retina, and the image is thus rendered distinct and strong. If you conceive, in fig. 3., every point of tlie tree to send fortli a pencil of rays, similar to those from A B, every part of the ti'ee will be as accurately represented on the retina, as the points a b. Emily. How admirably, how wonderfully, is this contrived! Caroline. But since the eye absolutely requires refracting humours, in order to have a distinct representation formed on the retina, why is not the same refraction equally necessary, for the images formed in the camera obscura ^ Mrs. B. It is; excepting the aperture through which we re- ceive the rays into the camera obscura, is extremely small; so that but very few of the rays diverging from a point, gain afdmit- tance; but when M'e enlarged the aperture, and furnished it with a, lens, you found the landscape more perfectly represented. Caroline. I remember how obscure and confused the image was, when you enlarged the opening, without putting in the lens. Mrs. B. Such, or very similar, would be the representation on the retina, unassisted by the refracting humours. You will now be able to understand the nature of that imper- fection of sight, which arises from the eyes being too prominent. In such cases, the crystalline humour, D, (fig. 5.) being extreme- ly convex, refracts the rays too much, and collects a pencil, proceeding from the object A B, into a focus, F, before they reach the retina. From this focus, the rays proceed, diverging, and consequently form a very confused image on the i-etina, at a b. This is the defect in short-sighted people. Emily. I understand it perfectly. But why is this defect remedied by bringing the object nearer to the eye, as we find to be the case with short-sighted people? M?-s. B. The nearer you bring an object to your eye, the more divergent the rays m\\ upon the crystalline humour, and consequently they are not so soon converged to a focus: this focus, therefore, either falls upon the retina, or at least ap- proaches nearer to it, and the object is proportionably distinct, as in fig. 6. Emily. The nearer, then, you bring an object to a lens, the further tlie image recedes behind it. 16. What causes a person to be short-sighted? fig. 5, plate 21. 17. Why does placing an object near the eye, enable such, to see distinctly ? fig. 6, '^f^^r' TLj\TE:xxnL OPTICS. 199 Mrs. B. Certainly. But short-sighted persons have another resource, for objects which they can not bring near to tlieir eyes; this is, to place a concave lens, C D, (fig. 1, plate 22.) before the eye, in order to increase ,the divergence of the rays. The eftect of a concave lens, is, you know, exactly the reverse of a convex one: it renders parallel rays divergent, and those which are already divergent, still more so. By the assistance of such glasses, therefore, the rays from a distant object, fall on the pupil, as divergent as those from a less distant object; and, with short-sighted people, they throw the image of a distant object, back, as far as the retina. Caroline. This is an excellent contrivance, indeed. Mrs. B. And tell me, what remedy w^ndd you devise for such persons as have a contrar}^ defect in their sight; tJiat is to say, who are long-sighted, in whom the cnstalline humour, be- ing too tlat, does not refract the rays sufficiently, so that they leach the retina before they are converged to a point? Caroline. I suppose that a contrary remedy must be applied to this defect; that is to say, a convex lens, L M, fio-. 2, to make up for the deficienfcy of convexity of the crystalline hu- mour, O P. For the convex lens would bring the rays nearer together, so that they would fall, either less divergent, or paral- lel, on the crystalline humour; and, by being sooner converged Jo a focus, would fall on the retina. Mrs. B. Very well, Caroline. This is the reason why elderly people, the humours of whose eyes are decayed by age, ^re under the necessity of usin^ convex spectacles. And when deprived of that resource, they hold the object at a distance from their eyes, as in fig 3, in order to bringc.the focus more forward. Caroline. I have often been surprised, when my grandfather reads without his spectacles, to see him hold the book at a con- siderable distance from his eyes. But I now understand the cause; the more distant the object is from the crystalline lens, the nearer to it, will the image be formed. Emily. I comprehend the nature of these two opposite de- fects very well; but I cannot now conceive, how any sight can be perfect: for, if the crystalline humour is of a proper degree ofconvexity, to bring the image of distant objects to a focus on the retina, it will not represent near objects distinctly; and if, on the contrary, it is adapted to give a clear image of near ob- jects, 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 18. A concave lens remedies this defect; how? fig. 1, platt 22. 19. What is the remedy, when a person is long-sighted? fig. 2. 20. Why Joes holding an object far from the eye, help such persons ? fig. 3. SOO OPTICS. defects, if we had it not in our power to adapt the eye, to the distance of the object; it is believed that this is accomplished, by our having a command over the crystalline lens, so as to pro- ject it towards, or draw it back from the object, as circum- stances require, by means of the two muscles, to which the crystalline humoUr is attached; so that the focus of the rays, constantly falls on the retina, and an image is formed equally distinct, either of distant objects, or 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 pro- trude, in that part of the eye wliich is exposed to view. 3Irs. 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 assistance of the cornea to bring them to a focus on the retina. Emily. Pray, what is the reason that we cannot see an ob- ject distinctly, if we place it very near to the eye? Mrs. B. Because the rays fall on the crystalline humour, too divergent to be refracted to a focus ort the retina; the confusion, therefore, arising from viewing an object too near the eye, is similar to that which proceeds from a flattened crystalline hu- mour; the rays reach the retina before they are collected to a focus, (fig. 4.) If it were not for this imperfection, we should be able to see and distinguish the parts of objects, which, from their minuteness, are now invisible to us; for, could we place them very near the eye, the 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 retracted to a focus on the retina? Mrs. B. The microscope is constructed for this purpose. The single microscope (fig. 5.) consists simply of a convex lens, commonly called a magnifying glassy in the focus of which the object is placed, and through which it is viewed: by this means, you are enabled to place your eye very near to the object, for 'the lens A B, by diminishing the divergence of the rays, before they enter the pupil C, makes them fall parallel on the crystal- line humour D, by which they are refracted to a focus on the retina, at R R. Emily. This is a most admirable invention, and nothing can be more simple; for the lens magnifiesthe object, merely by aU lowing us to bring it nearer to the eye. 21. How is the eye said to adapt itself to distant, and to near objects? 