PRINCIPLES THE SCIENCES. En Jamiliar letters. 1819. PRINCIPLES OF rirttrro ; OR, THE ELEMENTS OF HUMAN KNOWLEDGE CONNECTED WITH RELIGION AND MORALITY: IN A SERIES OF FAMILIAR LETTERS, FROM A FATHER TO HIS SON. Treating respectively of THEOLOGY, ZOOLOGY, MECHANICS, PNF.I MATIUS, MYTHOLOGY, BOTANY, MAGNETISM. ELECTRICITY, ASTRONOMY, GEOGRAPHY. NAVIGATION, GALVANISM, METEOROLOGY, HISTORY, OPTICS, LITERATURE. GEOLOGY, CHRONOLOGY, PHONICS, MUSIC, MINERALOGY, LOGIC, CHEMISTRY, PAINTING. PHYSIOLOGY, ONTOLOGY, HYDROSTATICS, SCULPTURE, ANATOMY, MATHEMATICS, HYDRAULICS, POETliY, ic. St.;. BY CECIL HARTLEY, M.A. Aullwr of " Principles of Punctuation," ad " Principles of Elocution.' " SCIENCE IS NOT SCIENCE 'TILL RKVIiAL'iJ." Eontion : PRINTED FOR EFF1NGHAM WILSON, ROYAL EXCHANGE. MDCCCXIX. at Stationers' Maurice, Fcnefaarcb*trect. PREFACE. THE advantages, arising from such a volume as the present, are many. By presenting a general and luminous, though circumscribed, view of the prin- ciples of the Sciences, the youthful mind will be excited to investigate each science from profourider sources. To promote this desirable object, re- ferences ha,ve been made, whenever it has been deemed necessary, to such valuable and standard works as relate to the respective subjects of illustra- tion. This cannot fail of being emi- nently serviceable ; as, next to the be- 1250811 IV PREFACE. nefit of being in actual possession of useful knowledge, is that of knowing where to look for the concealed trea- sure. Thus, although it is impossible that so small a volume should contain such a full or final explication of the Sciences, as to preclude the necessity of further research, it may be regarded as a key to the grand store-house of hu- man wisdom; independently of which, perhaps, a greater number of scientific facts, or a greater variety of informa- tion, can scarcely be found, in an equal number of pages, in any work in the English language. Another of the advantages, resulting from this little circle of the sciences, is the tendency which it possesses, to excite and determine the bearings of o genius to develope the tender germs of talent to shew to what particular branch of the wonders of creation the PREFACE. V application of each individual may be most successfully directed: as it should never be forgotten, that " Not alike to every mortal eye Is this great scene unveil'd. For, while the claims Of social life to different labours urge The active powers of man, with wisest care Hath Nature, on the multitude of minds, Impress'*! a various bias ; and to each Decreed its province in the common toil. To some she taught the fabric of the sphere, The changeful moon, the circuit of the stars, The golden zones of heaven. To some she gave To search the story of eternal thought; Of space, and time ; of fate's unbroken chain, And will's quick movement. Others, by the hand, She led o'er vales and mountains, to explore What healing virtue dwells in every vein Of herbs or trees. But some to nobler hopes Were destin'd : some within a finer mould She wrought, and temper'd with a purer flame. To these the Sire Omnipotent unfolds, In fuller aspects and with fairer lights, This picture of the world. Through every part They trace the lofty sketches of his hand: In earth or air, the meadow's flow'ry store, The moon's mild radiance, or the virgin's mien Dress'd in attractive smiles, they see pourtray'd VI PREFACE. (A far as mortal eyes the portrait scan) Those lineaments of beauty which delight The mind supreme." In these days, thanks to the general diffusion of knowledge, occasioned by improved modes of education, and, still more, by the numerous compendia which have been given to the world, even the illiterate experience a sense of shame, when discovered to be igno- rant of the elements of science; of those elements, which, when placed beneath the quick-glancing eye of un- taught genius, may be so rapidly ac- quired, so widely extended. To such; to those, the chief portion of whose time is necessarily devoted to pursuits of a less intellectual nature, but who yet pant for liberal information ; and to those, whose limited means will not allow them to encounter the expense of purchasing more voluminous pro- PREFACE. Vll ductious; these " Principles of the Sciences" will be found to constitute a vade-mecum of the most useful class. In addition to all that has been yet advanced, it must be remarked, that the inquisitive spirit of the age de- mands a constant supply of the result of learned inquiry. To meet this de- mand, is one of the aims of these Let- ters ; as, in their progress, they point out and record many of the latest dis- coveries in each branch of science ; and even notice, and place in a clear light, several interesting problems, the solution of which is yet pending. On these grounds, they will prove service- able to persons advanced in life and in knowledge ; not only in recalling to their memories facts and names, which, in the lapse of years, may have been forgotten, as it is well known, that, by the association of ideas, the most tri- PREFACE. fling circumstance is capable of repro- ducing a long train of dormant thoughts and reflections, subservient to useful purposes ; but also in directing their attention to recent improvements in science, and to the numerous disco- veries, which, of late years, have been effected in the secrets of nature and of art. It has been well observed, that "the seeds of knowledge must be sown, be- fore flowers can expand, and fruit can ripen and be gathered." Were any stimulant necessary to increase the existing thirst for knowledge, nothing stronger could be offered, than that " to know GOD is to worship him." So evident a position requires no greater proof, than that which is contained in the breasts of all his creatures. We know God in his works, which we study by the light of Revelation. Hap- PREFACE. IX py are we in this knowledge! how much happier than those heathens, who sought a Deity, and found him in his works also ! Yet shall we, because we possess this light, withdraw our eyes from the volume of science, which it but enables us to read the more dis- tinctly ? Forbid it, heaven ! The blue sky, the glorious sun, the pale but lus- trous moon, the glittering star, the fiery meteor, the blazing comet, wandering trackless in its orbit through innumera- ble worlds, and the whole beauteous form of the creation, were made for the glory of God, who made man with a capacity to comprehend the sublime arrangement, and a grateful soul to re- flect the praise due to its Author. Like the pleasures of the senses, or the delights of the imagination, science needs only to be tasted, to be sought after with avidity. How immense is X PREFACE. the difference between the enjoyment which is the bane, and that which is the food, of human intellect ! Let us then cherish in the soul that germ of true nobility, which shall one day shoot up to Heaven, and produce fruit worthy of HIM who placed it there. CONTENTS. Page. PREFACE iu LETTER I. Introductory 1 II. Theology, and the Christian Religion 6 III Mythology 15 IV. Astronomy and Astrology 21 V. Meteorology 34 VI. Geology and Mineralogy 41 VII. Physiology, Anatomy, &c. 54 VIII. Zoology 66 IX Botany 74 X. Geography 83 XI. History and Chronology 92 XII Logic 108 XIII. Ontology, or Metaphysics 112 XIV. Mathematics : Arithmetic, Algebra, Geome- try, and Trigonometry ... 117 XV Mechanics 125 XVI Pneumatics 131 XVII Hydrostatics and Hydraulics 136 XVIII Optics , 143 XIX. Phonics, or Acoustics, and Music 157 XX Chemistry 163 XXI Electricity and Galvanism 179 XXII. Navigation ; including Magnetism, and Ani- mal Magnetism 188 XXIII. Literature and the Fine Arts ; including Rhetoric, Poetry, Painting, and Sculpture 199 XXIV. Concluding Remarks 208 PRINCIPLES OF THE SCIENCES. LETTER I. INTRODUCTORY. MY DEAR FRANCIS, THE time has arrived, in which it will be emi- nently your advantage to obtain a view, in their first principles, of the several sciences, the possession of which constitutes one of the grand distinctions between bar- barian and civilized man. As the love of power is na- tural to our species, the great axiom of Lord Bacon, that" KNOW LEDGE is POWER", cannot be too frequently impressed upon the youthful mind, as a stimulant to in- dustry in the pursuit of learning. In that pursuit, however, there is a nobler object than power ; there is a lofty intellectual superiority, which not only raises us above the level of common mortals, but, in the words of the poet, enables us " To look through nature up to nature's God," 2 INTRODUCTORY. It is well observed, by an excellent writer of the pre- sent day, that, " the value of any science is to be estima- ted according to its tendency to promote improvement, either in private virtue, or in those qualities which ren- der man extensively useful in society. Some objects of pursuit have a secondary utility ; in furnishing ratio- nal amusement, which, relieving the mind at intervals from the fatigue of serious occupation, invigorates and prepares it for fresh exertion. It is the perfection of any science to unite these advantages, to promote the advancement of public and private virtue, and to supply such a degree of amusement, as to supersede the neces- sity of recurring to frivolous pursuits for the sake of re- laxation." You perceive, my dear boy, how useful, how important, how delightful, is the acquisition of know- ledge!' Desirous of giving you all the as istance in my power, and confidently relying on your attention to the remarks which I shall offer, I have determined occasionally to occupy a few hours, in attempting to unfold the princi- ples of science to your youthful apprehension. For this purpose, I shall avail myself of the epistolary form, agreeably to the following synoptical view. The knowledge to which it behoves mail to turn his " That knowledge only is of value, which exalts the virtue, multiplies the comforts, soothes the sorrow, and improves the general felicity, of human intercourse ; which accompanies the possessor in every condition, and through all the vicissitudes, of mortality ; which exhilarates and amends society ; which solaces and animates the retirement of domestic life." Wakefidd. INTRODUCTORY. 6 earliest attention, and tbat to which man, in his uncivi- lized state, does turn his attention, is the science of na- ture, or the study of the works of the Creator. That study will direct his view to the origin of those works, and suggest the existence of a first and universal cause. This is the beginning of natural religion, which ends in the full conviction of, first, the existence of God ; next, of his power ; and lastly, of his goodness. Hence, my dear child, the object that claims your first consider- ation is your eternal Creator j to the knowledge of whom all natural science must lead; in whose service, either di- rectly, in acts of worship, or indirectly, in offices of love to your fellow creatures, all the sublime faculties he has bestowed must be employed. This service is Religion, in the genuine sense of the word ; and, as " to know is to adore,"! shall naturally lead you, through the knowledge of God and his attributes, to the forms of worship which he, in compassion to the imperfections of his creatures, has ordained. Thus imbued with the knowledge of their Author, your mind will be prepared to admit the most evident princi- ples of natural science, in which Astronomy, as leading us more nearly to Him who framed the heavens, is enti- tled to the earliest notice. Next, what more wonderful can yen behold around yon, than the structure and beauty of the human body ! what study can impress the mind with emotions of awe and gratitude, more than that which demonstrates your pre-eminence over the beasts of the field ; especially when we reflect, that the mere ma- terial form of man is only the agent of his intellectual powers. From man, the transition is easy to the contem- 4* INTRODUCTORY. plation of the animal world, of which he is justly re- garded as the head. Zoology, or the natural history of animals, will embrace this point ; and Botany, or the science of plants ; and Mineralogy, or the doctrine of minerals ; may be considered as forming the second and third divisions on the scale of nature. A question occurs to me, as likely to proceed from the examination of the works of nature ; and it is not impro- bable that you will inquire, " wherefore these things are ?" " For the use of man, and the glory of God." The an- swer is connected with the inquiry, whether the end have 5! ways answered to the means? Look into the History of Man; its friendly light will enable you to view the in- structive events of past ages, and to collect wisdom from the conduct of others. For the right understanding of History, the study of Geography is necessary, and should precede it ; and, for its due arrangement and retention, Chronology, or the science of Time, is indispensible. Connected, these two sciences serve as land-marks, or clues, to guide the student of history through the labyrinth of events of which it is composed. They have, indeed, been emphatically termed, the eyes of history. These are the studies which belong to a Christian, and a lover of his species. I shall next consider those which regard your future commerce with men ; in which I shall begin with the principles of artificial science : such is Logic, correctly defined the right use of reason, because it is the art of employing the faculties of the mind, in the discovery of truth. The next, in usefulness, to Logic, and which forms the best exemplification of that science, is the Mathematics : under this head, I shall en- INTRODUCTORY. 5 deavour to make you sensible of its utility, and to define its principal branches. Chemistry, the science of sepa- ration and combination, by which we ascertain the prin- ciples of which bodies are composed ; Electricity, one of the principal agents of nature j and Magnetism, or the powers of the load-stone ; will follow, as deductions from the united principles of natural and artificial science. Navigation, or the art of traversing the ocean in ships, will be included in this division. I shall conclude with some observations on Taste, and on the Fine Arts. The principles of the former are in- planted in every mind distinguished by good sense ; and the latter hold a pleasing influence over the imagination, and furnish a constant variety of amusement and plea- sure. Thus, my dear boy, according to the arrangement which I have proposed, my series of letters will enable you to form a general estimate of the value and impor- tance of Theology, Mythology, Astronomy, Meteorology, Geology, Mineralogy, Physiology, Zoology, and Bota- ny j of Geography, History, and Chronology ; of Logic, Ontology, Mathematics, Mechanics, Magnetism, Navi- gation, Optics, Phonics, Chemistry, Hydrostatics, Pneumatics, Electricity, and Galvanism ; and of Lite- rature, Music, Painting, and Poetry ; with a variety of concomitant and minor subjects. If you happen to be unacquainted with any of the terms, which, in this preliminary sketch, I have found it ne- cessary to employ, do not be discouraged, as I shall ex- plain them in order, in the course of my succeeding let- tris. Youi's, &c. B 2 LETTER II. THEOLOGY, AND THE CHRISTIAN RELIGION. " The Deity, his being and attributes, justly claim the first place ; there is no other object of attention so excellent, none so impor- tant to the highest purposes of our existence and our happiness." Abernethy* I SHALL commence, my dear pupil, with Theology, or the science of Divinity; a science which instructs us in the knowledge of God and divine things ; which has God and the tilings that he has revealed for its objects; and which shews us what we are to believe of the Supreme Being, and the manner in which he ought to be wor- shipped. The antients had a three-fold Theology : first, the mythic, or fabulous, which flourished amongst the poets, and was chiefly employed in the Theogony or Genealogy of the gods ; secondly, the political, which was embraced, chiefly, by the politicians, priests, and people, as most suitable and expedient to the safety, quiet, and prosperity, of the state; and, thirdly, the physical or natural, which } THEOLOGY. 7 was cultivated, principally by the philosophers, as most agreeable to Nature and Reason. The physical theology acknowledged only one Supreme God, but admitted of the mediatorial agency of daemons. It is now more usual to consider theology as divided into two branches ; natural, and revealed or supernatural. Natural theology is the knowledge which we have of God, from his works, by the light of Nature and Reason; and supernatural theology is that which we learn from revelation. Again, positive, or as some term it expositive, theology is the knowledge of the scriptures and their signification, agree- ably to the opinions of the fathers and councils. Moral theology teaches us the divine laws, relating to our manners and conduct ; and scholastic or school theology proceeds by reasoning, or derives the knowledge of seve- ral divine things from certain established principles of faith. The existence of a Supreme God has been generally acknowledged, in all ages of the world. To prove that existence, two methods have been adopted. Some reason from the necessity of an infinite and eternal being : others, by tracing effects backward to their causes, demonstrate the existence of a first Great Mover. The latter is at once the easier and the safer method ; for if, from the existence of a watch, an organ, or a book, we may safely infer the existence of a watch-maker, an organ-builder, or a printer, more freely may we infer the existence of a Creator from the universe of animate and intelligent creatures. To form an idea of God, it is usual to accu- mulate every human excellence.to reject all imperfection, and to clothe the whole with the incommunicable attri- 8 THEOLOGY. butes of eternity and immensity. The Almigbty, how- ever, has been pleased to reveal his own character, in the sacred writings. Natural and moral appear to be the usual distinctions of the divine perfections. Amongst the former, are wisdom and power, of which we possess clear ideas : of the spirituality of the divine nature, we endeavour to obtain some notion, by forming the abstract idea of pure intelligence; but, when we endeavour to combine the attributes of wisdom, power, and unembodied spirit, with immensity and eternity, they instantly swell beyond the grasp of human imagination. A being un- limited by time or space, can be comprehended only by his own infinite mind. All questions and discussions on this subject must, therefore, terminate in disappointment. Of the moral perfections of Deity, justice, goodness, mercy, and truth, we seem to have tolerably distinct ideas, because we know what those terms mean, as ap- plied to ourselves; but there is a considerable difficulty in harmonizing them, so as to form an idea of the Great Supreme. Indeed, it is only in the divine scheme of hu- man redemption, that ''mercy and truth meet together," that " righteousness and peace salute each other." Taking the scriptures for your guide, and deducing from them the outline of a system of theology, it is necessary that you should be acquainted with the evidences of their authority, aud with the truth of Christianity. Amongst the numerous evidences of Uie truth of revelation, the following are the most obvious and striking: first, from the authenticity of the books of the New Testament; secondly, the character of our Lord and Saviour; thirdly, the prophecies, of which he was the subject, as well as THEOLOGY. 9 those which he delivered; fourthly, his miracles ; fifthly, the sublime morality of his precepts ; and sixthly, the rapid and extensive propagation of his religion, under circumstances the most hostile to its advancement. You will easily perceive, my dear pupil, that each of these heads would admit of a distinct treatise, of con- siderable length. The study of such treatises must, ne- cessarily, be reserved for a distant period. At present, I shall content myself with such a rapid sketch as will suffice to give yon a general view of the subject. Re- specting the authenticity of the books of the new testa- ment, we have the authority of many contemporary writers, who were not Christians, to the principal facts which they record. Josephus, the celebrated Jewish historian, who supports the authority of the old testa- ment, mentions also the existence and wonderful charac- ter of Christ, and his forerunner John the Baptist. Ta- citus, Suetonius, Pliny the younger, and many other heathen writers, also allude to the history of Jesus and his disciples. As Christianity is founded upon Judaism, and as our Saviour and his apostles uniformly appealed to Moses and the prophets, as divine authority, it may be proper to add, that the antiquity and authenticity of the old testament are supported by a variety of collateral evidences and corroborative facts. The books of Moses and the prophets are repeatedly quoted, and referred to, by numerous pagan writers ; and the history of the creation, the deluge, the destruction of Sodom, the conquest of Canaan, and the captivities of the Jews, with the destruction and dispersion of that people, are con- firmed by historical evidence, altogether irresistible; 10 THEOLOGY. The character of onr Saviour, as represented in the plain and energetic narratives of the Evangelists,* is marked by qualities the most extraordinary and the most transcendent. This character they were no less unable than unwilling to invent. The presumption, therefore, is fair, that they wrote from the immediate impressions of reality. They saw, they conversed with, the Saviour of mankind, and heard, from his sacred lips, the words of eternal liffc. Be it remembered, too, that the moral cha- racter of these writers has never been impeached ; and that the books, handed down to us as theirs, are the same that were formerly so esteemed, is ascertained by their having been publicly read, in all ages of the church, translated into most antient as well as modern languages, and quoted by a host of writers, Jewish, Christian, and heathen, from their earliest promulgation to our own days. I have already said, that Moses and the prophets have been referred to, and supported, by writers of all ages. With respect to prophecy, it is admitted that a man of accurate observation, and acute penetration, may forctel events which appear, at the time, improbable ; and also, that many vague and bold predictions may be so matched with subsequent circumstances, as to have very much the appearance of divine inspiration. The prophecies of scripture, however, constitute a regular chain, almost " How infinitely more interesting the simple narrative of the personal occurrences in the life of our Lord, as given in the four Evangelists, tlian the most eloquent and elaborate history of any empire or distinguished personage that ever existed on earth. THEOLOGY. 