22. Why are objects rendered indistinct, when placed very near to the eye? fig. 4, plate 22. 23. What is the single microscope, fig. 5, and how does it mi^gnify objects ? \ \ OPTICS. 201 Mrs, B. Those lenses, therefore, which have the shortest focus will magnify the object most, because they enable us to place it nearest to the eye. Emily. But a lens, that has the shortest focus, is most bulg- ing or convex; and the protuberance of the lens will prevent the eye from approaching very near to the object. 3Irs. B. This is remedied by making the lens extremely small: it may then be spherical without occupying much space, and thus unite the advantages of a short focus, and of allowing the eye to approach the object. There is a mode of magnifying objects, without the use of a lens: if you look through a hole, not larger than a small pin, you may place a minute object near to the eye, and it will be distinct, and gi-eatly enlarged. This piece of tin lias been per- forated for the purpose; place it close to your eye, and this small print before it. Caroline. Astonishing! the letters appear ten times as large as they do without it: I cannot conceive how this eftect is pro- duced. Mrs. B. The smallness of the hole, prevents the entrance into the eye, of those parts of every pencil of rays which diverge much; so that, notwithstanding the nearness of the object, those rays from it, which enter the eye, are nearly parallel, and are, therefore, brought to a focus by the humours of the eye. 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 wliich 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 M, for the purpose of magnifying the object, is called the object-glass, the other, N O, acts on the principle of the single microscope, and is called the eye-glass. There is another kind of microscope, called the solar micro- scope, 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 can show you the effect of this microscope; but for this purpose, 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 23, fig. 1.) which is a small insect, before tlie lens C D, and nearly at its focus: the image E F, will then be represented on the opposite wall, in the same manner, as the landscape was in the camera obscura; 24. How may objects be ma^ified without the aid of a lens ? 25. Why can an object, very near to the eye, be distinctly seen, when viewed through a small hole ? 26. Describe the double microscope, ai represented in fig, 6, plate 22. 202 OPTICS. with this difference, that it will be magnified, instead of being diminished. I shall leave you to account for this, bj examinino- the figure. '^ Emily. I see it at once. The image E F is magnified, be- cause it is farther 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, an- swers the purpose equally well, either for magnifying or dimin- ishiiig 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 pro- digious: 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 admit 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. Emily. But could you not by means of another lens, bring a large pencil of rays to a focus on the object, and thus concen- trate upon it the whole of the light admitted ? Mrs. B. Very well. We shall enlarge the opening, and place the lens X Y (fig. 2.) in it, to converge the rays to a focus on the object A B. There is but one thing more wanting to complete the solar microscope, which I 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 object: if a similar mirror were placed to reflect light upon the lens, would it not be a means of illuminating the object more perfectly? Mrs. B. You are quite ri^ht. P Q (fig. 2.) is a small mir- ror, 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 apparatus, let us examine the mites on this piece of cheese, which I place near the focus of the lens. 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 27. How does the solar microscope, (fig. 1 plate 23.) operate ? 28. Why may minute objects be greatly magnified by this instrument ? 29. In its more perfect form it has other appendages, as seen in fig. 2, what are they ? and what their uses ? OPTICS. 203 piece of cheese has fallen : one might imagine it an earthquake : some of the poor mites must have been crushed; how fast they run — the^ absolutely seem to gallop. But this microscope can be used onlj for transparent objects; as the light must pass tlirough them, to form the image on the wall? Mrs, B. Very minute objects, such as are viewed in a mi- croscope, 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 magic lanthorn constructed on the same principles? Mrs. B. Yes, with this difference; the objects to be magni- fied, are painted upon pieces of glass, and 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 tlie naked eye. But there are objects, which, though not really small, appear so to us, from their distance; 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 ettect 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 make the object approach the eye, cannot we by means of a lens bring an im- age 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 another lens, which will act as a microscope, enable us to bring the im- age close to the eye, and thus render it visible? 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 com- mon 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 invert- ed; and that is not the case in the telescopes I have looked through. 30. What is added when opaque objects are to be viewed ? %. 3. 31. In what does the ma^c lanthorn dififer from the solar microscope ^ 32. What are the use and structure of the telescope, as shown in %. 4 ' 204 OPTICS. 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 consequently upright. These additional glasses are used to view teiTestrial objects,- for no inconvenience arises from seeing the celestial bodies in- verted. Emily. The difference between a microscope and a telescope, seems to be this> — a microscope produces a magnified image, be- cause the object is nearest the lens; and a telescope produces a diminished image, because the object is furthest 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. These are called reflecting telescopes, because the image is reflected by metallic mirrors. 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 hun- dred feet: dn instrument of this kmd may, therefore, possess a high magnifying power, and yet be so short, as to be readily managed. 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 with the object, it is true; but it is magnified, if you compare it to the di- mensions of which it would appear without the intervention of any optical instrument; and this magnifying power is greater in reflecting, than in refracting telescopes. We must now bring our observations to a conclusion, for I have communicated to you the whole of my very limited stock of knowledge of Natural Philosophy. If it enable you to make further progress in that science, my wishes will be satisfied; but remember, in order that the study of nature may be productive of happiness, it must lead to an entire confidence in the wisdom and goodness of its bounteous Author. 33. When terrestrial objects are to be viewed, why are two additional lenses employed ? 