11 from the beginning to the end of time. Many of them relate to events so distant, and so improbable, that no human foresight could ever reach; some relate to dates and circumstances which required the most exact ac- complishment; and others are fulfilling, to the present time, and before our own eyes. The weight of evidence, in favour of their truth, is daily accumulating, and gathering strength as it accumulates. The events, which happened about thirty years after the ascension of our Saviour, completely verified the prophecies which he had delivered. The details of Josephus *re confirmed by Tacitus, Philostratus, and Dion Cassius. Though probably they were unacquainted with the works of the Jewish historian, they all corroborate his account, and unite to illustrate the prophecies of Christ. A miracle may be defined, as " a sensible change in the order of nature ;" or, as " an evident deviation from her known laws." Miracles, therefore, can be produced only by the Creator and Ruler of the world, and mnst give divine sanction to the cause in favour of which they may be performed. It has been well remarked, that the miracles of our Saviour, with which, from your new testament, you are well acquainted, " were so frequent, that they could not be the effects of chance; so public, that they could not be the contrivance of fraud and im- posture ; so instantaneous, that they could not result from any preconcerted scheme ; and so beneficial, in their im- mediate consequences, and so conducive to propagate the salutary truths he taught, that they could not pro- ceed from the agency of evil spirits. They must, there- fore, have been effected by the interposition of that Di- 12 THEOLOGY. vine Power to which Christ himself attributed them." The moral precepts of our Saviour are justly consi- dered as forming the most complete, the most intelligi- ble, and the most useful, system of Ethics, or moral phi- losophy. The powerful motives, by which our Lord en- forces the practice of his laws, are correspondent to the expectations of accountable beings ; and the internal evidence of the system proves that it came from God, because it is, in every respect, consistent with his wis- dom and goodness. The Christian code combines the noblest precepts of antient philosophy with others which are peculiarly and eminently its own. To this code " we are indebted for those rules of conduct which enjoin the sacrifice of self-interest, selfish pleasures, and vain-glory. By it alone we are taught, in the most ex- plicit language, and in the most authoritative manner, to check all violent passions, and to cultivate the mild and pure affections of the heart ; to forgive injuries ; to love our enemies ; to resist the first impulse of evil de- sires; to practise humility and universal benevolence ; and to prefer the joys of heaven to the pleasures and occupations of the world. Advancing to a degree of improvement far beyond the lessons of heathen morality, far beyond what was ever taught under the porches of Athens, or in the groves of the Academy ; we are in- structed to entertain the most awful veneration for the Deity, aud to express the most lively gratitude for his mercies ; we are supported by the firmest reliance on his grace ; and we are invited upon all occasions to re- sort, in earnest and fervent prayers, to his power, mercy, and goodness, for the supply of our numerous wants, for THEOLOGY. 13 the pardon of our sins, for security in the midst of dan- gers, and for support in the hour of death." * The rapid and extensive propagation of Christianity, at its first promulgation, under circumstances the most hostile to its success, is, perhaps, the most evident proof that can be adduced of its truth, and of its divine origin. Whether we view the qualifications of its preachers, as, in general, men of mean education, ignoble birth, rude manners, and without enterprize or ambition; whether we regard the scene of their labours, as the seats of learning and eloquence, and at a period when the world was enlightened with science ; or, whether we con- template the difficulties which they had to encounter from the unflattering nature of their doctrine and the sternness of their precepts, frem the interest and super- stition of priests, and from the rage and jealousy of princes; it cannot appear otherwise than miraculous, that, in the course of a few years, Judea, Greece, Italy, and nearly the whole of the known world, should be filled with the doctrines and adherents of the Christian religion. The rapid success of Mahomet has been absurdly men* * " Remember for what purpose you were born : through the whole of life, look at its end ; and consider, when that comes, in what will you put your trust ? Not in the bubble of worldly vanity; it will be broken : not in worldly pleasures; they will be gtme : not in great connexions ; they cannot serve you : not in wealth ; you cannot carry it with you : not in rank; in the grave there is no distinction : not in the recollection of a life spent in a giddy conformity to the silly fashions of a thought- less and wicked world; but in that of a life spent soberly, right- eously, and godly, ln "'' s present world." Bp. Watson. c II THEOLOGY. tioned as a parallel; but that impostor, although he adapted his system to the corrupt and carnal appetites of mankind, was under the necessity of inculcating his doctrines by the sword. Here yon may, probably, expect me to add an ex- planation of various doctrinal points of the Christian faith : but, in this place, I deem it not requisite ; as you will, necessarily, in the course of education, receive this information from other sources. To such informa- tion attend with patience and humility : then form your habits on the principles of Christian truth, fortitude, and benevolence; that you may at length exclaim, in the words of the great apostle of the gentiles, " The time of my departure is near : I have maintained the good contest, I have finished my course, I have kept the faith. Henceforth there is laid up for me a crown of glory, with which the Lord, the righteous judge, will reward me at that day : and not me only, but all those, like- wise, who love the prospect of his appearing." At a future period, yon may derive much information and great advantage on the subjects of this letter, from the perusal of such works as the following: Dr. Paley' Natural Theology, and his Evidences of the Truth of the Christian Religion ; Addison's, Jenyns's, Beattie's, and Porteus's, Evidences of Christianity ; the Bishop of Lincoln's Elements of Christian Theology, Lord Lyttletou on the Conversion and Apostleship of St. Paul; Doildridge's Three Sermons on the Evidences; Bishop Watson's Apology ;. &c. I am, &c. LETTER III. MYTHOLOGY. SOME knowledge of mythology, or the history of the fabulous gods and heroes of antiquity, is necessary, to enable yon properly to understand the antient authors, particularly the poets, whose works abound with descrip- tions of, and allusions to, the mysteries and allegories of pagan superstition. So far as can be ascertained, many of the antient nations adored the sovereign lord of the universe under the name of Fate, or Destiny ; to which, ideal being, Jupiter himself, and all the inferior deities, every animated existence, the heavens, the earth, and the whole frame of nature, were subservient, and nothing could reverse its decrees. The early sages, charmed with the study of the heavenly bodies, which they had the opportunity of contemplating under a cloudless sky, soon considered them as the residence of divine intelli- gence, and worshipped them, with the elements, as dei- ties ; attributing to them the government of the universe, instead of considering them as the unconscious instru- ments of the divine will. The historians of antiquity im- mortalized the heroes of their narratives, by translating them to the skies, and giving them the names of the heavenly bodies. The traditions, also, of the first ages 16 MYTHOLOGY. were magnified into the history of the gods and goddes- ses. Noah and his children have been considered as the prototypes of Saturn and his sons ; and much of the early scripture history may be traced in the fables of Greek mythology. Many of the incidents of the life of Moses are to be found in the history of Bacchus. Ham is supposed to have been converted into Jupiter; Tnbal Cain into Vulcan ; and Jubal into Apollo, the god of music and of song. Perhaps it may be considered, that mythology was first reduced to a system by the Hindoos ; and afterwards by the priests of Egypt, who were the depositaries of eastern learning. Some of the Hindoo fables relate to the creation, to the general de- luge, and to Vishnu, or Brahma; who, they tell us, was miraculously born, in Sweta, or the White Island, sup- posed to be Britain. At the age of thirty, he is said to have first promulgated his doctrines, which have since spread all over India and a great part of Asia. He inculcated the belief, that the souls of men, after death, migrated into the bodies of other animals ; a doctrine to which we may trace the origin of idols, in the shape of beasts, monsters, &c. According to the Chinese mythology, Fo, or Fohi, (supposed to be the Noah of scripture) the founder of their empire, was also born in a supernatural manner, and was inspired, by heaven, with every description of knowledge. The Chinese Confucius has, likewise, the credit of a supernatural birth ; and he is said to have reasoned profoundly, from the earliest hours of bis infant tate. The Egyptians founded their mythology on theadvcn- MYTHOLOGY. 17 hires and exploits of Osiris, Oris, Typhon, and Isis, and the transformation of the gods into various animals. Ac- cordingly, Jupiter Ammon was worshipped under the semblance of a ram ; Apis, as a cow ; Osiris, as a bull ; Pan, as a goat; Thoth, or Mercury, as an ibis, &c. Thoth, or Mercury Trismegistus, was also regarded as the inventor of letters, and of every branch of science and philosophy. There appears to be something philosophical in the belief of the antient Persians, that the world had been repeatedly destroyed, and repeopled by creatures of different conformation and habits, who had been succes- sively annihilated for disobedience to the Supreme Be- ing. Their mythology originated in the heroic actions of Tahmarus, one of their kings, who is said to have at- tacked and vanquished the daemon which opposed the happiness of the country. Rostan, another Persian chief, whose character was similar to that of Hercules, is amongst the Persian deities. It is evident that the mythology of Greece was partly founded upon that of Egypt. Their twelve superior deities were, Jupiter, or the Thunderer, the son of Sa- turn and Rhea ; Juno, the sister and wife of Jupiter, famous for her jealousy, her implacable revenge, and her quarrels with her husband ; Minerva, the goddess of wisdom, and the patroness of the arts, who sprang from the head of Jupiter; Neptune, the ruler of the seas; Venus, the goddess of love and beauty, said to have sprung from the froth of the sea ; Mars, the god of war, who is represented as a warrior, riding .in a chariot, drawn -by hor*cs,and driven by a fury; Vulcan, the god 18 MYTHOLOGY. of fire, and of workers in metal*; Vesta, the daughter of Saturn, and the goddess of fire ; Apollo, the son of Ju- piter and Latona, who presided over the fine arts, and was skilled in the practice of music; Diana, the sister of Apollo, goddess of chastity, of the chace, and of the woods; Ceres, the goddess of fertility, who is said to have taught the art of tilling the earth ; and Mercury, the messenger of the gods, and the inventor of letters, and the arts and sciences. Jupiter, who \vasthe Vishnu of the Hindoos, the Osiris of the Egyptians, the Belus of Babylon, and the Ammon of /Ethiopia, after deliver- ing his father from the tyranny of the Titans, became master of the universe, which he divided with his bro- thers, reserving heaven for himself, giving the sea to Neptune, and the infernal regions to Pluto. Polyphe- mus, one of the Cyclops, who devoured all who fell into his hands, was the child of Neptnne. Pluto, the son of Saturn and Ops, was considered as the ruler of the dead. Saturn, the father of Jupiter, Neptune, Pluto, &c. was considered as the son of Heaven and Earth. Many of these deities were worshipped under a plurality of names. Thus, Diana was adored as Luna, in heaven, and as Hecatl, in the infernal regions. Bacchus was the god of wine; Somnus, of sleep; Morpheus, of dreams ; Erebus, of darkness ; Nox, of night ; Plutus, of riches ; and Moniu*, of folly. Hebe was the god- dess of yonth. The nine muses were daughters of Jupi- ter, by Mnemosyne, or Memory, and the three graces, by Eurynome. Charon was the personage who conducted ' The workmen under Vulcan were the Cyclopes, or Cyclops. MYTHOLOGY. 19 the souls of the dead across the Styx, where they drank the waters of Lethe. Minos, Rhadamanthus, and ./Ecus, were the inexorable judges of the dead ; and Tisiphone, Alecto, and Megara, were the jailers and executioners of Tartarus, or Hell, armed with snakes and blazing torches. The three Fates were, Clotho, who held the distaff; Lachesis, who spun the thread of destiny ; and Atropos, who, with her scissars, cut it asunder, Amphi- trite, the wife of Neptune, was the mother of the Ne- reids. Hercules, or Alcides, the great hero of antiquity, was the son of Jupiter and Alcmaena. He performed twelve celebrated labours, amongst which were the killing of the Nemaean lion, and the cleansing of the Angzean stable of three thousand oxen, which had not been cleansed for thirty years. It now remains for me to say a few words on the my- thology of our Scandinavian * ancestors, of whom Odin, or Woden, was the chief divinity. He is supposed to have been a great military chief from the east ; and he is represented as the god of battles, and as killing thou- sands at a blow. His palace, called Valhalla, was situa- ted in a city, named Midgard, where the souls of heroes who bravely fell in battle enjoyed supreme felicityf . Of * " Scandinavia : A name given by the antients to that tract of territory which contains the modern kingdoms of Norway, Sweden, Denmark, Lapland, Finland, &c. supposed by them to be an island Plin. 4, c. 13." Lempriere. J* " The heroes," says the Edda, " who are received into the palace of Odin, have every day the pleasure of arming themselves, of passing in review, of ranging themselves in 20 MYTHOLOGY. Sleipner, the horse of Odin, many wonderful stories are related. From the Scandinavian " Hela" is derived our word, hell; and from " Sinna," the wife of Loke, the evil genins, comes our word, sin. The Voluspa, or book of prophecies, and the Edda, contain the history of the world, the mysteries of religion, &c. according to the Scandinavian belief. The mythology of the northern nations, which is exceedingly rude, is partly made up of giants, dwarfs, and fairies. You will find many curious particulars on the subject, in Turner's History of the Anglo-Saxons, Mallet's Northern Antiquities, &c. ; and, for farther elucidation of the Grecian, Roman, and Eastern, mythology, I refer yon to the works of the learn- ed Jacob Bryant, and of Sir William Jones ; to Spence's Polymetis, Lempriere's Classical Dictionary, &c. I am, &c. order of battle, and of cutting one another in pieces ; but, so soon as the hour of repast approaches, they return on horseback all safe and sound, to the hall of Odin, and fall to eating and drink- ing.' Though the number of them cannot be counted, the flesh of the boar Scrimner is sufficient for them all ; every day it is served up at table, and every day it is renewed again entire ; their beverage is beer and mead ; one single goat, whose milk is excellent mead, furnishes enough of that liquor to intoxicate all the heroes: their cups are the skulls of enemies they have slain. Odin alone, who sits at a table by himself, drinks wine. " A crowd of virgins wait upon the heroes at table, and fill their cups as fast as they empty them. Such was that happy state, the bare hope of which rendered all the inhabitants of the north of Europe intrepid, and which made them not only to defy, but even to seek, with ardour, the most cruel deaths." LETTER IV. ASTRONOMY AND ASTROLOGY. " The spacious firmament on high, With all the blue ethereal sky, And spangled heavens, a shining frame, Their great Original proclaim." Addison. ASTRONOMY, my dear Francis, is that science which treats of the heavenly bodies, and their phenomena. Some extend the term to the theory of the universe, with the primary laws of nature; but it is more properly considered as a mixed mathematical science, by which we become acquainted with the magnitudes and motions, distances, periods, and order, of the celestial luminaries. It is of all sciences the most sublime, the most beau- tiful, the most interesting, and the most admirably adapted to raise our minds, in admiration, to the great first Cause of all things. Well, indeed, might the poet exclaim, " An undevout astronomer is mad !" Some parts of the science are so useful to mankind, as to render the cultivation of it essentially necessary; as, by its assistance, geographers are enabled to ascertain the 22 ASTRONOMY true figure and size of the earth, and the situations aud extent of countries ; chronologists can compute the mea- sure of the year ; and navigators can determine the lon- gitude of places, and direct their courses through the trackless and stormy ocean with correctness and safety. Traces of this science arc discoverable amongst all nations ; but the foundation of a methodical system ap- pears to have been first laid by Hipparchus, about 147 years before the commencement of the Christian aera. That philosopher made many observations on the fixed stars, to which Ptolemy, who lived about 280 years after him, added several others. Astronomy, however, was much neglected, till the close of the thirteenth century, when Alphonsus, King of Castile, formed the most accu- rate tables that had been known. In the sixteenth cen- tury, by the system of Copernicus, after wards perfected by Kepler and Galileo, the science made great progress. Most of the antient philosophers had adopted the notion of the immobility of the earth. This, which was called the Ptolemaic system, from Ptolemy, an Egyptian as- tronomer of the second century, exhibits the earth in the centre of the universe, surrounded by the Moon, Mercu- ry, Venus, the Sun, Mars, Jupiter, and Saturn ; beyond which were the fixed stars. This system, abounding with innumerable difficulties, was superseded by that of Nicholas Copernicus, whose treatise on the subject was published in the year 1543. Copernicu's admitted the diurnal motion of the earth on her own axis, and also her motion in the ecliptic around the sun. This system, regarded as heretical, encountered much opposition : Galileo was thrown into the prison of the Inquisition for AND ASTROLOGY. 23 supporting it; nor could he regain his liberty without affecting to retract his opinion. Tycho Brahe, though a man of high talent, was not convinced of its correct- ness. He projected another system, in which he placed the earth, and made the Sun, Moon, and fixed stars, re- volve around her. Mercury, Venus, Mars, Jupiter, and Saturn, he placed in orbits round the Sun. The Coper- nican system, however, kept its ground ; and, having been fully established, by the observations of Sir Isaac Newton and other modern philosophers, it is now uni- versally received. The certainty of the principles upon which it rests has been finally determined, and is more particularly proved by the calculation of eclipses ; the astronomer being able to determine, not only that the Sun or the Moon will be darkened at some future time, but at what particular minute, and to what exact extent, such obscuration will happen. The Sun, which forms the centre of our system, is supposed to revolve upon its own axis, in twenty-five days, fourteen hours, and eight minutes. This immense globe, the diameter of which is 883,217 miles, while that of the earth is only 7,964, is now generally believed to be habitable. The celebrated astronomer, Dr. Hers- chel, considers it to be surrounded by two strata, the in- ternal one forming an atmosphere for the inhabitants, and the external one consisting of luminous matter of a phosphoric nature, which diffuses light through the whole system. This external stratum, he supposes, has several openings in it, through which we perceive the body of the Sun, and which form the spots that are so frequently seen upon its surface. 24- ASTRONOMY The solar system, as I have already remarked, consists of the Sun, in the centre : of seven primary planets; Mercury g , Venus $ , the Earth , Mars $ , Jupiter %, Saturn Tj, and Herschel $: of four asteroids, or minor planets, Ceres, Pallas, Juno, and Vesta : of eighteen secondary planets, the Earth's Moon, Jupiter's four satellites, Saturn's seven, and six belonging to Herscliel. If yon will take the trouble of committing the following lines of Chatterton to your memory, you will, at all times, be able to recal to your mind a lucid idea of this arrangement : " The Sun revolving on his axis turns, And, with creative fire, intensely burns ; First, Mercury completes his transient year, Glowing, refulgent, with reflected glare ; Bright Venus occupies a wider way, The early harbinger of night and day ; More distant still our globe terraqueous turns, Nor chills intense, nor fiercely heated burns. Around her rolls the lunar orb of light, Trailing her silver glories through the night. Beyond our globe, the sanguine Mars displays A strong reflexion of primeval rays ; Next, belted Jupiter far distant gleams, Scarcely enlighten'd with the solar beams ; With four MiitixM receptacles of light, He towers majestic through the spacious height : But farther yet, the tardy Saturn lags, And seven attendant luminaries drags ; Invested with a double ring, his pace He circles through immensity of space." AND ASTROLOGY. 25 Of the planet Herschel, which Chatterton has not mentioned, a more modern poet thus speaks : " From Earth, how large, how strong, the Sun's bright ball ! But, seen from thence, how languid and how small ! When the keen north with all its fury blows, Congeals the floods, and forms the fleecy snows, "Tis heat intense to what can there be known ; Wanner our poles than is its burning zone ; Who there inhabit, must have different powers, Juices, and veins, and sense, and life, than our's. One moment's cold, like their's, would pierce the bone, Freeze the heart-blood, and turn us all to stone. Strange and amazing must the difference be, 'Twixt this dull planet and bright Mercury : Yet reason says, nor can we doubt at all, Millions of beings dwell on either ball, With constitutions fitted for the spot Where Providence, all-wise, has fix'd their lot." It is necessary further to remark, that all the planets move round the Sun from east to west, at different dis- tances. The paths or orbits in which they move are elliptical, but they approach very nearly to circles. Whilst revolving in their orbits, the planets also move round their own axes, with various degrees of velocity ; the former motion producing the change of summer and winter, the latter the variety of day and night, and there- fore called diurnal rotation. These, with several other useful points in the science, will be seen by inspecting the table, on the next page. ASTRONOMY c g o-<*5* o ^Is-i .2 S < 0^ f4j3 a a a g IAS* its ! J5 M 3i g ** &. 5 ^rill 2p i i "^* K _ -B ~_2 M e -o ro * G Q o js ;-M o-a ^ ^S -2 ooo e I?* * * .2-O jT i. C i S tT 1 1 f 8 2S 5 252|ott>geM' i 05 s i I-T o^ ^v 5 }. !*! o I ^-^ l .^ >| ^i f j- >j ^s-S ~ - ^CC^C'C-^-^CMC^ Ilfl'l o- 5* B= DP llill o 2 o oo;co o jtif 3 g j r '5 -e 3 S "oto ""rt-t "CO E**3eO ""* I !-! 5 Te* eoscsceo |tccOT*co -S^ frflsa g JC bOJ) 1! ,5 a H CC ^ ".C CO "O CO tO ^J O C^ e t-o E ^ e M g 0^ CM jo * > *f5 t> ;'**, K E ^ C -o | 2 -i CO J= J E ~f S ^ =*-!_ ^ ^S ,* "fs s^ 6 -. E.S| -ja llllll g ? s s= -5 iaas S~S^ S j;" q" 3 "? Ill O O gW S.2 g-u'g-el l8.II-g ^ Q -w ) ^E?^i^ -^^lllili 2 lls ggii|sss g 's e | | - i-sii.'fli e_ w.tJ 2 S P 3 CO CO t^CO S -3 J5 ' b ra - C^ ; o> jj O o _. 2 J3 ^^s-sg w m Ss-figoig^iE'pO c ^ S o-o|K B * B ^ 'S ^"5 fc , i ^ fe S S *, r r ^ t* r ' < ^ ^ r ' "' ~* -? 5? **"* 5? - 9 9 1 5 g.&.'S E AND ASTROLOGY. 27 \ The plane of u planet's orbit is an imaginary surface, which is included within the orbit, and is supposed to cut through solid bodies. Thus, a card, of a circular form, may represent the orbit Of the planet, and the surface of the card, the plane of the orbit. The planets are retained in their orbits by the united operation of what is termed the centripetal force, by which a body is attracted to the centre of gravity; and the centrifugal force, by which it endeavours to persevere in a straight line. These two powers, balancing each other, retain the planets in their orbits, and compel them to perform their respective revolutions. The point of the line, in which a planet approaches nearest to the Sun, is called the perihelion ; that, at which it is most remote, the ap- helion. Its mean distance is equal to half the sum of its greatest and least distances from the focus in which the Sun is placed. The Earth, you will remember, may be considered as turning round a point, which is within the body of the Sun ; and therefore it turns around the Sun. Our planet has two motions : one, a daily motion of 24 hours about its own axis the other, an annual motion of 365 days, and nearly 6 hours, around the Sun. The former causes the changes of night and day ; for the side of the earth, which is turned towards the Sun, is enlightened by his rays, and then it is day. When the same side is turned away from the Sun, gradual darkness succeeds, and night comes on. Of all the celestial objects, next the Sun, the Moon is most remarkable, and most interesting to us. Her mean distance from the Earth is about thirty of the Earth's 28 ASTRONOMY diameters, or 240,000 miles. Her surface is to that of the Earth, nearly, as 1 to iSf , and their respective quan- tities of matter are nearly as 1 to 39. When the Moon is in opposition to the Sun, the enlightened side is turned towards the Earth, and she appears full ; when in con- junction, her dark side is towards us, and she is invisi- ble. As she advances in her orbit, a small part of her body is seen, and we have a new Moon. We gradually perceive more of her, till she becomes what is termed a full Moon : the waning of the Moon occurs in a similar manner, and is accounted for upon the same principle. If the plane of the Moon's orbit coincided with the eclip- tic, whenever she might be in opposition to the Sun, she wonld be darkened by the shadow of the Earth, and an eclipse would take place ; but, as that is not the case, an eclipse of the Moon can occur, only when she may happen to cross the ecliptic, at the time of her opposition to the Sun. If these two events occur at the same mo- ment, the eclipse is total ; otherwise, only a part of the Moon comes within the shadow of the Earth, and the eclipse will be partial. An eclipse of the Sun is produ- ced by the Moon passing between the Sun and the Earth, and intercepting its light. There will be a great, and very interesting, though not total, eclipse of the Sun in the autumn of the year 1820, to which you will do well to direct your attention. Of the primary planets, Mercury and Venns, it is evident, revolve within the orbit of the Earth, for they are never seen in opposition to the Sun : that is, they never appear to rise from the horizon when the Sun is setting. From this circumstance, they arc called inferior AND ASTROLOGY. 29 planets. All the others, which are seen in opposition to the Sun, are termed superior. Of the latter, the orbit of Mars is nearest to the earth. Jupiter, when viewed by a telescope, appears to be surrounded by several belts, which vary in their appearance, and are supposed to be ranges of clouds in the atmosphere of the planet. Saturn, known by his pale and steady light, is more par- ticularly distinguished, by being surrounded by what appears, at first, to be one large luminous ring, which, as well as the seven moons by which he is attended, re- flects the Sun's light. There are, however, two concen- tric circles about this planet. The Georgian planet, Herschel or Uranus, was discovered by Dr. Herschel, in 1781. This planet, which is the most distant that we are acquainted with, and which cannot be seen without a telescope, is accompanied by six moons, all of which move around him, nearly at right angles with the planes of his orbit. The motion of these moons is retrograde. Of the Asteroids, so called from two Greek words, signifying" the form of stars," Ceres was discovered by Piazzi, of Palermo, on the 1st of January, 1801 ; Pal- las was discovered by Dr. Olbers, of Bremen, on the 28th of April, 1802 ; Juno was discovered, on the 1st of September, 1804, by Harding, of Bremen; and Vesta was discovered, on the' 29th of March, 1807, by Dr. Olbers. Comets, so named from the Greek xoitw, the hair, in Latin coma, are opaque bodies, which revolve around the Sun, with a surprising rapidity, and in very eccentric ellipses. Sir Isaac Newton computed the heat of the comet, which appeared in the year 1680, when nearest D2 30 ASTRONOMY the Sun, to be two thousand times hotter than red-hot iron, and that it must retain its heat until it should come round again ; even should its period be more than 20,000 years, and it is calculated to be only 575. The tail of this comet was at least 100,000,000 miles in length. The tail of that beautiful comet which illuminated our hemis- phere for some months, in the autumn of the year 1811, and of which, I think, yon must have some recollection, was 30,000,000 miles in length : it was composed of two diverging beams of bluish light. It is remarkable, that, of the numerous comets which have been seen within the limits of our system from 350 to 500, the periods of only three are known with any degree of certainty. The first of these appeared in the years 1531, 1607, and 1682, and is expected to appear every 75th year ; the second, which appeared in 1532 and 1661, was expected to return in 1789, and every 129th year afterwards ; and the third, which appeared in 1680, and is thought to perform its revolution in 575 years, is expected to return in the year 2225. In for- mer ages, it was considered probable, that the approach of a comet might endanger the existence of the earth; but, as it is evident, that the planes of the orbits of co- mets make very large angles with the ecliptic, the opini- on now is, that, except when at or near the nodes, they are too far north or south of the orbits of the planets to affect the system by any shock. It is even thought, that no material or even sensible alteration has ever been produced in our system by the action of a comet. Far beyond the limits of the solar system are the fixed stars, each of which is supposed to be the sun of ano- AND ASTROLOGY. 31 ther system, shining from its own light. They are so j-emote, that the best telescopes represent them only as points. To the naked eye, about six hundred only of these luminaries are visible; but, as the number increases in proportion to the power of the telescope employed, it is probable that an infinite number may exist, beyond the reach of all human discovery. Those parts of the heavens which are brighter than the rest, and of a whitish hue, appear, when examined, to be clusters of stars, which are called nebulae. The most remarkable of these, is the galaxy, or milky way. In the month of February, 1814, Dr. Herschel read a paper to the Royal Society, containing the results of thirty years' observations on these nebulae, or clusters of stars. His general opinion is, that the stars form independent sys- tems amongst themselves; and that, by a power, which he calls the clustering power, they have a constant dis- position to unite more closely. The fixed stars are divided into several classes, or or- ders, on account of their apparent difference of size. Those, which appear largest, are denominated stars of the first magnitude ; and so on, to the sixth, which are the smallest visible to the naked eye. Those, which cannot be seen without the assistance of telescopes, are called telescopic stars. The fixed stars are farther dis- tinguished by different constellations, or collections. The principal of these form the signs of the Zodiac; so called, because the antients classed them under the names of the animals from which they take their names. The names and signs of these characters are as follow : Aries, y ; Taurus, ; Gemini, n ; Cancer, B ; Lro t 32 ASTRONOMY SI ; and Virgo, ttji; northward of the equator: Libra, ^t ; Scorpio, ttj.; Sagittarius, / ; Capricornus, yf ; Aquarius, t^ ; and Pisces, X ; southward of the equa- tor. The zodiac, which is supposed to encircle the heavens, is about 16 degrees broad ; so that it takes in the orbits of all the planets. The other constellations are fifty-seven in number, twenty-nine on the north, and twenty-eight on the south, side of the zodiac. The an- nual progress of the sun, through the twelve signs, is thus well described by Chatterton : " On the earth's orbit, see the various signs Mark where the Sun, our year completing, shines : First, the bright Ram his languid ray improves ; Next, glaring wat'ry thro* the Bull, he moves ; The am'rous Tvrins admit his genial ray ; Now, burning, through the Crab he takes his way ; The Lion, flaming, bears the solar power ; The Virgin faints beneath the sultry shower. Now, the just Balance weighs his equal force; The slimy Serpent swelters in his course ; The sabled Archer clouds his languid face ; The Goal with tempests urges on his race ; Now in tlie Water his faint beams appear, And the cold Fishes end the circling year." You have doubtlessly heard of ASTROLOGY, which is the art, now almost exploded, of foretelling future events from the aspects, positions, and influence, of the heavenly bodies. To that branch of the art, which is termed Natural Astrology, belongs the predicting of natural effects ; as the changes of weather, winds, storms, hur- ricanes, thunders, floods, earthquakes, &c. It is pre- AND ASTROLOGY. 33 tended, that inundations may be foretold, and numerous phenomena explained, by the contemplation of the stars; and the diversity of seasons is accounted for, from the different situations and habitudes of the planets, their retrograde motions, the number of fixed stars in the constellations, and other circumstances connected with atmospheric influence. The nature of the seasons, and of the weather, will, however, be much better understood from the study of Meteorology, which will form the sub- ject of my next letter. The other branch of Astrology, termed judicial, is that which pretends to foretel moral events, or such as have a dependance on the free-will and agency of man. We derive our knowledge of the art from the Arabs; but it is generally believed to have been invented in Chal- dica, and thence transmitted to Egypt, Greece, and Rome. It is now practised only by impostors, who de- rive a subsistence from the infatuation of the ignorant. Bonnycastle's Introduction to Astronomy, Pliillips's Astronomical Lectures, Mrs. Bryan's Astronomy, Vince's Complete System of Astronomy, and many other works, will assist you in the study of that noble and delightful science*. I am, &c. * Mr. Walker's very ingenious " Eidauranian, or Grand Transparent Orrery," which is usually exhibited in London, during Lent, should be visited by every person desirous to have the stupendous facts of Astronomy clearly explained and im- pressed on his memory, which cannot be effected by description alone. LETTER V. METEOROLOGY. METEOROLOGY may be considered as comprising the phenomena of the air and atmosphere, including mete- ors formed therein ; as, clouds, rain, snow, hail, dew, thunder, winds, vapours, exhalations, &c. The atmos- phere, or fluid mass, which surrounds the earth, partakes of its annual and diurnal motions. It is essential, not only to the comfort and convenience of life, but also to life itself, and to the constitution of matter in general. As the earth, and the circumambient parts of the atmos- phere, revolve uniformly together about the common axis, the different parts of both have a centrifugal force, whose tendency is more considerable, and the centripe- tal less, as the parts are more remote from the axis. The figure, therefore, of the atmosphere is that of an oblate spheroid, since the parts corresponding to the equator are farther from the axis than those at the poles. Were the atmosphere, in every part, equally dense, it would not be more than about five miles in height; because the weight of the atmosphere being equal to a column of water, 33 feet high, and water being 800 times heavier than air, the height of the air would be 800 multiplied METEOROLOGY. 35 by 33, equal to 26,400 feet, or 5 miles ; but, as the den- sity of air diminishes according to its height, it is found to be about 45 miles high. The chief component parts of atmospheric air are oxygen gas, and nitrogen gas; in the proportion of about one part of the former and three of the latter. Combustion, and animal life, are supported by the agency of the oxygen gas. The atmos- phere possesses a refracting power, by which we per- ceive the light of the sun, some minutes before it actually rises above the horizon, and some minutes after it has sunk below it; and it has also a reflective power, by which objects are enlightened on all sides. Without the latter property, we should perceive only those parts of objects, on which the sun's rays directly fall. There are various instruments for the purpose of ma- king observations on the atmosphere. In the barometer, which ascertains the weight of the air, the mercury rises and falls, according to the different degrees of den- sity of the atmosphere ; but it seldom rises higher than 30| inches, and is rarely so low as 28. The thermome- ter marks the changes in the temperature of the atmos- phere, with respect to cold and heat. With us, thermo- meters are graduated upwards and downwards : 32 de- grees are called the freezing point ; and water boils at the height of 212. It is temperate, at 55 ; hot weather, at 70; and summer-heat, at 76 ; but it has been, in this country, as high as 95 ; and it is, in winter, sometimes 50 degrees below the freezing point, or 18 degrees below zero; 40 degrees below which, mercury freezes. There are also, hygrometers, to measure the moisture of the air ; electrometers, to measure the electricity of the air ; 35 METEOROLOGY. anemometers, to measure the velocity of the wind ; and rain gauges, to take the depth of rain. The wind is jnst perceptible, when if moves at the rate of two miles in an hour; it is brisk, at fifteen ; high, at thirty-five; a storm, at fifty; and a hurricane, at the rate of one hundred miles, in an hour. Water, by the agency of air, is raised in vaponr from the Earth, and forms clouds, which seldom reach the height of more than three or four miles. These clouds, when sufficiently condensed, either by the colder re- gions of the atmosphere, by electricity, or other causes, fall down in rain. The seven principal modifications of clouds, abstracted from Mr. Howard's valuable nomen- clature, yon will find in the note below*. When the up- 1. CIRRUS, a light lofty cloud, resembling a lock of hair, or a feather, or a plume of feathers : parallel, flexuous, or diverg- ing, fibres, unlimited in the direction of their increase. Its appearance is a general indication of wind, &c. 2. CIRRO-CUMULUS, a connected system of small roundish clouds, placed in close order or contact, one above another, fre- quently reaching, to appearance, into the azure sky. This modi- fication is attended by an increased temperature, and usually found to accord with a rising barometer. 3. CUMULUS, a cloud which increases from below in dense convex or conical heaps, and frequently evaporates after sun-set. The CUMULUS is both the accompaniment and prognostic of fair weather. 4. STRATUS or STRATUM, an extended level sheet of cloud, increasing from beneath a creeping mist, appearing either be- fore or after sun-rise. This is the lowest modification, being METEOROLOGY. 37 per regions of the air are .sufficiently cold, the vapours, af- ter condensation, are frozen, and the particles descend in the form of feathery flakes of snow ; and, when the drops of rain happen to be frozen in their descent, they fall as hail. The dew, which falls in a summers evening, is part of the vapour raised in the day by the heat of the snn, condensed with the evening's cold, and impelled to the earth by its own gravity. In the nights of winter, the dew becomes frozen in the form of rime or hoar frost. formed in contact with the earth or water. The nocturnal visits of the stratus have been always held a presage of fair weather. 5. CIRRO-STRATUS, horizontal or slightly inclined masses, attenuated towards a part or the whole of their circumference, bent downward, or undulated, separate or in groups, consisting of small clouds having these characters. This modification in- dicates a decrease in temperature, wind and rain, and generally accords with a sinking barometer. 6. CUMULO-STRATUS, the cirro-stratus blended with the cumulus, and either appearing intermixed with heaps of the lat- ter, or superadding a wide-spread structure to its base. The cumula-ntratus is most frequent during a mean or changeable state of the barometer, when the wind blows from the west, with occasional deviations from the north and south. 7- NIMBUS, or CUJIUJLO-CIRRO-STRATUS, the rain-cloud; a horizontal sheet, above which the cirrus spreads, while the cumulus enters it laterally, and from beneath. A nimbus is fre- quently accompanied by cirro-strati lying near it (particularly the nimbus of thunder-storms) and on a level with the densest part of the cloud : it moves with the wind, and from the rapidity of its passage affords but little to the rain-gage. 38 METEOROLOGY. Meteors, or luminous bodies, which appear suddenly, at uncertain times, and with more or less motion in the atmosphere, may be reduced under three classes; viz. fire-balls, falling or shooting stars, and ignesfatui. In tropical climates, fire-balls are more common and more stupendous than in our more temperate regions. Two of them, however, appeared in England in the year 1783 ; several within these few years ; and one, in particular, which was observed by a scientific friend of mine, in the county of Suffolk, in the morning of the 8th of Decem- ber, 1817. "I was looking at Mars," observes my friend, in a letter to me, upon the occasion, " whose situation is near to the star of the Bull's northern horn, when, midway between the Bull's horns, I perceived a fiery body, resembling a red-hot ball of iron, about four inches in diameter, which, having passed three or four degrees in a direction between the principal stars of Capella and Canis Minor, suddenly burst into a spherical body of white light, nearly as large as a full moon, of so great a lustre as scarcely to be borne by the eyes ; throwing out a tailabont three degrees in length, of a beautiful rose colour, tinged around the edges with blue. It thus passed on, without any apparent diminution, to- wards the principal stars in the head of Hydra, where it suddenly disappeared, as J believed, with an explosion, as I distinctly heard a rumbling noise about ten seconds afterwards, resembling that of cannon at a considerable distance. Its duration was, as closely as I can guess, about ten or twelve seconds, during which it traversed a pace of nearly 60 degrees ; and, from the time of its disappearing to the time of my hearing the explosion, it METEOROLOGY. 39 unist have been about two miles off, and about a mile and a half, or rather more, in height. The barometer was rapidly falling at the time, and fell, altogether, during the night, an inch and one tenth. The thermome- ter was 42 degrees. Within a quarter of an hour after- wards, the atmosphere became entirely obscured by clouds, and violent tempests of wind and rain succeeded, although the stars were previously visible and the zenith free from vapour. It is scarcely possible to give an ade- quate description of the varied splendour which charac- terized this extraordinary phenomenon : it cast a light around, equal to the noon-day sun. I could compare it to nothing so wellasthe burning of phosphoric oxygen gas ; and its effects upon the organs of sight were analogous." Respecting these extraordinary meteors, Lalande, a ce- lebrated French astronomer, observes as follows : "One frequently observes globes of fire in the atmosphere ; I have mentioned above 36 instances of that kind in the Connaissance des Terns for the year 1779. That obser- ved on the 8th of March, at seven in the evening, was attended with some singular circumstances. The hea- vens were serene, and a large globe, as big as the moon, was seen to proceed from the east and to advance with rapid motion towards the west : it was followed by a train of light, the rays of which, collected into bundles, were terminated by small globes. Six or seven sprigs of stars were seen on each side of the train. At the end of some seconds, there was an explosion like a lengthened clap of thunder, or the loud report of a cannon. This globe was at such a height, that it might be seen in places seven or eight leagues asunder. Had it been at a greater 40 METEOROLOGY. distance, it would have appeared like those falling stars which are observed so frequently. The common cause of these phenomena appears to be hydrogenous gas, set on fire, by some means, in the atmosphere." After the explosion of some of these bodies, aerolites, or meteoric stones, have been found, from a few ounces in weight, to several tons. They are generally circular, and invari- bly covered with a rough black crust, resembling the oxyde of iron. Specimens of many of them have been subjected to chemical analysis ; and, in all instances, the result was, that these stony bodies differed com- pletely from every other known stone ; that they all re- sembled each other ; and that, their component parts- silica, oxyde of iron, magnesia, oxyde of nickel, sulphur, and lime, were all the same. Various conjectures have been formed, as to the origin of these stones ; some, with La Place, supposing them to be projected by volcanoes in the moon; and others, with Sir William Hamilton, and Mr. King, suppose them to be concretions formed in the atmosphere. What is termed the shooting, or falling, star, is a com- mon phenomenon ; but, though frequently observed, the great distance and transient nature of these meteors have hitherto frustrated every attempt to ascertain their cause, which is probably electric. They generally pre- cede a change of wind. The ignis-fatvus, or will-with-a-wisp, is caused by some volatile vapour of the phosphoric kind. I am, &c. LETTER VI. GEOLOGY AND MINERALOGY. THE science of Geology, my dear Francis, relates to the structure and formation of the earth, the considera- tion of the materials of which it is composed, the cir- cumstances peculiar to its original formation, the differ- ent states under which it has existed, and the various changes which it has undergone. This naturally leads to Mineralogy, or the science of mines and metals, stones and gems, with strata, clay, sand, &c. Geology, or the study of the earth, is comparatively a modern science ; for, until towards the close of the eighteenth century, it was very little understood. This might probably arise from the circumstance, that Che- mistry and Mineralogy, upon which it greatly depends, had not, until then, been reduced to the systematic forms which they now present. It is particularly deserving of your notice, however, that every genuine discovery which has been made, tends more and more to prove the truth of the Mosaic account of the Creation, so far as relates to the globe which we inhabit. Of some of the various theories which have, at differ- ent times, been started, I will endeavour to give you a E 2 42 GEOLOGY slight sketch. Burnet was of opinion, that the whole earth, at first, consisted of a uu ifonn light crust, which covered the abyss of the sea ; and that this crust, hav- ing been broken for, or by, the general deluge, formed the mountains by its fragments. Woodward considered the deluge to have been occasioned by a momentary sus- pension of cohesion amongst the particles of mineral bodies ; that the whole mass of the globe was dissolved ; and that the soft paste became penetrated by shells. Winston conjectured that the earth was originally a co- met, which, at the period mentioned in the Mosaic ac- count, as that of the Creation of the world, had its orbit rendered nearly circular, and such an arrangement form- ed of its component parts as rendered it fit for the exist- ence of the vegetable and animal creation. At a subse- quent period, he supposed a comet to have passed so near to the earth as to involve it in the vapour forming its tail, which, when condensed, fell in torrents, and oc- casioned the deluge; the action of the comet, on the earth itself, having been sufficient to produce, at the same time, those irregularities of its surface, which form chains of mountains and the vast beds of the ocean. Leibnitz and Des Cartes amused their imagina- tions, by conceiving the world to be an extinguished sun, or a vitrified globe ; upon which the vapours, con- densing, formed seas, and deposited calcareous strata. Tlie Egyptian philosophers were of opinion, that, at the beginning, the water had covered the whole surface of the world. This belief they considered to be proved by the remains of organized beings, which were so fre- <|ueiitly found in the substance of the earth. The wa- AND MINERALOGY. 