34. What part of the telescope performs the part of a mi- •roscope? 3.u. la what does the reflecting, differ from the refractin* tele- scope ? 36. What auvaatages, do reflectipg, possess over refracting t-^^^ scopes ? GLOSSARY. \CCELERATED MoTION. Motion IS said to be accelerated, when the ve- locity is continually increasing;. Accidental Properties. Those properties of bodies which are lia- ble to change, as colour, form, &c. Acute.: — See Angle. Air. Ar> elastic fluid. The atmo- sphere which surrounds the earth, is generally understood by this term, but there are many kinds of air. The term is synonymous with Gas. Air PirMP. An instrument by which vessels may be exhausted of air. Altitude. The height in degrees of the sun, or any heavenly body, above the horizon. Angle. The space contained be- tween two lines inclined to each other, and which meet in a point. Angles are measured in degrees, upon a segment of a circle described by fjlacing ope leg of a pair of com- passes on the angular point, ai^d with the other, describing the seg- ment between the two lines. If the segment be exactly l-4th of a circle, it is called a right angle, and con- tains 90 deg. If more than l-4th of a circle, it is an obtuse angle. If less, an acute angle. See plate 2. Angle of Incidence, is the space contained between a ray which falls obliquely upon a body, and a line perpendicular to the surface of the body, at the point where the ray falls. Angle op Reflection. The space contained between a reflected ray, and a line perpendicular to the re- flecting point. Angle op Vision, or visual angle. The space contained between lines drawn from the extreme parts of any object, and meeting in the eye. AwTARCTic Circle. A circle ex- tending round the south pole, at the distance of 23 1-2 degrees from it. The same as the south frigid zone. Aphelion. That part of the orbit of a planet, in which its distance from the sun is the greatest. Area. The surface enclosed be- tween the lines which form the boundary of any figure, whether regular or irregular. Aries. See Sign. Asteroids. The name given to the four small planets, Ceres, Juno,"* Pallas, and Vesta. -», Astronomy. The science which treats of the motion and other phe* nomena of the sun, the planets, the stars, and the other heavenly bo- dies. Atmosphere. The air which sur- rounds the earth, extending to an unknown height. Wind is this air in motion. Attraction. A tendency in bodies to approach each other, and to exist in contact. Attraction op Cohesion. That attraction which causes matter to remain in masses, preventing them from falling into powder. For this attraction to exist, the particles must be contiguous. Attraction of Gravitation. By this attraction, masses of matter, placed at a distance, have a ten- dency to approach each other. At- traction is mutual between the sun and the planets. Axis of the Earth, or of any oe the planets. An imaginary line passing through their centres, and terminating at their poles ; round this their diurnal revolutions are performed. Axis pp Motion. The imaginary line, around which all the parts of a body revolve, when it has a spin- ning motion. Axis OB A L^NS, OR Mirror. A i06 GLOSSARY. line passing through the centre of a lens, or mirror, in a direction per- pendicular to its surface. Balloon. Any hollow globe. The term is generally applied to those which are made to ascend in the air. Barometer. Commonly called a weather-glass. It has a glass tube, containing quicksilver, which by rfsing and falling, indicates any change in the pressure of the at- mosphere, and thus frequently warns us of changes in the wea- ther. Body. The same as Matter. It may exist in the solid, liquid, or aeriform state ; and includes every thing with which we become ac- quainted by the aid of the senses. Burning-glass, or Mirror. A lens, or a mirror, by which the rays of light, and heat, are brought to a focus, so as to set bodies on fire. Cambra Obscura, a darkened room; or more frequently a box, admitting light by one opening, where a lens is placed; which, bringing the rays of light, from external objects, to a focus, presents a perfect picture of them, in miniature. Capillary Tubes. Tubes, the bore of which is very small. Glass tubes are usually employed, to show the phenomenon of capillary attraction. Fluids in which tliey are immersed, rise in such tubes above the level of that in the con- taining vessel. Centre of a Circle. A point, equally distant from every part of its circumference. Centre of Gravity. That point within a body, to which all its par- ticles tend, and around which they exactly balance each other. A sys- tem of bodies, as the planets, may hav3 a common centre of gravity, around which they revolve in their orbits ; whilst each, like the earth, has it3 particular centre of gravity within itself. Centre of Motion* That point about which the parts of a revolv- ing body move, which point is, itself, considered as in a state of rest. Centre of MAttNiTUDE. The middle point of any body. Suppose a globe, one side of which is formed " of lead, and the other of wood, the centres of magnitude and of gravity, would not be in the same points. Central Forces. Those which either impel a body towards, or from, a centre of motion. Centrifugal. That which gives a tendency to fly from a centre. Centripetal. That which impels a body, towards a centre. Circle. A figure, the periphery, or circumference of which, is every where equally distant,, from the point, called its centre. Circle, Great. On the globe, or earth, is one that divides it into two equal parts, or hemispheres. The equator, and meridian lines, are great circles. Circle Lesser. Those which di- vide the globe into unequal parts. The tropical, arctic and antarctic circles, and all parallels of latitude, are lesser circles. Circumference. The boundary line of any surface, as that which surrounds the centre of a circle ; the four sides of a square, &c^ Comets. Bodies which revolve round the sun, in very long ovals, approaching him very nearly in their perihelion, but in their aphe- lion, passing to a distance immea- surably great. Cohesion. See Attraction. Compressible. Capable of being forced into a smaller space. Concave. Hollowed out ; the inner surface of a watch-glass is concave, and may represent the form of a concave mirror^ or lens. Convex, Projecting, or bulging out, as the exterior surface of a watch- glass, which may represent the form of a convex mirror, or lens. Cone. A body somewhat resembling a sugar-loaf; that is, having a round base, and sloping at the sides, until it terminates in a point. Conjunction. When three of the heavenly bodies are in a straight or right line, if you take either of the extreme bodies, the otlier two are ia conjunction with it; because a GLOSSARY. 207 straight line drawn fr6m it, might pass through the centres of both, and join them together. At the time of new moon, the moon and sun are in conjunction with the earth ; and the moon and earth, are in conjunction with the sun. CoNSTELLA-TidN, OR SiGN. A col- lection of stars. Astronomers have imagined pictures drawn in the hea- vens, so as to embrace a number of contiguous stars, and have named the group after the animal, or other article supposed to be drawn; an individual star is generally desig- nated by its fancied location ; as upon the ear of Z.eo, the Lion, &c. Convergent Rays, are those which approach each other, so as eventually to meet in the same point. Crystals. Bodies of a regular form, having flat surfaces, and well defin- ed angles. Nitre, and other salts, are familiar examples. Many mass- es of matter, are composed of crys- tals too minute to be discerned with- out glasses. Curvilinear, consisting of a line which is not straight, as a portion of a circle, of an oval, or any curved line. Cylinder. A body in the form of a roller, having flat circular ends, and being of equal diameter throughout Degree. If a circle of any size be divided into 360 equal parts, each of these parts is called a degi-ee. One quarter of a circle contains ninety degrees ; one twelfth of a circle, thirty degrees. The actual length of a degree, must depend upon the size of the circle. A de- gree upon the equator, uix)n a me- ridian, or any great circle of the earth, is equal to 69 1-2 miles. Straight lines are sometimes divided into equal parts, called degrees; but these divisions are arbitrary, bear- ing no relationship to the degrees upon a circle. Density. Closeness of texture. When two bodies are equal in bulk, that wliich weighs the most, has the greatest density. Diagonal. A line drawn so as to connect two remote angles of a square, or other four-sided figure. Dilatation. The act of increasing in size. Bodies in general, dilate when heated, and contract by cool- ing. Discord. When the vibrations of the air, produced by two musical tones, do not bear a certain ratio to each other, a jarring sound is pro- duced, which is called discord. Divergent Rays. Those which proceed from tlie same point, but are continuallyreceding from each other. Divisibility. Capability of being divided, or of having the parts sepa- rated from each other. This is called one of the essential properlics of matter; because, however minute the particles may be, they must still contain as many halves, quarters, &;c. as the largest mass of matter. Echo. A sound reflected back, by some substance, so situated as to produce this eifect. Eclipse. The interruption of the light of the sun, or of some other heavenly body, by the iiUervention of an opaque body. The moon passing between the earth and tlie sun, causes an eclipse of the latter. Ecliptic. A circle in the heavejis. The apparent path of the sun, through the twelve signs of the zo- diac. This is caused by the actual revolution of the earth, round the sun. It is called the ecliptic, be- cause eclipses always happen in the direction of that line, from the earth. Elasticity. That property of bo- dies, hy which they resume their dimensions and form, when the force which changed them is removed. Air is eminently elastic. Two ivory balls, struck together, become flat- tened at the point of contact ; but immediately resuming their form, they react upon each other. Ellipsis. An oval. This figure dif- fers from a circle, in being uneqaal in its diameters, and in having two centres, or points, called its/oa. The orbits of the planets are all el- liptical. EauATOR. That imaginary lino which divides the earth into north- 208 GLOSSARY. em and southern hemispheres, and .which is equally distant from each pole. EauiLiBRiUM. When two articles exactly balance each other, they are in equilibrium. They may, not- withstanding-, be very unequal in weight, but they must be so situat- ed, that, if set in motion, their mo- mentums would be equal. EauiNox. The two periods of time at which the nights and days are every where of equal length. Ttae vernal equinox is in March, when the sun enters the sign Aries; the autumnal equinox in September, when the sun enters Libra. At these periods, the sun is vertical at the equator. Exhalations. All those articles which arise from the earth, and mixing with the atmosphere, form vapour. Expansion. The same as dilatation, which see. Extension. One of the essential properties of matter ; that by which it occupies some space, to the exclu- sion of all other matter. ''"■iGURE. All matter must exist in some form, or shape; hence figure is defemed an essential proji^rty of matter. ■p'LUiD. A form of matter, in which its particles readily flow, or slide, over each other. Airs, or gases, are called elastic fluids, because they are readily reduced to a small- er bulk by pressure. Liquids, are denominated non-elastic fluids, be- cause they suffer but little diminu- tion of bulk, by any mechanical force. Focus. That point in which con- verging rays unite. Force. That power which acts upon a body, either tending to cre- ate, or to stop motion. FcxxNTAiN. A jet, qr stream of wa- ter, forced upwards by the weight of other water, by the elasticity of air, or some other mechanical pressure. Friction. The rubbing of bodies together, by which their motion is retarded. Friction maybe lessened, but cannot be destroyed. Frigid Zones. The spaces or areas, contained within the arctic and antarctic circles. Fulcrum. A prop. The point or axis, by which a body is supported, and about which it is susceptible of mo- tion. Gas. Any kind of air ; of these there are several. The atmosphere con- sists of two kinds, mixed, or com- bined with each other. Geometry. That branck of tlie mathematics, which treats of lines, of surfaces, and of solids; and inves- tigates their properties, and pro- portions. Globe. A sphere, or ball. It has a point in its centre of magnitude, from which its surface is every where equally distant. Gravity. That species of attrac- tion which appears to be common to matter, existing in its particles, and giving to them, and of course to the masses which thoy compose, a tendency to approach each other. By gravity a stone falls to the earth, and by it the heavenly bodies tend towards each other. Harmony. A combination of musi- sical sounds, produced by vibra- tions which bear a certain ratio to each other ; and which thence ai- fect the mind agreeably,when heard at the same time. Sounds not so related, produce discord. Hemisphere. Half a sphere or globe. A plane passing through the centre of a globe, will divide it into hemispheres. Horizon, 'l^his is generally divided into sensible^ and rational. The sensible horizon is that portion of the surface of the earth, to which our vision extends. Our rational horizon is that circle in the heavens which bounds our vision, when on the ocean, an extended plane, or any elevated situation. In the heavens our sensible, and our rational hori- zon are the same ; its plane would divide the earth into hemispheres at 90 degrees from us ; and a per- son standing on that part of the earth which is directly opposite to us, would, at the same moment, see in his horizon, the same heavenly «LOSSARY. 