43 ters, they supposed, had retired to the interior cavities of the world, remaining in this great abyss, ready to issue out and produce the most extensive inundations; to one of which, it was supposed, some of their records re- ferred. The Chaldaeans and the Indians, like the Egyptians, believed the earth to be hollow ; that, in the early ages of its formation, a perpetual spring existed ; and, that there was a vast abyss, in the centre of the earth, for the reception of the water which remained after the consolidation of the crust of the globe. A- mongst the moderns, Bertrand has also supposed the earth to be hollow, and that it contains within it a load- stone, which is dragged from one pole to the other, by the attraction of comets ; so as, by changing its centre of gravity, to drown alternately the two hemispheres. Marschall conjectured that the fragments, of which the surface of the earth is composed, fell from hea- ven. Buffon thought that the mass of our globe, with the masses of the other planets, was struck off by the Sun, iu a liquified state, or by a comet, at the same instant. With Bicher appears to have originated the belief of the existence of a central fire, by the influence of which, numerous mineral principles are raised in a state of va- pour, through the different clefts of the earth, until they arrive near to its surface, where they enter into various combinations. Buffou, Gensanne, and various other mo- dern philosophers, have embraced this opinion ; which, with many, is, at this time, gaining ground. Besides those who consider an inherent or central fire as neces- sary to the formation and continuation of our globe, there are others who refer the particular modification of 44 GEOLOGY the form of its surface to the operation of subterraneous fires, acting partially, by the incalescence of pyrites and volcanic eruptions, with accompanying earth-quakes. Some of the moderns, believing every thing to have been originally fluid, consider that this universal fluid gave existence to animals of the simplest kind ; that, in pro- cess of time, the races of these animals became compli- cated, and, dying, supplied calcareons earth, or lime ; that aluminous earth, or clay, was produced by the de- cay of vegetables ; that, by a final analysis, these two earths were re-dissolved into silex ; and hence, that the more antient mountains are siliceous. Demaillet, still more fanciful, conceived the globe to have been covered with water, for many thousands of years ; that the water gradually retired ; that all the terrestrial animals were, originally, inhabitants of the sea ; and that man himself commenced his career as a fish ! The celebrated astro- nomer, Kepler, believed the globe to be possessed of living faculties, and a circulating vital fluid ; that all its particles are alive, and possess instinct and volition, whence arise their attraction and repulsion; that the mountains constitute the lungs of this huge animal ; that mineral veins are abscesses ; and that metals are the produce of disease, decay, and rottenness. Sir Richard Phillips, who has distinguished himself by writing against the Newtonian system of philosophy, in the Monthly Magazine, attributes all the changes of the earth's surface, and the successive alrtila of land and ma- line remains, to the progress of the perihelion point round the ecliptic. That point, according to Sir Rich- ard's theory, revolves round the ecliptic in 20,900 years; AND MINERALOGY. 45 consequently, the extremes of nortli and south declina- tion take place every 10,450 years; and, during the in- termediate periods, the seas are encroaching and retreat- ing, as the declination increases or diminishes. A cen- tury ago, Dr. Halley and others imagined the hypothesis of a metallic or magnetic kernel in our globe ; and, in the course of the year 1818, Mr. Stheinhauser, a German philosopher, announced, in the Literary Gazette of Halle, a similar theory. To explain the variation of the magnetic needle (respecting which I shall offer you some information in a future letter) he supposes that, in the interior of our globe, at the depth of about 170 miles, there exists another small body, which performs, around the centre of the earth, a revolution from west to east, in a period of 440 years. This small globe, endued with a magnetic attraction, would be the cause of the varia- tion of the magnetic needle. The calculations of this gentleman, to say nothing of his hypothesis, appear to be sanctioned by experience. He had predicted, in the year 1805, that the needle would be, for a time, station- ary ; and that, subsequently, it would retrograde towards the east. This seems to be taking place. Mr. Sthein- hauser has given to his subterraneous globe, the name of Pluto. Mariners, he says, will have no more occasion to observe the celestial bodies to guide them at sea : it will be sufficient for them to understand the motions of Pluto, by observation of the magnetic variation. Some- what similar to this theory, is the notion conceived by an American captain, of the name of Symmes. He de- clares the earth to be hollow, and habitable within ; that it contains a number of solid concentric spheres, one 46 GEOLOGY within the other; and that it is open at the poles, twelve or sixteen degrees. This person, in the month of April, 1818, naming Dr. S. L. Mitchill, Sir Humphry Davy, and M. Alexander de Humboldt, as his protectors, advertised for one hundred brave companions, well equipped, to start from Siberia, in the fall season, with rein-deer and sledges, on the ice of the frozen sea, to ex- plore the hollow. He engaged, that they would find warm and rich land, stocked with thrifty vegetables, and brute animals, if not men, on reaching one degree northward of latitude 82. These, my dear pupil, are some only of the numerous theories relative to the origin and nature of our earth. The writings of M. Pallas, Dr. Hutton, De Luc, Patrin, Kirwan, De la Metherie, and others, will, at a future period, demand your attention. At present, I shall only add, that the theory of Werner is, at. this time, the most prevalent. According to this philosopher, all the su- perficial parts of the globe were once in a state of aqueous solution, from which the materials were, at first, separated by chemical deposition, in a crystalline state, and formed a thick mass of granite around the globe. Upon this mass of granite the primary rocks were successively deposited ; over the primary rocks, the transition rocks were placed ; and, over the transi- tion rocks, the earthy stratified rocks; each of these layers encircling the globe, and covering each other, in a manner resembling the coats of an onion. Whilst this process was going forward, the waters were gradu- ally retreating, and becoming turbid. Hence, the ma- terials which they deposited, to form the upper strata, AND MINERALOGY. 47 were more earthy than those of the primary rocks : they were also intermixed with fragments of the rocks which had been previously formed. Thus, mountains and val- leys were caused by the original inequality of the nucleus of the globe. Your view of this subject will be mate- rially enlarged by an attentive consideration of the fol- lowing admirably condensed " General Conclusions," drawn from Mr. William Phillips's " Outlines of Geo- logy and Mineralogy ;" an excellent elementary work : " 1. That the lowest and most level parts of the earth consist of horizontal strata, composed of various substances, many of them containing marine productions. " 2. That similar strata are found in hills to a great height. " 3. That shells are sometimes so numerous as to constitute entire strata. " 4. That shells are found in elevations far above the level of the sea, and on heights to which the sea could not be raised by any existing cause. " 5. That these shells once lived in the sea, and were depo- sited by it. " 6. That shells continue to be found, as we rise to the foot of great chains of mountains. " 7- That, at this elevation, the strata, instead of being hori- zontal as in plains, are of various degrees of inclination, and sometimes vertical. " 8. That, from these and other circumstances, it is inferred that there have been frequent irruptions and retreats of the sea. " 9. That, as we approach the summits of lofty mountains, the remains of marine animals and shells become rare, and even wholly disappear. 48 (JEOLOGV " 10. That their strata, are wholly different, and contain no vestige of a living 1 creature. " 11. That these strata are, by some, considered not to be precisely in die place in which they were formed. " 12. That, nevertheless, as they contain no vestige of animal remains, they are considered to be the oldest rucks, and there- forearecalledpnmVuT. " 13. That rocks, which, because they include no vestige of .icToi* .., ^ . animal remains, are termed primitive, are of various kiuds. " 14. That rocks, enclosing animal remains, are never found underneath, or supporting, those rocks which are termed primi- tive. * 15. That some primitive rocks alternate with each other, but that granite is found beneath all others, and frequently over- tops all the rest. " Ifi. That rocks which include organic remains must have been formed after the shells they contain ; and, therefore, not being considered primitive, are by some termed secondary rocks ; whence the terms, used by geologists, of primary and secondary formations. " 17- That there are many varieties of secondary rocks, each of which has received a geological appellation. " 18. That there exists another class of substances, not ap- propriately called rocks ; but which, being considered to be the debris or ruin of rocks, by their long exposure to the action of air and water, or both, are therefore termed (dhn-iuldqiosites. " 19. That the catastrophes to which the surface of the globe has been subject, have Ix-en numerous: and some of these have not been owing to irruptions of the sea, but to the agency of fresh water; and these irruptions of fresh and of suit ,-aferhave been alternate. " 20. That certain deposites are always found beneath, ncrcr above, certain other deposites. That rocks which contain no AND MINERALOGY. 49 animal remains are always found beneath, never resting upon, those rocks which do contain animal remains : and that those deposites which are termed alluvial, as gravel, sand, clay, &c. are never found beneath other rocks, but always resting upon them." It is evident, that every part of the dry land has at one time or other been covered with water. It is con- sidered, that three-fifths of the surface of the globe are, at this time, covered by the sea, the average depth of which has been estimated at from five to ten miles. Great changes, however, have taken place in the rela- tive positions of the present continents with the ocean, which, in former ages, rolled its waves over the summits of our highest mountains. It is further evident, that many genera of quadrupeds, once existing, have disap- peared from the earth ; and that, at the period when many of the great changes which I have mentioned took place, man was not an inhabitant of our planet. I shall now proceed to MINERAIOGY, the object of which is to describe the different appearances and cha- racters of minerals with accuracy and precision; and to arrange and class the several minerals, according to the most obvious relations which a clear and comprehensive description may suggest. The division of mineral productions, by Avicenna, a celebrated Arabian physician of the eleventh century, has been adopted by Kirwan and other writers, as fol- lows : the earthy, or stones; the saline, or salts; the inflammable, such as sulphur, &c. ; and the metallic, in- cluding metals and ores. 50 GEOLOGY Of the earthy minerals, the siliceous, or flint genus, embraces all those in which flint, or silex, predominates ; as, the quartz, garnet, ruby, common flint, and others. The clay genus comprises common clay, slates, and every substance which contains a predominant quantity of alumine. In the saline class arc found all the combinations of the acids with alkalies ; such as saltpetre, or nitrate of potash ; sal-ammoniac, or the muriate of ammonia ; common rock salt, or the muriate of soda, &c. In the third class are comprehended all combustible bodies, (the diamond and metals excepted,) including sulphnr, resins, bitumens, and graphite. The resin in- cludes amber; the bitumens include petroleum, mineral pitch, various sorts of coal, &c. ; and the graphites arc carburets of iron, of which plumbago, or black lead, is one. In the fourth, or metallic class, are found all mineral substances, composed either entirely of metals, or of those substances in which metals predominate. From minerals of this class all metals are extracted. Metals exist in ores, cither in a metallic state, combined with sulphur, in the state of oxydes, or combined with acids. Hence we have the four genera, alloys, sulphurets, oxydes, and salts. It may be added, that the genera of minerals are divided into species ; and that the species are again divided into tub-species and varieties, accord- ing to their agreement or differences in shape, colour, fracture, hardness, &c. Gold is at once the most precious and the most curious of metals. A single ounce of gold may be beaten till it covers one hundred and AND MINERALOGY. 51 fifty square feet ; and the wire-drawers will extend it to more than one thousand feet, as a cover to other me- tals, without a flaw being discoverable, even by the mi- croscope. The fusion of a mineral is effected by exposing it to the flame of a candle or lamp, concentrated by the in- strument called a blow-pipe. The hardness of minerals is ascertained, either by a comparison with each other, by their power of scratching glass, or by the applica- tion of the file. The phosphorescence of minerals is that faint light which they emit, either by exposure to simple heat, or in consequence of friction. The elec- tricity of a mineral is that property which, being ex- cited either by simple heat or by friction, shews itself in the attraction or repulsion of other substances with which the mineral is brought nearly into contact. By the specific gravity of a mineral, is understood the amount of its weight, when compared with the weight of a quantity of water of the same bulk with itself. The crystalline forms under which minerals present themselves are various ; but, with a few exceptions, they may be resolved into the six following primitive forms : the parallelepiped, either cubic or rhomboi'dal ; the octohedron; the tetrahedron; the regular hexhedral prism; the dodecahedron, having twelve equal and simi- lar rhomboi'dal surfaces ; the dodecahedron, consisting of two regular six-sided pyramids, applied base to base, or having twelve triangular surfaces. Before I conclude, I must inform you that all the sub- jects of Mineralogy are denominated fossils ; and that fossils are either native or extraneous. Native fossils 52 GEOLOOY are substances found either buried in the earth, or lying on its surface, of a plain, simple, structure, and shewing no signs of containing vessels or circulating juices. Of fossils, naturally and essentially simple, some are neither inflammable, nor soluble in water, as simple earths, talcs, fibrariae, gypsum, selenitae, crystal, and spars; others, though uninflammable, are soluble in water, as all the simple salts ; and others, on the contrary, are inflammable, but not soluble in water, as sulphur, auri- pigmentum, zarnich, amber, arnber-gris, gagates, as- phaltum, ampelites, lithanthrax, naphtha, and pissas- phalta. Of fossils that are naturally compound, but unmetallic, some are neither inflammable, nor soluble in water, as compound earths, stones, septariae, siderochita, scrnpi, semi-pellucid gems, lithidia, conissalae, and pel- lucid gems ; others are soluble in water, but not inflam- mable, as all the metallic salts ; and lastly, some are in- flammable, but not soluble in water, as the marcasites, pyrites, and phlogonia. Of metallic fossils, which are bodies naturally hard, remarkably heavy, and fusible in fire, some are perfectly metallic, as being malleable when pare ; such are gold, lead, silver, copper, iron, and tin ; others are imperfectly metallic, as not being malleable even in their purest state; such are antimony, bismuth, cobalt, zinc, and quicksilver, or mercury. Extraneous fossils are bodies of the vegetable or ani- mal kingdoms, accidentally buried in the earth. Of the vegetable, the principal kinds are trees, and herbaceous plants, or the parts of these. Of the animal, there are four kinds ; first, sea-shells ; secondly, the teeth, or bony palates, and bones of fishes ;> thirdly, complete fishes j AND MINERALOGY. 53 fourthly, bones of land animals. These extraneous fos- sils have excited the curiosity of several naturalists, who have framed different hypotheses to account for the ap- pearances of petrified sea-fishes, in places far remote from the sea, and on the tops of mountains ; of shells, in quarries of stone ; of elephants' teeth, of bones of va- rious animals peculiar to the southern climates, and of plants growing only in the east, found in a fossil state in our northern parts. Of the various theories upon this subject, I shall mention only that of Woodward : he maintains, that the earth, with every thing belonging to it, was dissolved at the time of the deluge ; and that a new earth was formed on the bosom of the water, con- sisting of different strata, arranged over each other, agreeably to the order of their specific gravities. By this means, he says, plants, quadrupeds, fishes, and shells, not dissolved with the rest, remained blended amongst the mineral and fossil matters. It is probable, however, that many bodies, usually considered as native fossils, have been formed by a slow process of nature, from ve- getable or animal substances buried in the flood. I am, &c. F 2 LETTER VII. PHYSIOLOGY, ANATOMY, &c. ' ' " Om life contains a thousand springs, And die* if one be gone ; Stiange! that a harp of thousand .strings Should keep in tnne so long." Watts* THROUGHOUT life, my dear boy, you will find, that, in a physical as well as in a moral sense, " The proper study of mankind is man. Physiology, respecting which I am about to offer some observations, comprehends, in its etymological significa- tion, the science of nature in general. Modern use, however, has restricted it to that department of physical knowledge which alone has relation to organic existence; and, when employed as a generic term, without any spe- cific indication, it is made exclusively to denote the sci- ence of animal life. It consequently embraces Zoology, or that part of natural history which relates to the vari- ous animal tribes, as well as to man. Anatomy is the science which specifically teaches the conformation of the human body, the solids and fluids of which are its parti- cular objects. Comparative Anatomy is that branch of the science, which considers the bodies of other animals, serving for the more accurate distinctions of several parts, ANATOMY, &C. 55 and supplying the defects of human subjects. Thus, comparative does not strictly stand in contradistinction to human anatomy ; but, whilst it embraces the entire circle of animated existence, it considers man as the standard of its comparisons, and the primary object of its inquiries. " The Creator," observes Linnaeus, " has given us eyes, by the assistance of which we discern the works of crea- tion. He has moreover endowed us with the power of tasting, by which we perceive the parts entering into the composition of bodies; of smelling, that we may catch their subtile exhalations; of hearing, that we may re- ceive the sound of bodies around us ; and of touching, that we may examine their surfaces ; and all for the purpose of our comprehending, in some measure, the wisdom of his works. The same instruments of sensation are be- stowed on many other animals, who see, hear, smell, taste, and feel ; but they want the faculty, which is granted us, of combining these sensations, and thence drawing universal conclusions. When we subject the human body to the knife of the anatomist, in order to line), in the structure of its internal organs, something which we do not observe in other animals, to account for this operation, we are obliged to own the vanity of our researches : we must therefore necessarily ascribe this prerogative to something altogether immaterial, which the Creator has given to man alone, and which we call soul." According to the Divine Revelation, when the Almighty called the universe into existence, he be- gan with the most simple elements, and proceeded, first, to the creation of the vegetable tribes ; then, to the in- 56 PHYSIOLOGY", ferior animals ; and, finally, to the human race. Man, the image of the Great Supreme, the last and noblest of all his works, is distinguished from other animals, no less by his external form than by his internal powers. His figure indicates him to be the lord of the creation : his body is upright, and touches the earth only with its ex- tremities : his countenance is stamped with the charac- ters of dignity and command. He is a thinking and a rational being. His body is divisible, extended, and pe- netrable, subject to disease, decay, and death; his soul is indivisible, unextended, and immaterial. With what wonderful sublimity has our inimitable poet expres- sed himself on this occasion! " What a piece of work is man ! How noble in reason ! How infinite in faculties ! Inform and moving, how express and admirable! In action, how like an angel! In apprehension, how like a God ! The beauty of the world, the paragon of animals ! " The animal body, to which we must now direct our attention, is composed of bones, muscles, nerves, arte- ries, veins, cartilages, membranes, and glands ; and also of chyle, blood, &c. Bone is a white, hard, brittle, and insensible, part, the support of the whole fabric. Hu- man bones contain oily matter, gelatine, cartilage, lime, and phosphorus. Bones are composed of solid fibres ; partly compact, or solid ; partly cellular, or spongy; and partly reticular, or like net-work. They have blood- vessels and absorbents. The number of separate bones in the human frame is about 248. Eight separate bones, in the skull, form a vault, or cell, in which the brain is enclosed. The vertebra; of the neck, so called from the ease with which they move, support the head, which, by ANATOMY, &C. 57 their means, is easily moved up and down, or round on either side. The vertebrae are parted from each other by an elastic substance. The chief bones in the trunk are as follow : the breast-bone, to which the seven true and five false ribs are united ; the spine, which extends from the skull to the end of the loins, and serves to con- tain and protect the spinal marrow, or oleaginous sub- stance in the cavities of the bones ; the pelvis, the office of which is, to support the abdomen ; and the thorax, reaching from the neck to the end of the breast-bone, which serves as a chest, or place of security, for