209 bodies, which would be seen in ours. HoRizoif TAL. Level; not inclined, or sloping. A perfectly round ball, placed upon a flat surface, which is placed horizontally, will remain at rest. Hydraulics. That science which treats of water in motion, and the means of raifiing, conducting, and using it for moving machinery, or other purposes. Hydrostatics. Treats of the weight, pressure, and equilibrium of fluids, when in a state of rest. Hydrometer. An instrument used to ascertain the specific gravity of difierent fluids, which it does, by the depth to which it sinks when floating on them. Image. The picture of any object which we perceive either by re- flected or refracted light. All ob- jects which ai-e visible, become so by forming images on the re- tina. Impenetrability, l^hat property of matter, by which it excludes all other matter from occupying the same space with itself at the same time. If two particles could exist in the same space, so also might any greater number, and indeed all the matter in the universe, might be collected in a single point. Incidence. The direction in which a body, or a ray of light, pioves in its approach towards any. substance, upon which it strikes. JifCLiJVED Plane. One of the six mechanical powers. Any platie surface inclined to the horizon, may be so denominated. Inertia. One of the inherent pro- perties of matter. Want of power, or of any active principle witliin it- self, by which it can change its own state, whether of motion, or of rest. Ikherent Properties. Those properties which are absolutely ne- cessary to the esdstence of a body ; called also essential properties. All others are denominated accidental. Colour is an accidental — extension, an esseutial property of matter. Latitude. Distance from the equa- S2 tor, in a direct line towards either pole. This distance is measured in degrees and minutes. The degree of latitude cannot exceed ninety, or one quarter of a circle. Places to the south of the equator, are in south latitude, and those to the north, in north latitude. Latitude, Parallels of. Lines drawn upon the globe, parallel to the equator, are so called ; every place situated on such a line, has the same latitude, because equally distant from the equator. Lens. A glass, ground so that one or both surfaces form segments of a sphere, serving either to magnify, or diminish objects seen through them. Glasses used in spectacles are lenses'. Lever. One of the mechanical pow- ers. An inilexible bar of wood or metal, supported by a fulcrum, or prop ; and employed to increase the efiect of a given power. Libra. One of the twelve signs of the zodiac. That into which the sun enters, at the autumnal equinox. LiGRT. That principle, by the aid of which we are able to discern all visible objects. It is generally be- lieved to be a substance emitted by luminous bodies, and exciting vision by passing into the eye. Longitude. Distance measured in degrees and minutes, either in an eastern, or a western direction, from any given point either on the equa- tor, or on a parallel of latitude. Degrees of longitude may amount to 180, or half a circle. A degree of longitude measured upon the equator, is of the same length with a degree of latitude; but as the poles are approached, the degrees of longitude diminish in length, be- cause the circles upon which they are measured, become less. Lunar. Relating to Luna, the moon. Lunation. The time in which the moon completes its drcuit. A lunar month. Luminous Bodies. Those which emit light from their own substance; not shining by borrowed, or reflect- 210 GLOSSARY. Ma^chine. Any instrument, either simple or compound, by which any mechanical effect is produced. A needle, and a clock, are both ma- chines. Ma»ic Lanthorn, or Lantern. An optical instrument, by which transparent pictures, painted upon glass, are magnified and exhibited on a white wall or screen, in a dark- ened room. The phantasmagoria, is a species of magic lanthorn. Mathematics. The science of numbers and of extension. Com- mon arithmetic, is a lower branch of the mathematics. In its higher departments, it extends to every thing which is capable of being ei- ther numbered or measured. Matter. Substance. Every thing with which we become acquainted by the aid of the senses ; every thing however large, or however minute, which has length, breadth, and thickness. Mechanics. That science which investigates the principles, upon which the action of every machine depends ; and teaches their proper application in overcoming resist- ance, and in producing motion, in all the useful purposes to which they are applied. Medium. In optics, is any body which transmits light. Air, water, glass, and all other transparent bo- dies, are media. Medium also de- notes that in which any body moves. Air is the medium which conveys sound, and which enables birds to fly. Melody. A succession of such single musical sounds, as form a simple air or tune. Mercury. That planet which is nearest to the sun. Quicksilver, a metal, which remains fluid at the common temperature of the atmo- sphere. It is capable of being ren- dered! solid, by intense cold. Meridian. Midday. A meridian line, is one which extends directly from one pole of the earth to the other ; crossing the equator at right angles. It is therefore half of a great circle. The hour of the day is the same at every place situated on the same me- ridian. Longitude is measured from any given meridian, to the oppo- site meridian. Places at the same distance in degrees, to the east or west of any meridian, have the same longitude. Microscope. An optical instrument, by which mirmte objects, are mag- nified, so as to enable us to perceive and examine such as could not be seen by the naked eye. Mineral. Earths, stones, metals, salts, and in general all substances dug out of the earth, are denomi- nated minerals. Minute. In time, the sixtieth part of an hour. In length, the sixtieth part of a degree. A minute of time, is an unvarying period; bat a minute in length varies in extent, with tiie degree of which it forms a part. The degrees and minutee are equal in number, upon a common ring, upon the equator of the earth, or, on any circle of the heavens. Mirrors. Polished surfaces of metal, or of glass coated with metal, for the purpose of reflecting the rays of light, and the images of objects. Common looking-glasses, are mir- rors. Those used in reflecting te- lescopes, are made of metal. Mobility. Capable of being moved from one place to another. This is accounted one of the essential pro- perties of matter, because we can- not conceive of its existence without this capacity. Momentum. The force, or power, with which e. body in motion acts upon any other body, or tends to preserve its own quantity of mo- tion. The momentum of a body, is compounded of its quantity of mat- ter, and its velocity. A body weigh- ing one pound, moving with a velo- city of two miles in a minute, will possess the same momentum with one weighing two pounds, moving with a velocity of one mile in a mi- nute. Motion. A continued and success- ive change of place, either of a whole body, or of the particles of GLOSSARY. 