the heart, lungs, and other thoracic viscera. The muscles are parts of the animal body, composed of flesh and tendinous fibres, and contain vessels of all kinds. They are susceptible of relaxation arid contrac- tion, and are, with the aid of the tendons, the instruments of motion. Muscles are either voluntary or involun- tary. Those which perform voluntary motions receive nerves from the brain, or spinal marrow ; those which are involuntary, from the cerebellum, which is the hinder part of the brain. The motions of the voluntary muscles are subject to the will, as in the case of the arms, legs, &c. A muscle cannot act when its nerve is cut asunder. Tendons are white, firm, and tenacious, parts, contiguous to, and generally forming the extremities of, the muscles. They are also connected to the bones. The nerves, of which nine pairs are called cranial, and thirty pairs vertebral, nerves, are cylindrical whitish parts, like cords, composed of fibres, deriving their origin from the brain and the spinal marrow. The nerves conduce to all the enjoyments and sufferings of life, and to the intelleo 58 PHYSIOLOGY, tual faculties of man ; and they may, therefore, be con* sidered as the organs of sensibility. The heart, the principal organ of life, is ahollow mus- cle, which, as well as the stomach, intestines, &c. acts upon its contents, by means of muscular fibres. This in- voluntary muscle contains four cavities, for receiving the blood, and for giving it a fresh impulse through the arte- ries. Its figure is that of an inverted cone ; its size various, generally about four or five inches at the base, and about six inches in length. The younger the subject, the lon- ger is the heart in proportion to the body. It has an al- ternate motion of contraction and dilation, called sys- tole and diastole, for the purpose of circulating the blood throughout the whole body. Its weight, when removed from the body, with its pericardium, is from ten to fifteen ounces. The arteries are hollow canals, which originate in the heart ; and through them the blood is carried from the heart to all parts of the body, for the maintenance of life and heat, for the conveyance of nutrition, and for the secretion of the different fluids. Of the two prin- cipal arteries, the pulmonary artery arises from the right ventricle of the heart, and carries the blood through the lungs only. The aorta, or great artery, furnishes all the other parts of the body. The arteries terminate in small, and, to the naked eye, invisible, veins, which bring back the blood from the extremities to the heart. The pulse, which is felt at the wrist, tem- ples, and various parts of the body, is occasioned by the reciprocal action of tfie heart and arteries, when the blood is driven from the former into the latter, to be dis- ANATOMY, &C. 59 tributed through the body. lu children, the pulse gives 120 strokes in a minute ; at twenty years of age, about 75 ; at thirty, about 70 ; and, in old age, seldom more than 60, or even 50. Each cavity of the heart is called into action about 4000 times in every hour ; and all the blood in the body passes through the heart fourteen times in every hour. The double circulation of the blood, by means of the two grand arteries, is this : one, from the heart to the lungs, for the purpose of receiving oxygen from the air ; the other, over all parts of the body, to supply it with nutritive and vital properties. The veins, all of which originate at the extremities of the arteries, gradually increase in size as they approach the heart. In the veins there is no pulse ; for the blood is thrown into them with a continual stream, and moves from a nar- rower to a wider channel. Numerous valves prevent the return of the blood. Cartilages are smooth, solid, flexible, elastic, parts, such as those which are contained in the nose, ears, and eye-lids. They are softer than bone, of the nature of which, however, they partake. A membrane is a thin, white, flexible, expanded, skin, employed to cover and protect certain parts of the body, and for various other purposes. Glands, which appear to the eye as whitish membran- ous masses, are organic parts of the body, employed for the secretion or alteration or the different fluids, and are composed of blood-vessels, nerves, and absorbents. The office of absorbents is to take up whatever fluids are effused into the different cavities of the body, and to pour out their contents for particular uses. They con- GO PHYSIOLOGY, stitute a set of small, colourless, vessels, which pervade the entire surface of the body, both externally and inter- nally; and they are divided into lymphatics and lacteals, according to their respective offices. Modern anatomists allow of only one kind of flesh ; viz. the muscular, consisting of little tubes, or vessels, containing blood ; so that fleshy and muscular parts of the body are, with them, the same. In the flesh, the blood-vessels are so small, as to retain only a sufficient quantity of blood to preserve the rednejs of their co- lour. The lungs are the organs of respiration. The chest contains two distinct membranous ba?s, called the right and left bags of the pleura : each of these holds the long of its own side. The lungs follow all the motions of the sides of the chest. They are distended by the influx, of air, when the chest is enlarged ; and the air is expel- led from the lungs, when the chest is diminished. The lights, which are sometimes used in families to feed cats, are the lungs of sheep or oxen, and are exactly similar to the lungs of man. Any rupture in their fabric, or de feet in their action, tends to the production of consump- tive disease. The act of respiration is performed about twenty times in a minute ; and about forty cubic inches of air are respired every time : of this, two inches of oxygen are absorbed by the blood in the lungs, produ- cing at the same instant 98 of vital heat, and preserving the bright red colonr of the veinous blood. The wind- pipe is the tube which conveys the external air into the lungs : it is divided into three parts; the larynx, the tra- chea, and the bronchi. The first of these is a hollow, ANATOMY, &C. 61 cartilaginous, organ, at the top of the trachea. The air which passes through it, in respiration, to or from the lungs, produces the voice. The cavity of the larynx opens above, at the root of the tongue, and below in the trachea. It is covered by the epiglottis, an oval-convexo- concavo cartilage, which acts upon the air as it issues from the lungs, like the key of a wind-instrument. It is attached to the root of the tongue ; and, during the act of deglutition, it is pushed back over the aperture of the larynx, so as to prevent food or drink from entering the wind -pipe. In women, the larynx is smaller than in men, in the proportion of two to one. In this principally consists the sexual difference of voice. The trachea, which lies between the larynx and the bronchi, is a cylin- drical tube, about three-fourths of an inch in diameter, and consisting of sixteen or eighteen cartilaginous rings. It runs along the middle of the fore-part of the neck, and has the large blood-vessels of the neck on each side. The bronchi are two branches, into which the trachea di- vides, for the two lungs : they ramify through the lungs, dividing into smaller and smaller branches, and ulti- mately communicate with the air-cells. The brain, which is a small pulpy mass, of a whitish colour, is enclosed in the cranium, or skull, in which all the nerves or organs of sense terminate. It is believed to be the receptacle of all onr sensations, and the source of all our ideas. It is encompassed by two membranes, called dura mater, and pia mater. Its figure is roundish, oblong, and flat on the sides. The cerebrum, or brain, strictly so calkd, the cerebellum, and the medulla oblon- gata, are its three principal parts. The medulla pinalis, 62 PHYSIOLOGY, or spinal marrow, is a continuation of the medulla ob- longata through the centre of the spine. The stomach is a large, bag-shaped, reservoir, the grand receptacle for food, and for retaining it until a certain change , called digestion, is produced. The liver, which stretches itself over the right side of the stomach, is a collection of glands, or a large glandulous mass, of a red sanguine colour, wherein the bile is separated. The office of the liver is to purify the mass of blood, by forming a secretion of the bilious humour which it con- tains. The gall-bladder, or reservoir for the bile, is a bag, shaped like a pear. It is firmly connected to the liver. Bile is supposed to contribute to the separation of the chyle, exciting the peristaltic motion, carrying off gross matter from the blood, &c. The spleen, which is connected by the blood-vessels to the stomach and left kidney, is situated between the spurious ribs and the stomach. It is a soft, vascular, body, full of blood. Its use is not accurately known ; but it is believed to be a subordinate organ, assisting the circulation. The kidneys are two glandular bodies for separating the urine. They have arteries, veins, and nerves. Beneath them are situ- ated the intestines, which are attached to the mesentery. The capacity of the human stomach varies, from about five to eleven pints. It has two openings ; one called the oesophagus, through which the food passes into it ; the other, called the intestinal canal, for carrying away the digested substance. The chief agent in digestion is the gastric juice, a most powerful solvent, which acts on all substances indiscriminately, even on the stomach itself, when unsupported by food. It is an anti-septic, and, ANATOMY, &C. 63 contrary to vulgar opinion, possesses no fermentative quality. By the vascular nature of the stomach, the food, when properly digested, is propelled through the intestinal canal into the intestine, which is a membranous tube, four or five times the length of the human body. In its digested state, the food is called chyme : in this state it enters the intestine, where it undergoes another change, and the chyle, a milk-like fluid, is separated from it. It appears., that chylification commences by chewing the food in the mouth, and mixing it with sali- va : when reduced into a sort of pulp, and received into the stomach, it is subjected to the action of the gastric juice, by which it is reduced to chyme : in the latter state, it is mixed with bile ; and, from this mixture, the lacteals of the mesentery and thoracic duct absorb a fluid, which is perfect chyle. The blood, as you must be already aware, is a red fluid, which circulates through every part of the body, serving for the support of life, and the nourishment of all its parts. The origin of the blood is in the chyle : passing the lacteals, and mixing with the blood, the chyle proceeds to the right ventricle of the heart, and thence, converted into blood, circulates through the body. The blood acquires its heat and colour by the absorption of oxygen from the atmosphere, by means of a slow com- bustion in the lungs. Its temperature is from 94 to 100 degrees; and, in some females, it amounts to 104 degrees of Fahrenheit. The quantity of blood usually found in the adult subject, varies from 25 to 30 pounds; of which about four parts are contained in the veins, and a fifth in the arteries. The circulation of the blood was 64 PHYSIOLOGY, discovered by Dr. Harvey, a native of Folkstone, in the year 1628 ; and it has been remarked, as a very extra- ordinary circumstance, that his system was not accredi- ted, or adopted, by any professional man of that period, whose age exceeded forty. Some physiologists sup- pose, that the red globules of the blood possess vitali- ty. Blood, when let out of the body, gradually separates into two parts ; the one, red, which coagulates into a mass, and is called the cruor ; the other, thin and trans- parent, which retains its fluidity, and is called the scrum. This separation is similar to that of milk, when reduced into curds and whey. The blood of quadrupeds is com- posed of nine different parts : its peculiar aroma, or odorous part ; fibrin, or fibrous matter, the chief sources of the muscular fibre ; gelatine, or jelly ; albumen, or matter like the white of an egg ; the red particles ; and iron, sulphur, alkalies, and water. From what are termed the five senses, flow all our sen- sitive perceptions, and all our various habits, qualities, passions, and powers. Feeling, by which we acquire ideas of solidity, hardness, heat, cold, &c. is the most univer- sal of our senses ; but many inferior animals possess it in a more excellent degree than man. The immediate organs of feeling are the pyramidal papilla;, which are little, soft, medullary, nervous, prominences, lodged every where under the outermost skin. Some consider the four other senses of hearing, seeing, tasting, and smelling, merely as modifications of feeling. The ear is placed in the most convenient part of the body, near the brain, the common seat of the senses. In the human subject, it it of a form proper for the erectness of the body : in ANATOMY, &C. 65 quadrupeds, its form is, in some, large, erect, and open; in others, covered : in subterraneous quadrupeds, it is short, and lodged deep : in birds, it is of a form proper for flight, and not protuberant. The structure of this organ is ad- mirably contrived to collect the undulations of sound, and to convey them to the sensory in the brain, The first part is the auricle, or external ear, formed for the purpose of stopping and collecting the sonorous undula- tions, and conveying them to the concha, or large round cell, at the entrance of the ear. In the interior is the auditory passage, curiously tunnelled and turned, to give sounds an easy passage, and yet to prevent them from too furiously assaulting the more tender parts of the in- terior. To prevent the entrance of noxious insects, this passage is secured by a bitter nauseous substance, called ear-wax. Amongst the numerous other parts of the ear, is the membranum tympani, with its inner mem- brane, &c. I shall be better enabled to make you comprehend the nature of the eye, when I come to treat of the science of optics. The taste is that sensation which all things give to the tongue ; but some consider the palate, or up- per part of the roof of the mouth, to be the organ of taste. The chief instruments of the sense of smelling are the nostrils and olfactory nerves ; the ramifications of which pervade the nostrils in every direction. The odorous effluvia in the air, when drawn into the nostrils by inspiration, strike with such force against the olfai 1 - tory nerves, as to shake them, and thus occasion ideas of aromatic, sweet, sour, or foetid. I am, &c. C-2 atiih < to LETTER VIII. shflul ,11 on ha* sm ,K' ^f&ii i:uj|w - . ZOOLOGY. fo,PC i ^:>bl(. id! " Whoever loves to contemplate the works of God, will trace him not only ia those immense globes which compose the system of the universe, bnt also in the little worlds of animals, insects, plants, &c. He will acknowledge and adore Divine Wisdom as much in the spider's web, as in the power of gravitation which attracts the earth towards the gun." Muri/i. ZOOLOGY, in its details, is eminently interesting; and I shall offer you a sketch of the systematic arrangement of the science, upon the principle of Linnaeus, the cele- brated professor, of Upsal, in Sweden. He distribntes the animal kingdom into the six following classes: I. Mammalia, or animals which suckle their young ; includ- ing man, the quadrupeds, and the whale kind. II. Aves, or birds. III. Amphibia, or amphibious animals. IV. Pisces, or fishes. V. Inseda, or insects. VI. Vermes, or worms. I. In this class, the heart has two ventricles and two auricles ; the blood is red and warm, and the animals are viviparous. It comprises the following seven orders, which are chiefly regulated by the number and situation of the teeth, l. Primates, or animals with two canine and four cutting teeth in each jaw. This order, compri- sing four genera, includes man, and all the ape, monkey, and bat, kinds. 2. Bruta, or animals which have no cut- ting teeth in either jaw; as, the elephant, ant-cater, &c.: ZOOLOGY. 67 this order includes seven genera. 3. Ferae, or animals whose cutting-teeth vary from ten to two. This order, which includes ten genera, comprehends most of the ra- pacious quadrupeds ; as, the dog, cat, and bear, kinds. 4. Glires, or animals which have only two cutting and no canine teeth ; as, the mouse, squirrel, hare, &c. It has ten genera. 5. Pecora, or animals which are hoofed, and have no cutting teeth in the upper jaw. This order, having eight genera, includes the camel, deer, sheep, and ox, kinds, &c. 6. Bellucc, or quadrupeds with catting teeth in each jaw; as, the horse, hog, &c. This order contains four genera. 7. Cetee, or animals whose teeth vary greatly in the four different genera of which it is composed. This order comprehends all the whale tribes, having pectoral fins, and fistula or spiracula upon the head. Besides the generic characters of this class, taken from the teeth, some of the genera have other distinguished marks. Thus, the whole order ofpccora are distinguished by having, or being destitute of, horns, and by peculiari- ties in the horns themselves : the hystrix is covered with sharp spines j and the vespertilio has a membrane attach- ed to its feet and sides, by which it is enabled to fly. The specific characters of the different genera and species vary greatly, and are taken from any parts of the body which possess a peculiar mark of distinction. II. The characters of this class are the same with those of the preceding, except that the animals are ovi- parous. It is divided into the six following orders; the distinctive characters of which are derived chiefly from the bills and the feet. 1. decilitres, eagle or hawk kind birds with hooked bills and short robust limbs ; the m- 68 ZOOLOGY. perior mandible, near the base, being extended on each side beyond the inferior one ; and, in some, it is armed with teeth. Under this order, which contains four genera, are comprehended vultures, falcons, owls, &c. They build their nests in the rocks, and other lofty situations. 2. Piece, or pies, birds with bills shaped like a knife, and convex on the backs j their short and strong limbs fitted for walking. They build their nests in trees. This order comprises twenty-three genera. 3. .itisins, ducks, have smooth bills, broadest at the point, covered with a smooth skin, and furnished with teeth ; the tongue fleshy, and the toes palmated, or webbed. They generally build their nests on the ground. This order has thirteen genera. 4. Grallce, or birds with obtuse and nearly cylindrical bills, short tails, and naked thighs. The limbs are fitted for walking. To this order, which comprises twenty genera, belong cranes, snipes, Sec. which generally build their nests on the gronnd. 5. Gallnue, or birds with convex bills, the superior mandible vaulted over the inferior one, the nostrils half covered with a convex cartilaginous membrane, and the feet divided, but connected at the inmost joint. This order, including ten genera, comprehends the pheasant, grouse, peacock, &c. 6. Passeres, or birds with conical sharp-pointed bills, the nostrils oval, wide, and naked. To this order, containing seventeen genera, belong the pigeon, swallow, thrush, &c. The characters, -distin- guishing the species of this class, are much varied : for example, crests, or feathers on the head, disposed in different manners ; the colours of particular feathers, or parts of feathers ; the shape and length of the tail ; ZOOLOGY. 69 the colour of the feet ; the number, situation, &c. of the toes ; the colour and figure of the bill ; the colour of the cere, or wax, &c. III. The animals of this class, which are enabled to live either in air or water, have but one ventricle to the heart, and one auricle : the blood is cold and red ; and the animals have the command of their lungs, so that the intervals between inspiration and expiration are, in some measure, voluntary. This class is divided into two orders. 1. Replilia pedata, or reptiles furnished witli feet, including turtles, lizards, frogs, &c. This order lias four genera. 2. Serpentes Apodes, or reptiles without feet. This order, which has six genera, comprehends all the serpent and snake kinds. The generic cha- racters of this class are taken from the general figure of the body; from having covered or naked bodies; from the number and situation of the spiracula; from the situation of the mouth ; from having teeth or no teeth in the mouth ; from the number, situation, and figure, of the scutae and scales; from being covered ornot covered with a shell; from having tails or no tails, &c. The specific characters are exceedingly numerous. IV. Animals of this class resemble those of the pre- ceding, in the structure of the heart and the qualities of the blood ; but they are distinguished from amphibia, by having no voluntary command of their lungs, and by having external bronchia, or gills. This class is divided into six orders, the characters of which are taken from the ventral, or belly, fins. 1. Apodes f or fishes which have no ventral fins. This order, having ten genera, comprehends all the eel tribes, whether they inhabit 70 ZOOLOGY. seas, lakes, or rivers. 2. Jugulates, or fishes with the ventral placed before the pectoral fins, as in the had- dock, whiting, ling, &c. This order comprises six ge- nera. 3. Thoracici, or fishes with the ventral situated under the pectoral fins, as in the feather-lasher, halibut, plaice, &c. This order includes nineteen genera. 4. Abdominales, or fishes with the ventral situated behind the pectoral fins, as the pike, mullet, herring, &c. This order has sixteen genera. 5. Branchiostegi, or fishes whose gills are destitute of osseous matter, as the sun- fish, pike-fish, frog-fish, &c. This order includes ten genera. 6. Clwndropterygii, or fishes with cartilaginous gills, as the balance-fish, sturgeon, dog-fish, &c. Of this class the generic characters are taken from some pecu- liarities in the head, mouth, teeth, nostrils, general figure, form of the tail, situation of the spiracula, eyes, rays in the membranes of the gills, &c. The specific characters, also, are chiefly taken from some peculiari- ties in those parts. V. The animals of this class have but one ventricle to the heart, and uo auricle 3 the blood is white and cold ; and the animals are furnished with antenna, or feelers, in the fore-part of their heads. The seven orders of which it is composed are chiefly derived from the wings. 1. Coleopteru, or four-winged insects, whose upper pair consists of a crustaceous, or horny, substance, and joins in a straight suture or ridge. These cover and defend the under pair, which are of a soft and flexible texture. This order, containing fifty-five genera, includes the whole scaraliai, or beetle tribe. 2. Hemiptera, insects which have also four wings ; but the upper pair, instead ZOOLOGY. 71 of being hard and horny, have a resemblance to fine vel- lum ; as in grass-hoppers, locusts, crickets, &c. This order comprehends fourteen genera. 3. Lepidoptera, or insects, whose four wings are covered with imbricated scales. This order, divided into three genera, embraces ail the butterfly and moth tribes. 4. Neuroptera, or insects with four membraneous wings, so interspersed with delicate veins that they resemble beautiful net- work ; as, the spring-fly, dragon-fly, &c. This order has seven genera. 5. Hymenoptera, or insects with four mem- braneous and naked wings, as the wasp, bee, &c. la some of the fifteen genera, however, which this order comprises, the -neuters, and in others the males, or even the females, have no wings. The tails of the female sex only are armed with a sting. 6. Dipteru, or two-winged insects, having two elevated halters, orbalanccs, behind each wing; such as the gad-fly, gnat, &c. This order has twelve genera. 7. Aptera, or insects which are destitute of wings, as the flea, scorpion, &c. VI. The characters of this class, which inclndes not only all the insects, commonly called worms, but all tes- taceous animals, and the zoophytes, or plant animals, are the same as those of the preceding class, except that the animals have no antennae, and are furnished with tenta- cula. It is divided into five orders. 1. Intestina, the most simple animals, perfectly naked, and without limbs. In this order are twenty-one genera. 2. Mollusca, compri- sing thirty-one genera, are also simple, naked animals, hut they are brachiated, orfurnished with a sort of Jimbs. 3. Testacea have the same characters as the Mollusca, excepting that they are covered with a shell. This 72 ZOOLOGY. order has thirty-six genera. 4. Zwphyta, compound ani- mals furnished with a sort of flowers, and having a vege- table root and stem. This order includes fifteen genera. 5. Infusoria, consisting of very small simple animals, in- cludes fifteen genera. Of quadrupeds, it is understood, there are already known to man about 230 species 5 of birds, about 1000 ; of amphibious animals, about 100 ; of fishes, about 500 ; of insects, about 2000; of worms, about 800. Besides these animals, there are animalcules, or in- sects, so minute, many of them] as to be invisible to the naked eye. Lewenhoeck mentions insects seen with a microscope, of which 27,000,000 would be equal only to a mite, and 4,000,000 to a single grain of sand. How must onr wonder and admiration of the Creator be exci- ted, when we learn that each of these animalcule has an organized body, provided with a heart, lungs, muscles, glands, arteries, and veins, and with blood and other fluids passing through them! It has been ascertained, also, that their vigour and powers of action are generally su- perior to those of larger animals ; that their length of life ia great, in proportion to their size; that a mite makes five hundred steps in a second ; that animalcules, in a drop of water, swim about with as much freedom as whales in the ocean ; and that those which feed on the leaves of trees may be adequately compared with oxen grazing in large pastures. There can, I think, be little reason to doubt, that creatures may still exist so incon- ceivably minute, as not to come within the power of our best microscopes. Amongst the numerous writers upon Natural History, ZOOLOGY. 73 you will find Linnaeus, Buffon, Pennant, Shaw, Latham, &c. particularly deserving of your notice. I shall conclude this letter, my dear pupil, with a re- mark which may prove serviceable in your physiological and zoological pursuits. The celebrated German pro- fessor, Camper, has shewn, that, amongst inferior animals, as we descend from man to the lower orders of the animal kingdom, the face advances, and the forehead recedes, in proportion as the species dimi- nishes in intellect. Thus, (he forehead of the negro is more depressed than that of the European, " the fore- head of the monkey is more depressed than that of the negro, that of the dog more depressed than that of the monkey, that of the horse more depressed than that of the dog, that of the bird more depressed than that of the horse, and that of the fish more depressed than that of the bird. The reason of all this is, that the brain, or organ of thinking, diminishes, and the organs of sense pro- portionally increase, as we descend among animals*." The consideration of this interesting fact may perhaps induce you, when your powers of observation shall be matured, to enter upon the study of the infant and somewhat doubtful sciences of Physiognomy and Cranio- logy- . I am,&c. * " So well were the Greeks aware of the importance of this law of the brain diminishing: with the diminution of intellectual power that, in their immortal sculptures, they have given even an unnatural expansion to the head, and especially to the fore- head, in order to confer the most august character on their heroesj demi-gods, and gods. " H LETTER IX. BOTANY. " Meadows and trees in their cheerful verdure, flowers in their bloom, and all the vegetable parts of creation in their most advan- tageous dress, inspire the heart with grateful joy." Seed. TAKING an enlarged view of the subject, the science of botany may be considered as treating of the kinds, forms, virtues, and uses, of plants ; but, in its restricted sense, it chiefly regards the classification of plants, or that systematic arrangement by which, from general character, we are enabled to trace the class, order, genus, and species, to which each plant belongs*. Lin- naens, whose system is universally adopted, thus dis- * The botanical classifications are of essential consequence, with regard to food and medicine; lor, though many animals possess the instinctive faculty of discerning the difference be- tween salutary or noxious food, mankind, who are not so en- dowed, can have recourse only to the deductions of science. BOTANY. 75 tinguishes the mineral, vegetable, and animal, kingdoms : "stones grow /vegetables grow, and live; and ani- mals grow, live, and feel." The general belief is, that the existence of all vegetables is mechanical, or similar to that of an animal, when asleep, during which time his functions proceed without consciousness. Poets, how- ever, of all ages, have indulged the pleasing idea of at- tributing to vegetables many of the properties, passions, and actions, of animals ; and it is by no means clear to me, that they are not blessed with a sentient existence, though of a nature altogether beyond the limits of our comprehension. Without attempting to discuss this question, I shall proceed to offer you a sketch of the Lin- naean, or sexual, system of plants; to which I shall subjoin a few such remarks and illustrations as may convey some idea, though only a faint one, of the innumerable beau- ties and unbounded interest of this delightful science. Linnaeus has divided the vegetable kingdom into twenty -four classes; these classes into about one hun- dred and twenty orders ; these orders, comprising about two thousand genera, or families ; and these fami lies about twenty thousand species, besides innume- rable varieties, which the accidents of climate or cul- tivation have added to these species. The names of the classes are formed from Greek words, and express the characteristics of each class. The first twelve are named from Greek numerals, and the suffix " andria " is used to denote stamina. The following table presents, at one view, the twenty-four classes, with the number aud names of their respective orders : 76 BOTANY. I. Monandria : 2. Monogynia, digynia. II. Diandria : 3. Monogynia, digynia, trigynia. III. Triandria 3. Monogynia, digynia, trigynia. IV. Tetrandria : 3. Monogynia, digynia, trigynia. V. Pentandria : 6. Monogynia, digynia, trigynia, tetragynia, pentagynia, polygynia. VI. Hexandria : 5. Monogynia, digynia, trigynia, tetragynia, polygynia. VII. Heptandria: Monogynia, digynia, tetragynia, heptagy- nia. VIII. Octandria : 4. Monogynia, digynia, trigynia, tetragy- nia. IX. Enneandria : 3. Monogynia, trigynia, hexagynia. X. Decandria : 5. Monogynia, digynia, trigynia, pentagynia, decagynia. XI. Dodecandria, twelve stamens: 5. Monogynia, digynia, trigynia, pentagynia, dodecagynia. XII. Icosandria, twenty stamens: 5. Monogynia, digynia, trigynia, pentagynia, polygynia. XIII. Polyandria : Monogynia, digynia, trigynia, tetragynia, pentagynia, hexagynia, polygynia. XIV. Didynamia, two powers : 2. Gymniospermia, angios- permia. XV. Tetradynaraia, four powers : 2. Siliculosa, siliquosa. XVI. Monadelphia, one brotherhood: 8. Triandria, pentan- dria, octandria, enneandria, decandria, endecandria, dode- candria, polyandria. XVII. Diadelphia, two brotherhoods : 4. Pentandria, hex- andria, octandria, decandria. XVIII. Polyadelphia, many brotherhoods : 4. Pentandria, dodecandria, icosandria, polyandria. XIX. Syngenesia, stamens united by the anthers : 6. Poly BOTANY. 77 gamia aequalis, polygamia snperftua, polyg-amia frustrania, polygamia recessaria, polygamia segregata, monogania. XX. Gynandria, pistils and stamens together : 9. Diandria, triandria, tetrandria, pentandria, hexandria, octandria, decandria, dodecandria, polyandria. XXI. Monoecia, one house : 1 1. Monandria, diandria, trian- dria, tetrandria, pentandria, hexandria, heptandria, poly- andria, monodelphia, syngenesia, gynandria. XXII. Dioecia, two houses : 15. Monandria, diandria, trian- dria, tetrandria, pentandria, hexandria, octandria, en- neandria, decandria, dodecandria, icosandria, polyandria, monadelphia, syngenesia, gynandria. XXIII. Polygamia, many kinds of flowers : 3. Monoecia, dioecia, trioecia. XXIV. Cryptogamia, invisible flowers: 4. Filices musci, algae, fungi. APPENDIX : 1 Palmse. You will observe, my dear boy, that the fourteenth and fifteenth classes are founded on the proportion of the stamina; that the next five are founded on the connection of the stamina ; that, from the twentieth to the twenty- third inclusive, the classes are formed from the situation of the stamina; that the twenty-fourth consists of such plants as have the parts of fructification impossible to be accurately observed, and includes ferns, mosses, lichens, mushrooms, &c. ; and that the appendix, which consists of trees and shrubs, may be all arranged in the preceding classes of the system : they have always a simple stem, not branched, bearing leaves at the top. The orders are, in general, deduced from the number 01 difference of the pistiltum, or female part of fluctuation; 78 BOTANY. the genera agree in the general characters of fructuation; the species differ in proportion, figure, &c. ; and the varieties differ in colour, size, or some accidental cir- cumstance. The parts which carry on the process of fructification are as follows : 1. The calyx, or flower- cup ; 2. the corolla, situated within the calyx, consist- ing generally of the coloured leaves of flowers ; 3. the stamina, situated within the corolla, varying in number, in different flowers, from one to several hundreds ; 4. the pistils, standing in the centre of the circle formed by the stamens ; 5. the pericarpium, or seed vessel ; 6. the si men, or seed ; 7. the receptaculum, which is the com- mon base of the parts of fructification. In many plants, all of these are found ; but, in others, some are wanting. It is necessary for you further to observe, that every plant is divided by Linnaeus into the following parts : I. The root ; consisting of two parts, the caudex and the radicula. The caudex, or stump, is the body or knob of the root from which the trunk and branches ascend, and the fibrous roots descend, and is either solid, bulbous, or tu- berous : solid, as in trees, &c. : bulbous, as in tulips, &c. : tuberous, as in potatoes, &c. The radicula is the fibrous part of the root, branching from the caudex. II. The trunk, which includes the branches, is that part which rises immediately from the caudex, in either herbaceous, shrubby, or arborescent, plants, and admits of several other distinctions, according to its shape, substance, sur- face, &c. III. The leaves are either simple, as those which adhere to the branch singly ; or compound, as when several expand from one foot-stalk. Leaves are further described by various terms indicative of their form and outline. BOTANY. 79 IV. The props, or external parts which strengthen, support, or defend, the plants on which they are found ; or which serve to promote some necessary secretion : as, the petiobts or foot-stalk of the leaf; the peduncidw or foot- stalk of the flower ; the stipula, or husk, that is, the small leaves which generally surround the stalk at its divisions ; the cirrhuS) or tendril ; the pubes, or down ; the anna, or defensive weapon ; as, thorns. V. The fructification, or mode of fruit bearing. VI. The inflorescence, or mode by which the flowers are joined to the several peduncles. The number of species of plants already known, is from twenty-five to thirty thousand ; and as we are but slightly acquainted with the interior parts of Africa, Arabia, North and South America, New Guinea, New Zealand, New Holland, and various other regions, it is probable that , at least double that number may remain to be discovered. It appears that the first step taken by nature, towards endowing a creature with motion, constitutes the con- nection between the animal and vegetable kingdoms. The oister has generally been considered as forming this connection. The various kinds of zoophytes also appear to be possessed of animation. The polypus ranks as the first of plants, and the last of animals, if it be true that its propagation can be effected by cuttings similar to the multiplication of plants by slips and suckers. Whether plants are sentient substances or not, some of them pos- sess an organical motion. The dinncca muscipula, or 80 BOTANY. Vonus's fly-trap, closes its leaves the instant a lly settles upon them. The hedysarum girans, a native of Bengal, has the appearance of voluntary motion; two small leaflets, or appendages, situated on each side of the foot-stalk, alternately meeting and receding during the greater part of the day. Many plants point their flow- ers to the san, as though they were eager to draw nou- rishment from his rays ; and mimosa, the sensitive plant, is well known to shrink at the touch. Flowers always turn towards the light ; under a serene sky, they expand ; but rain and storms cause many of them to contract ; and, at night, they hang down their heads, and fold np their leaves, as though they were yielding to the power of sleep. Some of them, however, as the cactus gran~ diftorut, or night-flowering cereus, sleep during the day, and wake daring the night. The resemblance which some flowers bear to the in- sect tribes is exceedingly curious. One species of the orchis resembles a bee ; a second, a wasp ; and a third, a spider. The cypripcdium of South America, in its nec- tary, resembles the body, and in its petals the legs, of a large spider ; and this ambiguous appearance is said to deter the humming bird from extracting honey from its flowers. When you come to examine the interior formation of plants by the microscope, you will find that their anato- my discovers the same infinite wisdom in the construc- tion of canals, or vessels, for the circulation of the sap, as we see in the wonderful mechanism of animals. The various tribes of mosses, though exceedingly minute, are BOTANY. 81 of such an admirable structure, as to excel the stately palms of India, or the sturdy oaks of Britain*. How admirable are the lines of Thomson, suggested by the sublime wonders of vegetable nature ! " Hail, Source of Being ! Universal Soul Of heaven and earth ! Essential Presence, hail ! By Thee, the various vegetable tribes, Wrapt in a filmy net, and clad with leaves, Draw the live ether, and imbibe the dew : By Thee, disposed into congenial soils, Stands each attractive plant, and sucks and swells The juicy tide, a twining mass of tubes : At Thy command, the vernal sun awakes The torpid sap, detruded to the root By wintry winds ; that now in fluent dance, And lively fermentation, mounting, spreads All this innumerous colour'd scene of tilings." By chemical analysis, we discover the same constitu- ent principles in all vegetables, viz. carbon, water, and air ; otherwise, calcareous earth, oil, water, and air, with a portion of iron, to which they owe their beautiful co- lours. The saccharine and oily productions of vegetables are parts of their sap or juices : the turpentine, the bit- * " From a little grain, we behold grass spring up, stalks and ears of corn ; but we are ignorant how it is done. We still less comprehend, how from a little fruit stone there can grow a plant, and then a great tree, under whose shade the birds make their nests ; which tree is covered with leaves and blossoms, to shade us, and afford us fruit to eat, and wood for our use and conve- nience." Sturw. 82 BOTANY. tcr, and the acid principles, are considered as the effect of preparation or secretion; The green colour of vege- tables arises from the oil which they contain ; the rays of the sun extracting the oxygen from the outer surface, and leaving the carbon and hydrogen, the constituent parts of oil. It is considered, that healthy vegetables perspire water, by the under part of their leaves, equal to one-third of their weight, every twenty-four hours. Amongst the numerous elementary works upon this science which have been published within these few years, I cannot mention one more deserving of your notice than a single volume, with plates, entitled " Con- versations on Botany." Linnaeus, Curtis, and Withering, are authors whose works may be studied to great advan- tage ; and the information which they convey ought to be confirmed and enlarged by actual observation, and by conversation with experienced botanists. I am, your's, &c. LETTER X. GEOGRAPHY. GEOGRAPHY, my dear pupil, is that science which exhibits the results of onr investigations respecting the earth ; whether we consider its figure, and the disposi- tion of the lands and water upon its surface; or the subdivisions which the different nations who inhabit it have made ; such as kingdoms, states, &c. The divisions of this science are : physical geography, or geology, of which I have treated in a former letter ; sacred geogra- phy, which is employed solely in the illustration of the Scriptures ; ecclesiastical geography, which describes the government of the church, as divided into patri- archates, archbishoprics, &c. ; antient geography; and modern geography ; to which hist I shall at this time chiefly confine my observations. The study of geography is of so much practical impor- tance in life, that it must have commenced in the early ages of the world. It was regarded as a science by the Babylonians and Egyptians, from whom it passed to the Greeks, and from the Greeks to the Romans, the Arabians, and the western nations of Europe. The me- thods of measuring the circumference of the earth, in use among the antients, appear to have been nearly the 84 GEOGRAPHY. same as those employed by the moderns, deficient only in the accuracy of their instruments. Hipparchus, who died about 125 years before the hirth of our Saviour, made a closer connection between Geography and As- tronomy, by determining the latitudes and longitudes of places from celestial observation. The fundamental principles of Geography are the spherical figure of the earth, its rotation on its own axis, its revolution around the sun, and the position of the axis or line around which it revolves, with regard to the celestial luminaries. The earth and sea constitute one vast sphere, or rather oblate elliptic spheroid. That the earth is globular is evident; 1st. from the circum- stance, that, by persons returning from sea, the tops of high buildings are first perceived ; whilst, from the land, the masts of ships are the parts first discerned ; and, 'jilly. because, during an eclipse of the moon, the sha- dow is bounded by a circular line. Many of the terres- trial phenomena depend upon the rotundity of the earth, and the position of its axis with respect to the sun ; par- ticularly the rising and setting of the celestial bodies ; the length of the days and nights ; the long moon-light which the inhabitants of the polar regions enjoy, &c. On the surface of the artificial globe are various ima- ginary lines ; as, the two polar circles, the tropics, and the equator. The equator divides the globe into the northern and southern hemispheres. The torrid cone is between the two tropics ; the northern temperate zone extends from the tropic of Cancer to the northern polar circle; the southern temperate zone reaches from the tropic of Capricorn to the southern polar circle ; and GEOGRAPHY. 85 the two frigid zones are within the polar circles. It is only in the torrid zone that the sun is vortical ; and the days or nights are not, in any place, more than twenty- four hours long, except within the polar circles. Every circle is supposed to be divided into 360 parts, called degrees : hence, there are 90 from the equator to each pole 23 from the equator to each of the tropics 43 between each of the tropics and each of the polar circles and 23| between each of. the polar circles and the poles. Each of these degrees, on an average, is computed to contain 69 English miles. It is necessary for yon to remark, that our globe or earth is considered, geographically, to consist of land and water. The land is divided into continents, islands, peninsulas, isthmuses, promontories, and capes : the wa- ter is divided into oceans, seas, gulfs, bays, lakes, and rivers. The following definitions of these terms you should commit to memory, and make yourself fully ac- quainted with them by the examination of maps, and the artificial terrestrial globe. A continent is a large portion of land, comprehending several regions, or king- doms, which are not separated from each other by seas ; as, Europe, Asia, Africa, and America. An island is a portion of land, surrounded by the sea ; as, Britain, Ireland, Madagascar, c. A peninsula is a tract of land, almost surrounded by the sea, and joined to a continent only by a narrow slip, or neck ; such as, the Morea, in the Mediterranean sea, connected to Greece by the isth- mus of Corinth. An isthmus is a narrow neck of land, which joins a peninsula to a continent, as I have just described; or one continent to another; as, the isthmus 86 GEOGRAPHV. of Suez, which joins Africa to Europe; and tbe isthmus of Darien, which unites North to South America. A promontory is a high part of land, which projects into the sea : the extreme point of a promontory is called a cape ; as, Cape Horn and the Cape of Good Hope. An ocean is a vast body of salt water, surrounding a con- siderable part of a continent} as, the Atlantic, the In- dian, and the Pacific, oceans. A sea is a smaller collec- tion of salt water, communicating with an ocean by means of a strait ; as, the Mediterranean, the Baltic, the Euxine, and the Red, seas. A gulf is a part of an ocean, or sea, which is nearly surrounded with land ; as, the gulfs of Venice, Persia, and Mexico. A bay is an inlet of the sea between two capes; not so narrow in proportion to its length as a gulf, and approaching in shape to a greater or smaller segment of a circle ; such as the bays of Biscay, Naples, and Dublin. A lake is a body of water, generally fresh, entirely surrounded by land, and having no visible communication with the sea ; such as, Keswick lake, in Cumberland ; the lake of Ge- neva, &c. A rirer is a stream of water, rising from fountains or sources inland, and Sowing into a lake, the sea, or the ocean; as, the Thames, the Mississippi, &c. To facilitate your geographical studies, it is essential that yon should be acquainted with the nature of maps. Latitude, yon will recollect, is the distance of a place, north or south from the equator; and longitude is the distance of a place, east or west from the meridian, or noon-line. In English maps, the first meridian is drawn through London, or through Greenwich, the site of the royal observatory. A map may be considered as an GEOGRAPHY. 