211 which a body is composed ; the earth in revolving upon its axis only, would not change its place as a body, but all the particles of which it is co;npo3ed, would revolve round a common axis of motion. In revolving in its orbit, its whole mass is constantly occupying a new portion of space. Natural Philosophy. That sci- ence which enquires into the laws which govern all the natural bodies in the universe, in all their changes of place, or of state. Neap Tides, Those tides which occur when the moon is in her quadiatures, or half way between new, and full moon; at these pe- riods the tides are the lowest. Nodes. Those points in the orbit of the moon, or of a planet, where it crosses the ecliptic or plane of the earth's orbit. When passing to the north of the ecliptic, it is called the ascending node ; wh^n to tlie south of it, the descending node. Oblate. See Spheroid. OcTAGOKT. A figure with eight sides, and consequently with eight angles. OPAauE. Not transparent ; refusing a passage to the rays of light. Optics. That branch of science which treats of light, and vision. It is generally divided into two parts. Catoptrics, which treats of the re- flection of light, and Dioptrics, which treats of its refraction. Orbit. The line in which a primary planet moves in its revolution round the sun ; or a secondary planet, in its revolution round its primary. These orbits are all elliptical, or oval. Parabola. A particular kind of curve ; that which a body describes in rising and in falling, when thrown upwards, in any direction not per- pendicular teethe horizon. Parallelogram, A figure with four sides, having those which are opposite, parallel to each other. A square, an oblong square, and the figure usually called a diamond, are Parallelograms. Parallel Lines. All lines, whe- ther straight or curved, which are every where at an equal distance from each other, are parallel lines. Parallel oe Latitude. See La- titude. Perihelion. That part of the orbit of a planet, in which it approaches the sun most nearly. Pendulum. A body suspended by a rod, or line, so that it may vi- brate, or oscillate, backwards and forwards. Pendulums of the same length, perform their vibrations in the same time, whatever may be their weight, and whether Uie arc of vibration, be long or short. Percussion. The striking of bodies against each, other. The force of. this, depends upon the momentum of the striking body. Period. The time required for the revolution of one of the neavenly bodies in its orbit. Perpendicular. Making an angle of 90 degrees with the horizon. When two lines which meet, make an angle of 90 degrees, they are perpendidicular to each other. Phases. The various appearances of the disc, or face of the moon, and of the planets; that portion of them which we see illuminated by the rays of the sun. Phenomenon. Any natural appear- ance is properly so called; the term, however, is usually applied to ex- traordinary appearances, as eclip- ses, transits, &c. Piston. That part of a pump, or other engine which is made to fit into a hollow cylinder, or barrel ; and to move up and down in it,^ in order to raise water, or for any otlier purpose. Plane. A perfectly flat surface. The plane of the orbit of a planet, is an imaginary flat surface, extend- ing to every part of the orbit. Planet. Those bodies which re- volve round the sun, in orbits near- ly circular. They are divided into primary, and secondary ; these lat- ter are also called satellites, or moons ; they revolve round the pri- mary planets, and accompany them in their courses round the sun. 212 GLOSSARY. Plitmb-hne. a string, or cord, by which a weight is suspended ; it is used foi* the purpose of finding a line perpendicular to the horizon ; the weight being always attracted towards the centre of the earth. Pneumatics. That branch of natu- ral philosophy, which treats of the mechanical properties of the atmo- sphere, or of air in general. Poles. The extremities of the axis of motion either of our earth, or of any other revolving sphere. The poles of the earth have never been visit- ed; the regions by which they are surrounded, being obstructed by impassable barriers ot ice. Power. That force which we apply to any mechanical instrument, to effect^ given purnose, is denomi- nated power, from whatever source it may be derived. We have the power of weights, of springs, of hor- ses, of men, of steam, &c. Prism. The instrument usually so called, is employed in optics to de- compose the solar ray: it consists of a piece of solid glass, several inches in length, and having three flat sides; the ends are equal in siie, and are of course triangular. Precession of the EairmoxES. Every equinox takes place a few seconds of a degree, before the earth arrives at that part of the ecliptic in .which the preceding equinox oc- curred. This phenomenon is called the precession of the equinoxes. There is consequently a gradual change of the places of the signs of the zodiac : a fact, the discovery of which has thrown much light on ancient chronology. Projection. That force by which motion is given to a body, by some power acting upon it, independ- ently of gravity. Pulley. One of the six mechanical powers. A wheel turning upon an axis, with a line passing over it. It is the moveable pulley only, which gives any mechanical advantage. Pump. An hydraulic, or pneumatic instrument, for the purpose of rais- ing water, or exhausting air. Quadrant. A quarter of a circle. An instrument used to measure the elevation of a body in degrees above the horizon. Qdadratures of the Moon. That period in which she appears in the form of a semicircle. She is then either in her first, or her last quarter ; and exactly half way, be- tween the places of new, and of full moon. Radiation. The passage of light or heat in rays, or straight lines ; these being projected from every lumi- nous, or heated point, in all direc- tions. Radius. The distance from the cen- tre of a circle, to its circumference; or one half of its diameter. In the plural denominated radii. Rainbow. An appearance in the at- mosphere, occasioned by the de- composition of solar light, in its re- fraction, and reflection, in passing through drops of i-ain. The bow can be seen, only when the sun is near the horizon, when the back is turned towards it, and there is a shower in the oppoite direction. Ray. a single line of light, emitted in one direction, from any luminous point. Reaction. Every body, whether in a state of motion, or at rest, tende to remain in such state, and resists the action of any other body upon it, with a force equal to that action. This resistance, is called its re- action. Receiver. This name is applied to glass vessels of various kinds, ap- pertaining to the air pump, and from which the air maybe exhaust- ed. They are made to contain, or receive, any article upon which an effect is to be produced, by taking off the pressure of the atmosphere. Refraction, of the rays of light, is the bending of those rays, when they pass obliquely from one medi- um into another of different density. A stick held obliquely in water, ap- pears bent or broken at the surface of the fluid. Refrangibility. Capacity of be- ing refracted. Light ia decomposed by the prism, because its compo- GLOSSARY. 