87 exact portrait of the surface of tbe earth, as seen from au eminence, or laid down according to a scale, in which every part retains its exact proportion. The fop of a map is the NORTH; the bottom, the SOUTH ; the right hand, EAST ; and the left hand, WEST. The figures which run from north to south, down the sides of a map, indicate tbe latitude, or distance in degrees and minutes from the equator. The lines across, from west to east, connecting the figures on each side, are for the purpose of guiding the eye, and are called the " parallels of latitude." Re- member that, when the figures increase upwwds, the latitude is NORTH; when downwards, SOUTH. The fi- gures at the top and bottom of the map indicate the longitude from the first meridian, and the lines which join them are called " meridians of longitude." When the distance increases from left to right, the longitude is EAST ; when it increases from right to left, it is WEST. The anticnts considered the earth under the three grand divisions of Asia, Europe, and Africa. Modern discoveries have added a fourth division ; that of Ame- rica, which might have formed two grand and distinct denominations, limited by the isthmus of Darien. A fifth specific division is now called Austral-asia, the greater portion of which is Australia, containing New Holland and New South Wales, with the island called Papua or New Guinea, and many smaller isles. It is supposed, that if the surface f the earth were divided into 100 parts, Europe would contain 2 ; Asia, 6 ; Afri- ca, 6 ; America, 7 ; and Austral-asia, 6 ; and that the remaining 73 parts are water. The general inclina- tions and levels of the continents are discovered by the 88 GEOGRAPHY. coarse of the rivers; of which the principal, and near- ly in the order of their magnitude, are, the Maranon, the Senegal, the Nile, the St. Lawrence, the Mississippi, the Wolga, the Ob6, the Amour, the Orinoko, the Gan- ges, the Euphrates, the Danube, the Dneiper, and the Dwina. However, if we class the rivers according to the length of country through which they run, the order will be somewhat different. Taking the length of the Thames for 1, the Maranon will be 15 ; the Kian Ku, 15| ; the Hoang-ho, 13 ; the Nile, 12| ; the Lena, llf ; the Amour, 11 ; the Ob, lOf ; the Jenisei, 10; the Gan- ges, Bnrrampooter, Ava, and Wolga, each 9| ; the Eu- phrates, 8f ; the Mississippi, 8 ; the Danube, 7 ; the In- dus, 5|; and the Rhine, 5 J. The total population of the earth has been variously estimated, at from 400 to 1000 millions. Mr. Carey, a missionary in the East Indies, considers it to be 731 millions ; of which there are, of Pagans, 420 millions ; of Mahometans, 130 millions ; of Roman Catholics, 100 millions ; of Protestants, 44 millions; of Greeks and Armenians, 30 millions ; and of Jews, 7 millions. Ta- king the population at 800 millions, and considering this entire number to be renewed every 32 years, 25 millions of human beings must, upon an average, die and be born every year; which is equal to 3000 every hour, or 50 every minute ! What must have been the aggregate number since the Creation! Hotv eminently does such a statement excite onr admiration and astonishment at the goodness of the Almighty, in bringing into existence so many myriads of beings, most of them for purposes of happiness and enjoyment here, and, through his di- GEOGRAPHY. 89 vine mercy, for eternal felicity hereafter ! Another calculation gives the superficial contents of the earth at 199,512,595 square miles ; of which 4,456,065 are apportioned to Europe; 10,768,323, to Asia; 9,654,807, to Africa ; 14,110,874, to America; and 160,522,026, to seas and unknown parts. Mr.Bristed, in his recently published table of Population and Territory of the present civilized world, gives to China 200,000,000 of population, and 1,200,000 square miles of terri- tory ; to Great Britain, 20,000,000 of population, and 100,000 square miles ; and, to the United States of Ame- rica, 10,000,000 of population, and 25,000,000 miles of territory; so that the United States have the largest home territory of all the nations, except Russia. The entire population of the world, Mr. Bristed estimates at 435,800,000 ; and of territory 9,687,000 square miles. Britain possesses 150,000,000 of subjects in her colonial empire, and covers a dominion equal to nearly one fifth of the whole surface of the globe ! It is considered that, in Europe, there are 34 inhabitants to each square mile ; in Asia, 36; in Africa, 16; and in America, 3: in Rus- sia, 17 j in Italy, 170 ; and in the Netherlands, 275. This great disproportion arises from the difference with respect to climate, agriculture, commerce, &c. I shall close this letter, my dear boy, with a brief no- tice of the different families of man, which are scattered over the earth. You will not fail, however, to bear in mind, that the whole human race sprang from one stock, as recorded in scripture ; and that, in the opinion of the wisest philosophers, the differences which we perceive are merely the effects of habit, climate, aiid education. I 2 90 GEOGRAPHY. The Laplanders, the Esquimaux, the Samoi'eds, the Greenlanders, the Nova-Zemblans, and the Kamstcliat- kadales, appear to be one family, inhabiting the northern frigid zone. Their stature is diminutive; their com- plexion, a deep brown, approaching to black ; their heads, large; their cheek-bones, high; their eye-lids, drawn aside ; their mouth, large ; their lips, thick and turned outward ; and their voices, thin and squeaking. The Chinese, Japanese, and Tahtars, who inhabit all that vast space of Asia, from the Great Ocean to the Caspian Sea, constitute another family. They are thick and short in stature, with olive complexions: their foreheads are broad, and their chins narrow ; with high cheek-bones; short and flat noses; small sunken eyes; and large and separated teeth. The black and swarthy inhabitants of India, and of the islands of the Indian ocean, have European features; long, black, straight, hair; and slender shapes. The American Indians, which spread over the whole of the new continent, are of a dark copper colour; ha- ving black hair ; small black eyes ; in many instances, flat noses ; and high cheek-bones. After the Indians may be mentioned, as the most re- markable of any, the negroes of Africa. Their com- plexion is black; their heads, woolly ; their noses, flat; and their lips, thick. The European nations the English, French, Ger- mans, Italians, Spaniards, &c. appear to derive their origin partly from the antieut Swedes and Goths, dis- tinguished by blue eyes and light hair ; and from the Celts, characterized by black hair and black eyes. GEOGRAPHY. 91 The greatest improvements in modern geography have been made within the last fifty years. The illustrious Captain Cook, in his three successive voyages, Vancou- ver and Perouse on the north-west of America, with Ross and Buchan in the north, have altogether thrown a light on this science which could not have been antici- pated. Within the same period, both France and Eng- land have produced many splendid specimens of land- maps, on a most extended scale ; and there is reason to expect that, in half a century more, geography will be nearly complete. Guy's School Geography, Goldsmith's Geography for Yonng Persons, Pinkerton's Geography, Bigland's View of the Worl J, D'Anville's Maps of Antient Geography, &c. are some of the most useful publications relating to this science. I am, &c. LETTER XL HISTORY AND CHRONOLOGY. " History Is philosophy, teaching by example." Bolingbroke* HISTORY and Chronology are so intimately connected, that I shall consider them as one subject, and shall dis- pose of that subject, interesting and important as it is, in a single letter. History, in the general sense of the word, signifies a true relation of facts and events. It is the office of the historian to trace the progress of man, from the savage state, and through the intermediate degrees of civiliza- tion, to the nearest approach to perfection of which social institutions are capable. It falls within his pro- vince, to note the effects of laws and political regulati- ons, and to record the revolutions which have been pro- duced in states by external violence, or by the gradual corruption of antient systems of government. Thus, in a moral point of view, history is eminently useful, as it points out the issues of things, and exhibits, as its gene- ral result, the reprobation consequent upon vice, and HISTORY. 93 the glory attendant upon virtue. History may also be regarded as the hand-maid of religion as opening to us the most wonderful prospects of the Divine interposi- tion in the government of the world. With respect to the nature of its subjects, history may be divided into general and particular ; and, with respect to time, into antient and modern. General his- tory relates to nations, and to events of a public nature, connected with them: particular history refers to indivi- dual countries, or particular periods. Antient history commenced with the creation of the world, and extends to the reign of Charlemagne, in the year 800 of the Christian era. Modern history begins at that period, and comes down to the present day. General history may be sub-divided into sacred, ecclesiastical, and pro- fane. Particular history consists of biography, or the account of the lives of eminent persons, memoirs, let- ters, &c. History may further be considered as resolving itself into certain periods, at each of which a great revolution occurred, either with respect to the world at large, or to one or more of its considerable parts. Theirs* of these periods comprehends transactions from the creation of the world to the deluge. In the course of that period, men made some progress in agriculture and the useful arts. The second period, commencing at the deluge, and extending to the beginning of profane history, in- cludes the attempt to build the Tower of Babe), the history of Noah's sons, the foundation of the kingdoms of Babylonia and Assyria, the migration and history of the Israelites, the history of the Greeks, and their ex- 94 HISTORY pedition against Troy, and the founding of Carthage and Rome. The third period commences about the year 668 before Christ; and includes the destruction of the kingdom of Assyria by the Medesand Babylonians, and the overthrow of the kingdom of Judaea by Nebuchad- nezzar. The fourth period, embracing only 31 years, includes the conquest of Babylon by'Cyrus, in the year 538 before Christ. In the fifth period, the Jews, under Cyrus, returned to their native land, re-built their tem- ple, and re-established their worship. The reign of Alexander the Great, and the overthrow of the Persian empire, are events of this period. The sixth period comprehends the rise and progress of the four empires, which bad arisen out of the vast empire of Alexander, and the history of the exploits of the Romans and Car- thaginians, till the fall of Carthage, about 150 years be- fore the birth of our Saviour. The seventh period in- cludes the conquests of the Romans, till the termination of the Roman republic ; the conquest of Britain, by Claudius ; the destruction of Jerusalem, by Vespasian ; and the death of Trajan. Theeighth period extends from the death of Trajan, to the division of the empire under Constantine. The ninth period displays the decline and fall of the western portion of the Roman empire, with several important revolutions in France, Spain, Italy, and Britain. It is deserving of remark, that, during this period, Africa changed its masters three times : the Vandals expelled the Romans, and erected an indepen- dent kingdom; that kingdom was overturned by the emperors of Constantinople ; and, from those emperors, it was takvu by the Goths, in the 620tb year of the AND CHRONOLOGY. 95 Christian era. The tenth period, commencing from tlie flight of Mahomet, in the year 622, embraces the rise, progress, and victories, of that impostor, and the fall of his empire ; the conquest of the Turks and Saracens, and the establishment of the Pope's temporal power, ex- tending to the time of the Crusades. The eleventh period commences with the Crusades, and comes down to the present time. It is probable, however, that future his- torians will date the commencement of a. twelfth period from the French Revolution, in 1789; an event which has materially influenced the fate of every power in Europe, and of many states in more distant regions. From what I have already said, it will be evident to you, my dear boy, that the student of history must make himself master of the details of Geography, the princi- ples of statistical calculations*, and the minutice of chro- nological researches. He ought also to be animated by a spirit of philosophical inquiry, that may distinguish truth from falsehood, and be able to deduce useful con- sequences from the facts which pass in review before him, in the obscure records of former times, or amidst the misrepresentations of factious malignity. For infor- mation on the subject of sacred history, he must apply to the scriptures, as the highest authority ; to the works * Statistics, a branch of study, which, of late years, has been much cultivated in Germany, comprises the actual power of nations ; the nature of their governments ; the state of their civi- lization, sciences, and arts ; their natural and artificial advanta- ges ; their population, produce, commerce, and relative import- ance on the scale of political greatness. 96 HISTORY of Josephus ; and to tlie annals of Archbishop Usher. The branch of general history, next in importance, is that of antient Greece, which exhibits a nation of heroes, philosophers, poets, orators, historians, and ar- tists, who spoke the noblest language that ever graced the tongue of man ; and who have been the guides and the instructors of all succeeding nations. Among the Greek historians, most entitled to notice, are Thucy- dides and Polybins. The history of the Romans next demands our atten- tion, in the order of excellence, as well as in that of time. The military prowess of those people, with their skill in the arts, next to that of the Greeks, renders them the boast of history and the glory of mankind. Livy and Tacitus are amongst their most eminent historians. The history of England abounds with such events and transactions, and displays such characters and actions, as cannot fail to excite the most powerful interest in your bosom. Clarendon, Robertson, Henry, and Hume, are some of our leading historians j and, by Turner, in his History of the Anglo-Saxons, a flood of light is thrown upon the early peiiods of our history. One of the most useful compendiums of antient and modern history is that of Dr. Mavor. Ty tier's Elements of General History, upon a much smaller scale, is also a valuable book. In the study of history, Chronological and Historical Charts and Tables, such as Blur's, Priestley's, Guy's, &c. are highly serviceable. The ex- amination of coins, medals, inscriptions, gems, and sta- tues, is also extremely useful, in developing the progress of antient arts, in determining dates, and in ascertaining AND CHRONOLOGY. 97 many curious particulars, respecting characters, instru- ments, buildings, ami ceremonies. It has, indeed, been well observed, ihat coins are, to general history, what miniatures are to historical pictures ; and that, when ar- range,d in exact order, they form a chronological abstract of an emblematic and lively kind. CHRONOLOGY, to which I shall now particularly re- quest your attention, is the science which treats of time, and the method of measuring its parts, and adapting them to past transactions, for the illustration of history. Its first division relates to the proper measurement of time, and the adjustment of its parts ; the second fixes the dates of the various events recorded in history, and arranges them according to the several divisions of time, in the order in which they happened. Thus, by the aid of this science, we are enabled to calculate the rise and fall of empires, the length of lives, and the dates of all remarkable occurrences in remote periods. Chronology includes dates, periods, epochas, and eras. A date is a relative mark of time, reckoned from some fixed period ; a period, from which marks of time refer- able to events are reckoned, is called an epocha ; the reckoning of time from the epocha is called the era of the epocha ; and the date is the part icular year of the era. Thus, you will perceive that an epocha and an era differ from each other, in chronology, as much as a point differs, in geometry, from a line which is drawn from it. For example, the birth of Christ is an epocha, or point : the years reckoned from that epocha, or point, constitute the Christian era, or line. K 98 HISTORY Different modes of computing time have been adapted by different nations. The most antientis that of Moses. In his description of the Deluge, he calculates by months, consisting each of thirty days ; and by years, consisting of three hundred and sixty days each*. According to Herodotus, the Egyptians reckoned in the same manner, and from them, it is probable, Moses adopted his sys- tem. The Greeks calculated by Olympiads. An Olym- piad is a space of four years, after the expiration of which, that is, in the fifth year, games in honour of Jupiter Olympius -were celebrated with great pomp near * The proof of this, by which the details of Scripture, rela- tive to the deluge, in Genesis, chap. vii. amount exactly to the sum total in Genesis, chap. viii. v. 13, is thus satisfactorily made out : 2 months 17 days. The time when the fountains of the deep were dried up. 40 Continuance of rain. 40 Increase of the deluge. 150 Its continuance. 40 Its decrease. 7 The dove sent from the ark the first time. 7 The dove sent out the second time. 301 days divided by 30 = 10 months 1 day. Add the 2 months 2 12 months 1 day. That is, the frat month in, the first day of the month, as sta- ted in chap. viii. v. 13, when the face of the ground was dry, and the waters had entirely subsided. AND CHRONOLOGY. 99 Olympia, a city in Peloponnessus. These games were fully established in the 3228th year of the world, 766 years before Christ. This mode of computation appears to have ceased after the 364th, which ended A.D. 440, as we have 110 further mention of them in history. The Romans reckoned by lustra, periods of five years, so de- nominated from a tax which became due every fifth year. Their more usual mode of computation, however, ap- pears to have been, by the years which had elapsed from the building of the city, anno urbis condtte, expressed briefly by the letters A. U. C. This event took place A.M. anno mundi, or in the year of the world, 3252; A.C. ante Christum, B.C- or before Christ, 752*. The gene- ral mode of reckoning the years of the world is, to take 4004 B.C. for the epoch of the Creation, which is adopted from the Scriptures. Christians compute from the most memorable of all epochs, the birth of Christ, wliitfi hap- pened in the 27th year of the reign of Augustus, A.U.C. 749*. The Turks compute from the Hegira, or flight of Mahomet from Mecca, which happened A.D. 622, when Heraclius was emperor of the East. The Julian, or old style, is so named from Julius Caesar, who regulated the Roman calendar. He added a day immediately after * There is a slight difference, of four years, between the true epocha of our Saviour's birth, and that assumed in the vul- gar era ; the true epocha having been found, upon examination, to be four years earlier than the common reckoning supposes it to be. So that the true date for the present year, 1819, of the vulgar Christian era, would be 1823. This accounts for the disagreement between the two dates, to which this note refers. 100 HISTORY the 24th of February, called by the Romans the 6th of the Kalends of March : thus it was reckoned twice ; auu the year, in which it was introduced, was termed Bis- sextile, or, what we call Leap-year. This calendar was re- formed A.D. 1582, by Pope Gregory XFII. The Julian year was too long by nearly 11 minutes, an excess which amounts to about 3 days in 400 years. The Pope, there- fore, by the advice of able astronomers, ordained, that a day, in every three centuries out of four, should be omitted; so that every century, which would otherwise be a bissextile year, is made to be only a common year, excepting only such centuries as are exactly divisable by 4, which happens once in 4 centuries. This reformation of the calendar, which is now observed in every Euro- pean state, except Russia, commenced in the countries under Papal influence on the 4th of October, 1582, when 10 days were omitted at once, which had been over-run since the council of Nice, A.D. 325, by the overplus of 11 minutes each year. In England, this new style commenced only in 1752, when 11 days were omitted at once ; the 3rd of September, in that year, being reckoned the 14th, as the surplus minutes had then amounted to 11 day?. The Christian clergy, it should be remarked, formerly began the year on the 25th of March; a method observed in England, particularly in civil affairs, until this alteration of the style in 1752, when, our year commenced on the 1st of January. Be- fore that time, it was usual to write the double date thus, February 13, 1744-5, or thus, February 13, 174f, from the 1st of January to the 25th of March, for February 13, 1745. The calendar, thus reformed, comes very AND CHRONOLOGY. 101 nearly to the accuracy of nature ; for, by an Act of Par- liament, in the 24th of George II. which directed it to be observed, it is ordered that Easter Sunday, on which the other moveable feasts depend, is always the first Sunday after the full moon, which happens upon, or next after, the 21st of March ; and, if the full moon hap- pen on a Sunday, Easter day is the Sunday after. The greatest difficulty in Chronology has been to ac- commodate the methods of computing time, by the course of the moon and of the sun, to each other; the nearest division of the your, by months, being 12, and yet 12 lunar months falling short of a complete year by 11 days. This gave rise to many cycles among the anti- ents : that which approached the nearest to correctness was called the Metonic, from Melon, an Athenian as- tionomer. It consisted of 15 years, in which 7 months were intercalated. This brought the two methods so near to an agreement, that, after the expiration of the pe- riod, not only do the new and the full moon return on the same day of the year, but very nearly at the same hour. This cycle was adopted at the council of Nice, for the purpose of settling the time for keeping Easter. This period, however, falling short of 19 years, almost an hour and a half, it happens that the real new and full moons have anticipated those in the calendar of our books of Common Prayer 4| days. These last are, therefore, called calendar new moons. Connected with this branch of the science, you will find the following rules very useful : To find whether any given year be leap-year. Divide the K 2 102 ^ HISTORY given year by 4 ; if remains, it is leap year ; but if 1, 2, 3, re- main, it is so many years after. To find the dominical letter before 1800. To the given year, add its fourth part, omitting fractions ; divide the sum by 7 ; the remainder taken from 7 leaves the index of the letter in the reckoning of the common years. 12 3 4 567 A B C D E F G But in leap-years this letter and its preceding one (in the re- trograde order which these letters take) are the dominical letters. The dominical is that which points out in the calendar the Sundays throughout the year ; thence called the Sunday letter. To know on what day in the week any proposed day of the month will fall. First find the dominical letter, then the day of the week the first of the proposed month falls upon, which may be known by these lines : At Dover Dwell George Brown, Esquire, Good Christopher Finch, And David Fryer. Where the first letter of each word answers to the letter belong- ing to the first day of the months in order from January to De- cember. To find the year of the solar, lunar, and golden, number, and indiction cycles. To the year add 9 for the solar, 1 for the lunar, 3 for the indiction : divide the sums by 28, 19, and 15, the remainder in each denotes the years of its respective cycle. The solar cycle is a period of 28 years, in which time all the varieties of the dominical letters will have happened, and the 29th year of the cycle begins again, when the same order of the letters will return At the birth of Christ, nine years had pas- sed in this cycle. The lunar cycle, or golden number, is a period of 19 years, containing all the variations of the days on wliich the new and full moons happen; after which time, they AND CHRONOLOGY. 