213 nent parts are refrangible in differ- ent degrees, by the same refracting medium. REPtTLSiow. The reverse of attrac- tion. A tendency in particles, or in masses of matter, to recede from each other. The matter of heat within a body, appears to counter- act the attraction of its particles, so as to prevent absolute contact. RETiJ»fA. That part of the ball of the eye, upon which the images of visible objects are formed; and from which, the idea of such forms, is 1 conveyed to the mind. Revolution, of a planet ; is either diurnal, or annual ; the former, is its turning upon its own axis; the latter, is its passage in its orbit. Satellites. Moons, secondary planets. Segment of a circle. A portion, or part of a circle ; called also, an arc of a circle. Semi-diameter. Half the diame- ter. The semi-diameter of the earth, is the distance from its sur- face, to its centre. SiBERiAL. Belonging to the stars. A siderial day, is the time required for a star to reappear on a given meridian. A siderial year, the period in which the sun appears to have travelled round the ecliptic, so as to have arrived opposite to any particular star, from which his course was calculated. * Signs, or Constellations. Col- lections, or groups, of stars. Those of the zodiac are twelve, corres- ponding with the twelve months in the year. In the centre of these the ecliptic is situated. The sun appears to pass in succession tlirough these signs ; entering the first de- gree of aries, which is accounted the first sign, about the 21st of March. Sky. That vast expanse, or space, in which the heavenly bodies «are situated. Its blue appearance is supposed to arise from the particles of which the atmosphere is compos- ed, possessing tlie property of re- flecting the blue rays, in greatest abundance. Sola?,. Appertaining tp, or govern- ed by, the sun : as the solar sys- tem, the solar year, solar eclip- ses. Solid. Not fluid. Having its parts connected so as to form a mass. So- lid bodies, are not absolutely so, all undoubtedly containing pores, or spaces void of matter. Solstices. The middle of summer and the middle of winter ; those two points in the orbit of the earth, in which its poles point most di-. rectly towards the sun. Sonorous Bodies. Those bodies which are capable of being put into a state of vibration, so as to emit sounds. Specific Gravity. The relative weight of bodies of different spe- cies, when the same bulk of each ia taken. Water has been chosen as the standard for comparison. If we say that the specific gravity of a body is 6, we mean, that its weight is six times as great as that of a portion of water, exactly equal to it in bulk. Spectrum. That appearance of differently coloured rays, which is produced by the refraction of the solar ray, by means of a prism, ia called the prismatic spectrum ; it exhibits most distinctly, and beauti- fully, all the colours seen in the rainbow. Sphere. A globe, or ball. Spheroid. Spherical ; a body ap- proaching nearly to a sphere in its figure. The earth, is denominated an oblate sphermd ; it not being an exact sphere, but flattened at the poles, so as to cause the polar di^ ameter to be upwards of thirty miles less than the equatorial. Ob- late, is the reverse of oblong, and means shorter in one direction, than in another. Spring Tides. Those tides which occur at the time of new, or of full moon. The tides then rise to a greater height than at any other period. SauARE. A figure having four sides of equal length, and its angles all right angles. In numbers ; the product of a number 214 GLOSSARY. multiplied into itself; thns, the square of 3 is 9, and the square of 8 is 64. Star. The fixed stars are so called, because they retain their relative situations; while the planets, by re- volving in their orbits, appear to wander amongst the fixed stars. Subtend. This term is applied to the measurement of an angle; when the lines by which it is bounded recede but little from each other, they are said to subtend ; that is, to be contained under, a small an- gle. SirPERFiciES. The surface of any figure. Space extended in length and width. System. The mutual connexion, and dependance of things, upon each other. The solar, or Coper- nican system, includes the sun, the planets, witli their moons, and the comets. Tawgewt. a straight line touching the circumference of a circle ; but which would not cut off any portion of it, were it extended beyond the touching point, in botli directions. Telescope. An instrument by which distant objects may be dis- tinctly seen ; the images of ob- jects being brought near to the eye, and greaUy magnified. Temperate Zones. Those portions of the surface of the earth situated between 23 1-2 and 66 1-2 degrees of latitude. Within these bounda- ries, the s\^ is never vertical ; nor does he ever remain, during a whole day, below the horizon. Thermometer. An instrument for measuring the temperature of the atmosphere, or of other bodies. Torrid Zone. That portion of the earth which extends 23 1-2 degrees on each side of the equator, to the tropical circles; within this limit, the sun is vertical, twice in the year. Transit. Mercury or Venus, are said to transit the sun, when they pass between the earth and that luminary. They then appear like dark spots, upon the face of the sun. Transparent. Allowing the rays of light to pass freely through. The reverse of opaque. Glass, water, air, &c. are transparent bodies. Tropics. Two circles on the globe on either hemisphere, at the dis- tance of 23 1-2 degrees from the equator. Beyond these circles, the sun is never vertical : and the countries within them, are denomi- nated tropical. Twilight. That portion of the morning or evening, in wliich the light of the sun is perceptible, al- though he is below the horizon. Vacuum. Space void of matter. Such is supposed to be the space in which the planets revolve. We are said to produce a vacuum, when we exhaust the air from a receiver. Valve, A part of a pump, and of some other instruments,which opens to admit the passage of a fluid in one direction, but closes when pressed in the opposite direction, so as to prevent the return of the fluid ; a pair of bellows is furnished with a valve. Vapour. Exhalations from fluid or solid substances, generally mixing with the atmosphere. The most abundant, is that from water. Vertical. Exactly over our heads : ninety degrees above our horizon. Vibration. The alternate motion of a body, forwards and backwards; swinging, as a pendulum. Visual, Belonging to vision; tis the visual angle, or that angle formed by the rays of light which enter the eye, from the extremities of any object. Undulation. A vibratory, or wave-like motion communicated to fluids. Sound, is said to be pro- pagated by the undulatory, or vi- bratory motion of the air. Weage. One of the mechanical powers ; the form of the wedge is well known. It is of extensive use ; serving to rend bodies of great strength, and to raise enormous weights. Wheel and Axle. One of the mechanical powers, used under ya«. GLOSSARY. 215 riotts modifications. Cranes for raising weights, the wheels and pinions of clocks and watches, windlasses, &c. are all applications of this power. Zodiac. A broad belt in the hea- vens, extending nfcarly eight de- grees on each side of the ecliptic ; the planes of the orbits of all the planets are included within this space. This belt is divided into twelve parts or signs, each contain- ing 30 degrees. These signs are : Aries; the Ram. Taurus; the Bull. Gemini; the Twins. Cancer; the Crab. Leo; the Lion. Virgo ; the Virgin. Libra; the Scales. Scorpio ; the Scorpion. Sagittarius ; the Archer. Capricornus ; the Goat. Aquarius; the Waterer. Pisces; the Fishes. The first six are called northern signs; because the sun is in them, during that half of the year, in which he is vertical to the north of the equator; tlie last six, are called southern signs ; because, during his journey among them, he is vertical to the south of the equator. The sun enters Aries, at the time of the vernal equinox ; Cancer^ at the summer solstice ; Libra, at the autumnal equinox ; and Capricor- nus^ at the winter solstice. The sun is said to enter a sign, when the earth in going round in its orbit, enters the opposite sign. Thus, when the sun appears in the first degree of Libra, it is in conse- quence of the earth having arrived opposite to the first degree of Aries. A line then drawn from the earth, and passing through the centre of the sun, would, if extended to the fixed stars, touch the first degree of Libra. Zone. The earth is divided into zones, or belts. See Frigid, Tem- perate, and Torrid Zones. INDEX. A. Air, 11, 15, 28, 50, 136. Air-pump, 31, 145. Angle, 44. acute, 44. obtuse, 44. right, 44. of incidence, 45, 154, 160,173. of reflecUon, 45, 154, 160, 173. visual, 168, 169, 170. Angular velocity, 171. Antarctic circle, 92. Aphelion, 75. Arctic circle, 92. Atmosphere, 28, 104, 129, 136, 144, 150, 163. colour of, 193. reflection of, 193. refraction of, 182. Attraction, 10, 14, 23, 25, 179. of cohesion, 15, 19, 118. capillary, 18. of gravitation, 18, 23, 29, 70,80,96,116, 136. Avenue, 170. Auditory nerve, 151. Axis, 78. of motion, 48. of the earth, 22, 99. of mirrors, 176. of a lens, 184. B. Balloon, 30. Barometer, 140. Bass, 155. Bladder, 138. Bodies, 10. elastic, 40. fall of, 23, 26, 30, 36-. luminous, 157. opaque, 157. sonorous, 152, 155. transparent, 157. Bulk, 16. C. Camera obscura, 184, 197, 201. Capillary tubes, 18. Centre, 48. of gravity, 48, 51, 52, 115. of magnitude, 48, 53. of motion, 48, 55, 115. Centrifugal force, 49, 72, 95, 115. Centripetal force, 49, 72. Ceres, 84. Circle, 44, 94. Circumference, 94. Clouds, 129. Colours, 23, 185. Comets, 86. Compression, 42. Concord, 155. Constellation, 86. Convergent rays, 175, 177. Crystals, 12. Cur\'ilinear motion, 47, 72. Cylinder, 52. D. Day, 78, 105, 106. Degrees, 44, 94, 99, 169, 170. of latitude, 94,112. of longitude, 94, 112.. Density, 16. Diagonal, 47. Diameter, 94. Discords, 155. Diurnal, 78. Divergent rays^ 175, 177. Divisibility, 10, 12. E. Earth, 18, 70, 84, 88, 95. Echo, 154. Eclipse, 110, 159. Ecliptic, 86, 92, 99. Elasticity, 41. Elastic bodies, 28, 40. fluids, 28, 41, 118, 136. Ellipsis, 75. Equinox, 100, 107. precession of, 107, Equator, 92, 99. Essential properties, 10, Exhalations, 13. Extension, 10, 11. 218 INDEX. Eye, 166, 195. Jupiter, 85. F. Fallofbodies, 24, 27,31. Figure, 10, 12. Fluids, 118, 128. elastic, 28, 41, 118, 136. equilibrium of, 120, 122, 132. non-elastic, 119. pressure of, 121. Flying, 40. JPocus, 176. of concave mirrors, 177. of convex mirrors, 175, 177. of a lens, 184. imaginary, 176. virtual, 176. Force, 33. centrifugal, 49, 72, 95, 115. centripetal, 49, 72. projectile, 47, 49. of gravity, 47, 49 Fountains, 135. Friction, 68, 69, 135. Frigid zone, 93. Fulcrum, 54. General properties of bodies, 10. Georgium Sidu«, 85. Glass, 183. burning, 188. refraction of, 183. Gold, 119, 126. Gravity, 18, 23, 78, 97. H, Harmony, 155. Heat, 16, 29, 103. Hemisphere, 92, 100. Herschel, 85. Hydraulics, 118. Hydrometer, 128. Hydrostatics, 118. I. Imaige on the retina, 165, 172. reversecl, 167. in plain mirror, 172. in concave do. 175. in convex do. 175. Impenetrability, 10. Inclined plane, 54, 66. Inertia, 10, 14, 32. Inherent properties, 10,^ funo, 84. L. Lake, 133, 135. Latitude, 94, 112. Lens, 184. concave, 184. convex, 184. meniscus, 184. plano-concave, 184. plano-convex, 184. Lever, 54, 55. first kind, 58. second kind, 60. third kind, 60. Light, 157. pencil of, 158. ofthemoon, 162, 163. absorption of, 188. reflected, 160. refraction of, 179. Liquids, 118. Longitude, 94,112. Luminous bodies, 157. Lunar month, 108. edipse, 110. M. Machin©, 54, 66. Magic lanthom, 203. Mars, 84. Matter, 10, 13. Mechanics, 32. Mediums, 137, 180. Melody, 156. Mercury, (planet) 83, 85, 114. Mercury, or quicksilver, 16, 140, 141 Meridians, 93. Microscope, 200. single, 200. double, 200. solar, 202, 203. Minerals, 12. Minutes, 94. Momentum, 38, 56. Monsoons, 149. Month, lunar, 108. Moon, 78, 79, 80, 82, 85. Moonlight, 162, 163. ^lotion, 14, 32, 36. accelerated, 36. axis of, 48. centre of, 48, 55. compound, 46. curvilinear, 47, 49. diurnal, 78. INfiEX. 219 Motion, perpetual, 35. retarded, 35. reflected, 43. uniform, 34. Mirrors, 172. axis of, 176. burning, 177. concave, 174, 176, 209. convex, 174, 175. plane or flat, 172. reflection of, 173. N. Neap tides, 1 16. Nerves, 166. auditory, 151, 166. olfactory, 166. opticv 164, 166. Night, 78. Nodes, 110. O. Octave, 156. Odour, 13. Opaque bodies, 157, 158. Optics, 157. Orbit, 86. Pallas, 84. Parabola, 51, Parallel lines, 25. Parallel of latitude, 94. Pellucid bodies, 157. Pencil of rays, 158. Pendulum, 98. Perihelion, 75. Perpendicular lines, 25. Phases, 109. Piston, 143, 145. - Plane, 92, 93. Planets, 76, 81, 83. Poles, 92, 99, 100. Polar star, 100, 112. Porosity, 42, 126. Powers, mechanical, 54. Projection, 49, 50,-71. Precession of equinoxes, 107. Pulley, 54, 63. Pump, 31. sucking and lifting, 143. forcing, 144, 145. air, 31, 145. Pupil of the ©ye, 164. Rain, 17, 129. Rainbow, 188. Rarity, 16. Ray of light, 158, 179. reflected, 160, 161. incident, 161. Rays, intersecting, 165. Reaction, 39. Receiver, 31. Reflection of light, 160, 163. angle of, 45, 161, 173. of mirrors, 173. of plane mirrors, 174. of concave do. 174. of convex do. 174. Reflected motion, 43. Refraction, 179, 186. of the atmosphere, 182. of glass, 183. of a lens, 184. of a prism, 185. Resistance, 54. Retina, 165. image on, 166 Rivers, 134. Rivulets, 131. S. Satellites, 80, 111, 113. Saturn, 85. Scales, or balance, 56. Screw, 54, 67. Shadow, 110, 111. Siderial time, 106. Sight, 165. Signs of the zodiac, 86, 93. Smoke, 14, 29. Solar microscope, 202. Solstice, 100, 102. Sound, 151. acute, 155. musical, 155. Space, 33. Specific gravity, 123. of air, 140. Spectrum, 190. Speaking-trumpet, 154. Sphere, 26. Springs, 130. Spring tides, 116. Square, 81, 85. Stars, 77, 8^, 102. Storms, 147. 220 INDEX. Substance, 10. Vibration, 98, 152. Summer, 76, 100. Vision, 164, 168. Sun,71,75,78,162, 182. Vision, angle of, 168, 170. Swimming, 41. double, 171. Syphon, 132. U. T. Undulation, 153. Tangent, 49, 73. ■Unison, 155. Telescope, 203, 204. reflecting, 204. W. refracting ,204. Water, 118, 130. Temperate zone, 92, 101. spring, 130. Thermometer, 142. rain, 130. Tides, 114, 116. level of, 120. neap, 116. Wedge, 54, 66. spring, 1 16. Weight, 23. jerial, 150. Wheel and axle, 54, 65. Time, 105, 107. Wind, 146. siderial, 107. trade, 147. equal, 107. periodical, 148. solar, 107. Winter, 76, 101. Tone, 155. Torrid zone, 93, 147. ,182. Y. Transit, 114. Year, 107. Transparent bodies, 157. siderial, 107. Treble and bass, 155 solar, 107. Tropics, 92. •y V. Zodiac, 86. Valve, 143. Zone, 93. Vapour, 17, 29, 104, 129. . torrid, 93, 147, 182. Velocity, 33, 67. temperate, 93, 101. ' Venus, 84. frigid, 93, 100. Vesta, 84. THE END. L cJ(m^ d