103 fall on the same day again. But when a centesimal or hun- dredth year falls in the cycle, the new and full moon, according to the new style, will fall a day later than otherwise. The birth of Christ happened in the 2d year of the cycle. The Roman in- diction is a cycle of 15 years, which first began the 3d year be- fore Christ. Tojind the epact, till A.D, 1900 Multiply the golden number for the given year by 1 1 ; divide the sum by 30 ; and from the remainder take 1 1 , which leaves the epact. If the re- mainder be less than 11, add 19 to it. To Jind the moon's age. To the epact, add the number and day of the month ; the sum, if under 30, will be the moon's age ; but, if it exceed 30, the excess in months of 31 days, or the ex- cess above 29 in a month of 30 days, is the age. The moon's age taken from 30, leaves the day of the next new moon. When the solar and lunar cycles begin together, the moon's age, in the first of each month, or the monthly epacts, are called the num- bers of the month, thus, Jan. Feb. Mar. April. May. June. 2 1 2 3 4 July. Aug. Sept. Oct. Nov. Dec. 5 6 7 8 9 10 Tojind when Easter day will happen. See on what day of March the new moon falls nearest the 21st in common years, or the 20th in leap years ; then the Sunday next after the full, or 15th of that new moon, will be Easter-day. If the 15th fall on a Sunday, the next is Easter-day. You are now, ray dear boy, in possession of nearly every thing that can be serviceable in the science of chronology, independently of charts and tables, to which I have alluded in a former part of this letter. The two 104 HISTORY following General Views of Antient and Modern History* are copied, with a slight variation, from a valuable little work, entitled, " The Bioscope, or Dial of Life, ex- plained," by Granville Penn, Esq. There are some lit- tle discrepancies between them, and the periods of his- tory which I have described ; notwithstanding which, you will find them of material assistance, at the com- mencement of your historical and chronological studies. I shall conclude, with giving you the dates of some important events in antient and modern history ; which, if committed to your memory, will be very serviceable. Creation A.C. or B.C. 4004 Deluge 2348 Call of Abraham 1921 Departure from Egypt 1491 Troy taken by the Greeks 1183 Building of Solomon's Temple 1012 Rome founded 753 Death of Cyrus 526 Battle of Marathon .". 490 Death of Socrates 396 Death of Alexander the Great 323 Destruction of Carthage 14f> Julius Caesar slain by Brutus 44 London founded by the Romans A.D. 49 Christianity first preached in Britain 63 Destruction of Jerusalem by Titus 70 Commencement of the Eastern Empire 339 Christianity established in France 49 winch instantly embraces the full meaning of a proposition, and penetrates the ambiguity of a sophism, is the first requisite of a sound logician ; but quickness of perception requires to be accompanied by soundness of judgement ; and sound- ness of judgement must be derived, either from our in- tuitive knowledge of first principles, which is the gift of nature ; from experience, which is acquired by time and observation; or, from the testimony of others. The methods of reasoning, or argumentation, may be reduced to two ; syllogism, and induction. A syllogism is an argument, containing three propositions ; the two first of which are laid down, as axioms, from which the third, or conclusion, is deduced. The previous proposi- tions are divided into mqjor and minor. Thus, let the LOGIC. 109 question be, Whether God be an intelligent Being? Here the major, or principal proposition, proceeds from the word intelligent, and the minor relates to God. The syl- logism may then be arranged, as follows : Maj. To dispose things in right and perfect order, is the work of an intelligent being. Afin. God has disposed creation in right and perfect order ; Conclusion. Therefore, God is an intelligent being. Propositions, you will remember, may be either simple or compound, universal or particular, affirmative or ne- gative, absolute or conditional ; and they may be com- bined and arranged in the various forms of epichirema, di- lemma, sorites, enthymem, &c. The epichirema is an argu- ment which contains the proof of the major and the mi- nor, or both, before it draws the conclusion. Cicero's oration for Milo may be reduced to this figure: "It is lawful for a person to kill those who lie in wait to kill him, as is allowed by the law of nature and the practice of mankind : Clodius laid in wait for Milo, with that intention, as appears from his guard of sol- diers, and his travelling armed; therefore, it was lawful for Milo to kill Clodius." The dilemma divides the whole argument into all its parts or members, by a dis- junctive proposition, and then infers something con- cerning each part, which is finally inferred concerning the whole. Thus Cicero argues, to prove that all pain ought to be borne with patience : " All pain is either violent or slight: if slight, it may easily be endured ; if violent, it will certainly be short ; therefore, all pain ought to be borne with patience." In the sorites, seve- L 110 LOGIC. ral middle terms are used to connect each other succes- sively, in several propositions ; till the last proposition connects its predicate with the first subject. Thus Themistocles, attempting to show that his little son governed the world, says : " My son governs his mo- ther, his mother governs me ; I govern the Athenians, the Athenians all Greece ; Greece governs Europe, and Europe the world; therefore, my son governs the world." In an enthymem, one of the premises is expres- sed, while the other is reserved in the mind ; thus : ' There is no true religion without good morals ; there- fore, a knave cannot be truly religious." The method of reasoning by induction is more simple, and of more general use. It consists in distributing a general idea into its species, and ascribing to the whole the property found in the species ; or rather, in inferring one truth, or proposition, from another. On most subjects, there are also two methods of rea- soning, termed the synthetical, and the analytical. In the former, we rise gradually from first principles to the sublimest truths : iu the latter, we reduce sublime and compound truths to their first principles. Thus the architect lays his foundation in the earth, and rears the summit of his building to the skies ; whilst the chemist, by analytical process, reduces compound substances to their simple elements. Pormerly, this science, wrapped up in technicality and obscurity, formed nearly the whole of a university edu- cation. Of late years, it has been, in a great measure, very judiciously superseded by the practice of rea- aoning, in the study of mathematics, by the various LOGIC. Ill branches of philosophy, and by the pernsal of the clas- sics. Watts's Logic is perhaps the best introductory book upon this subject. Locke's Essay on the Human Understanding presents an analysis of the human mind, and constitutes the basis of all that is known of genuine logic. In the words of a celebrated writer, " Mr. Locke has displayed the human soul in the same manner as an excellent anatomist explains the springs of the human body. He always takes the light of physics for his guide. He sometimes presumes to speak affirmatively, but then he presumes also to doubt. Instead of conclu- ding at once what we know not, he examines gradually what we would know. He takes an infant at the in- stant of his birth, and traces, step by step, the progress of his understanding." I am, yonr's, &c. LETTER XIII. ONTOLOGY, OR METAPHYSICS. OF the subtle, indefinite, and evasive, science of On- tology, or Metaphysics, I can convey to you only a very faint idea, in the circumscribed limits of a letter. Some writers define metaphysics to be a science which refers to spirits and immaterial beings, and is otherwise called pneumatology. Others, who adhere to the etymology of the word, explain metaphysics by transnatural, pre- ternatural, or postnatural, philosophy. Some, perhaps with more propriety, confine metaphysics to what is termed ontology, i. e. the doctrine or science of being, in the abstract. By a writer, deeply read in antient philosophy, metaphysics has been defined as the science of the principles and causes of all things existing. Hence it is, that mind, or intelligence, especially the supreme intelligence, which is the cause of the universe, and of every thing which it contains, is the principal subject of this science. Being is generally divided into matter and spirit. The sacred writers, however, speak of body, soul, and spirit ; thus dividing the human constitution into three distinct ONTOLOGY. 113 parts. Pure spirit can, perhaps, be predicated only of the Deity ; for all created spirits seem to have relation to time and place ; that is, they have no existence, till created ; and, when called into being, cannot exist in two places at once ; nor can two spirits exist in the same individual place. We are accustomed to form our ideas of spiritual being, by subtracting the known pro- perties, of matter, and leaving only that of thought, or consciousness ; and, as we can conceive of no property but as existing in some substance, we call this, immate- rial being. Regarding the terms in a philosophical sense, the essential properties of matter are extension and so- lidity } for we can conceive of no material being with- out extended form or shape, and density or adhesion of its parts. Other properties have been added ; but it is doubtful whether they are equally essential. It is, how- ever, extremely difficult to conceive that thought can be a property of matter ; and revelation, which teaches man that he possesses an immortal soul, assures us that it is not. The soul is defined to be a spirit, proper to inform and animate an organized body ; by its union with which a leasonable animal is constituted. Philosophers have not agreed in what part of the body the soul re- sides : some suppose it to be equally diffused through every part ; others, that it influences all parts, but has its principal residence in some particular one. Des Cartes maintains its residence to be in the pineal gland of the brain, where all the nerves terminate. Of what the soul may be capable, when severed from the body, it is difficult to conceive. By some, who have reasoned it 1 14 ONTOLOGY, from analogy, man has been compared with the humble caterpillar. " When arrived at its proper age, it sleeps, as it were, the sleep of death, losing all its powers of action or sensation for a season, till, at length, it bursts its shell, stretches its wings, and becomes the inhabitant of another element. Such may be the enlargement of the soul at death such the deliverance of the body at the resurrection." All this, however, is merely conjec- tural. The principal faculties of the soul are, the understand- ing, the tcill, and the memory ; to which, unless they should be distinctly classed, may be added the passions. Understanding, or the power by which we perceive, compare, and reason, precedes intelligence and knowledge; thus we say, a man of tolerable understanding, of great intelligence, of consummate knowledge. Whatever is perceived by the mind is an idea ; and the comparison of these ideas is that, process which we call reasoning. The Sources of our ideas are, sensation and reflexion. Sensation is the perception of an object, by the organs of sense, as, hearing, seeing, feeling, &c. Reflexion is the result of the mind's perception of its own faculties and operations. The trill, says Malebranche, is to the soul, what mat unt is to the body. According to Locke, it is that faculty which the soul has of beginning, forbear- ing, continuing, or ending, various actions of the mind and motions of the body, merely by a thought or prefer- ence of the mind. The memory is a faculty of the mind, whereby it retains the ideas which it has once received. By retention, we deposit and preserve, in the seat of memory, such ideas, conceptions, and facts, as have OB METAPHYSICS. 115 been communicated to the mind ; so that, though they be not always present to the perception, we are able to recal them on particular occasions. Recollection is that power by which we call forth, to our mental perception, some portion of that knowledge which has been accumulated by memory. This is effected by a direct effort of the mind ; or, more usually, by the association of ideas. Attention, or a voluntary exertion of our mental powers, and repetition, materially assist in the fixing of ideas in our memories ; but ideas, which make the deepest and most durable impressions, are those which are associated with pleasure or pain. Dr. Watts defines the passions to be l< sensible com- motions of our whole nature." Objects will excite various passions in the mind,' according as they appear good or evil, present or future, certain or doubtful. Thus, good, or what is considered to be good, if present, excites love and joy ; if future, hope ; and, according to its degree of certainty, confidence or doubt. On the other hand, evil provokes hatred and grief, fear and horror. All the other passions may be traced to the same sources of good or evil, pleasure or pain, variously considered, associated, and combined. In the proper management of the passions consists almost all human wisdom. The state called pension is violent and transi- tory j emotion is less so ; and affection is the least violent, and most permanent. Thus we distinguish between the lowest and highest degrees of feeling, by the terms pas- sion, emotion, and affection, which are always employed to express the sensible effects of objects, or ideas con- cerning them, on the mind. The five grateful passions, 116 ONTOLOGY. ai they have been termed, of love, desire, hope, joy, and pleasing recollection, increase each other ; as do the five ungrateful ones of hatred, aversion, fear, grief, and dis- pleasure. Harris's Philosophical Arrangements, Dr. Reid's Es- say on the Intellectual Powers of Man, Hartley's Obser- vations on Man, Beattie's Essay on Truth, and Dugald Stewart's Essay on the Human Mind , are all valuable works, relating to, and connected with, this science. I am, your's, &c. LETTER XIV. MATHEMATICS: Arithmetic, Algebra, Geometry, and Trigonometry. MATHEMATICS, according to the strict and modern definition of the term, constitute the science of quan- tity, either as subject to measure or number. Their various branches are adapted to the common uses of life, and to the deepest and most abstract speculations. " I have mentioned mathematics," observes Mr. Locke, "as a way to settle in the mind a habit of reasoning closely, and in train : not that I think it necessary that all men should be deep mathematicians; but that, having got the way of reasoning, which that study necessarily brings the mind to, they might be able to transfer it to other parts of knowledge, as they shall have occasion." Mathematics are either pure or mixed: the former consider quantity abstractedly, without regard to mat- ter, or particular bodies, as arithmetic, algebra, geome- try, and trigonometry; the latter treat of quantity, as subsisting in bodies, and, consequently, they are inter- mixed with the consideration of physics, or experi- 118 MATHEMATICS. mental philosophy, as, mechanics, pneumatics, hydro- statics, optics, astronomy, navigation, &c. My observations upon ARITHMETIC, with which you are already acquainted, shall be very brief. Its five fun- damental rules are, notation, addition, subtraction, mul- tiplication, and division. Out of these, for the purposes of science and commerce, have arisen vulgar and decimal fractions, the extraction of the square and cube roots, logarithms, &c. The method of reckoning by tens, which probably arose from the obvious mode of counting by the fingers, was in use with the Egyptians, Greeks, and Romans. The present mode of computation, and its characters, were introduced into Europe by the Saracens, when they had possession of Spain, in the tenth century. ALGEBRA is an Arabic word, supposed to be derived from the particle al, and gcbcr, which signifies the reduc- tion of broken numbers to a whole number. Properly speaking, it consists of two parts : first, the method of calculating magnitudes or quantities, as represented by letters, or other characters ; secondly, the manner of applying these calculations in the solution of problems. This science proceeds by rules and operations similar to those of common arithmetic, and is founded upon the same principles. It has, however, great advantages over common arithmetic, in its abstract mode of resol- ving questions ; and, by the concise method in which its problems are worked, it spares the labour and prevents the perplexity of long and intricate calculations. Its principal characters arc as follows -. MATHEMATICS. 119 more, as 3 + 4, is 3 added to 4. less, as 4 3, is 3 taken from 4. I am, your's, &c. LETTER XV. MECHANICS. MECHANICS constitute that branch of mixed inathe - matics, which treats of forces, motion, and moving pow- ers, with their effects in machines. It is divided into practical and rational : the former treating of the me- chanical powers and their various combinations ; the latter containing the whole theory and doctrine offerees, with the motions and effects thereby produced. The first step, in the study of this science, is to ac- quire a knowledge of the nature of motions and forces ; so far, at least, as will give clear ideas of the various terms by which they are expressed. 1. Body is a mass, or quantity, of matter. 2. Matter is a substance, the object of our senses, always having length, breadth, and thickness. 3. Farce is a power exerted on a body to change its state of motion, or rest. If it act instantaneously, it is called percus- sion, or impulse ; if constantly, it is an accelerative force. 4. Motion is a continual and successive change of place. If the body move equally, the motion is called equable, or uniform. If it increase or decrease, it is called accelerated or retarded motion. When it is compared with some body at rest, it is M 2 126 MECHANICS. called absolute motion ; but, when compared with other bodies in motion, it is called relative motion. 5. Velocity is a property of motion, by which a body passes over a certain space in a certain time ; and it is greater or less, as the body passes over a greater or a less space in a certain time. Thus, if a machine move 20 or 15 miles in an hour, it moves with a velocity of one mile in 3 or in 4 minutes. 6. Direction of motion is the time in which the body tends to move. 7. The momentum of a body, or the quantity of its motion, is the motion which a body has, considered both in regard to its velocity and quantity of matter. Thus, if two balls of equal weight be so struck, that the velocity of one be double that of the other, the force of the swifter will be double that of the slower. 8. Inertia is the innate property of matter, by which it re- sists any change, striving to preserve its present state of rest or motion. A body can never put itself in motion. A bowl on the ground, or a cannon-ball going through the air, passes from a state of motion to a state of rest, by the friction of the earth, by its own weight or gravity, or by the resistance of the air. 9. Gravity is the force with which a body endeavours to fall downwards. It is called absolute gravity in empty space, and relative gravity when immersed in a fluid. Specific gravity is the greater or less weight of bodies of the same magnitude, or the proportion between their weights. The centre of gravity is a certain point of a body, upon which the body wheu suspended will rest in any position. Thus, if I balance a stick on ray fin- ger, I know that the centre of gravity rests upon the finger. 10. Equilibrium is the balance of two or more forces, so as to remain at rest. 11. A murlihn; or engine, is any instrument to move bodies, and is constructed of levers, wheels, &c. MECHANICS. 127 12. Friction is the resistance which a machine suffers by its parts rubbing one against another. In treating of this science, it is necessary at first to take some things for granted, which are not strictly true ; and, after the theory has been established, to make the proper allowances for them. 1. That a small portion of the earth's surface, wliich is sphe- rical, may be considered as a plane. 2. That all bodies be supposed to descend in lines parallel to each other. 3. That all planes be considered as perfectly smooth ; levers to be inflexible, and without thickness or weight ; cords, per- fectly pliable ; and machines, without friction and inertia. The mechanical powers, of which I shall now proceed to give you some idea, are certain simple machines, which are used for the purpose of raising greater weights, or overcoming greater resistances, than could be effected by the natural strength of man. They are six in num- ber, viz. the lever, the wheel and axle, the pulley, the inclined plane, the wedge, and the screw. They may, per- haps, be reduced to two ; for the pulley, and the wheel and axle, are only assemblages of levers, and the wedge and the screw are inclined planes. A lever, the simplest of all the mechanical powers, is an inflexible bar of wood, iron, &c. which is used chiefly to raise large weights : it is supported by a prop, or fulcrum, on which all the other parts turn, as the centre of motion. From the different modes of using the lever, it is called a lever of the first, second, or third, kind ; 128 MECHANICS. viz. -of the first kind, when the weight is on one side of the prop, and the power on the other. Thus, a poker, when employed to stir a fire, is a lever ; the bar of the grate being the fulcrum; the coals, the weight to be moved ; and the hand, the power. Pincers, shears, and snuffers, are composed of two levers of this kind. A lever of the second kind has the weight between the prop and the power. Nut-crackers, knives employed in cutting chaff, an oar in the act of moving a boat, are levers of the second kind ; as is every door which turns on hinges ; the hinges being the fulcrum, the door the weight to be moved, and the hand in opening it, the power. A lever of the third kind is, when the power ia between the fulcrum and the weight. A ladder, to be raised by the strength of a man's arms, is a lever of this kind ; the end fixed against the wall being the fulcrum ; the top of the ladder, the weight ; and the power, the strength applied. The bones of a man's arm, and the greatest number of the move-able bones of animals, arc levers of this kind. In this, as in all the other mecha- nical powers, it is a maxim, that the advantage gained is in proportion to the space passed over by the moving power. The wheel and axle is an engine composed of a wheel fixed upon the end of an axle, so that they both turn round together at the same time : the power being ap- plied at the circumference of the wheel, the weight to be raised is fastened to a rope, which coils round the axle. Windlasses, capstans, cranes of all sorts, and those axles which are turned by means of winches, be- long to this power. MECHANICS. 129 A pulley is a small wheel, moveable round an axis, called its centre-pin, with a rope passing over it. Its chief use is to change the direction of the power, from upwards to downwards, and to convey bodies to a great height, or distance, by a person not moving from his place. The inclined plane is formed by planks, &c. laid in a sloping direction, on which large and heavy bodies may be more easily lowered, or raised, than by a mere lift. The increase of power is in the proportion of the length of the plane to its height, when the direction of the power is parallel to the plane. The wedge, which resembles two inclined planes, is very serviceable to drive in below heavy weights ; to raise them only a small height ; to cleave blocks of wood, stone, &c. It is generally used, not by pressure, but by percussion ; as, by the blow of a hammer, or mallet. The screw is a kind of perpetual inclined plane, used with a lever, or winch, to assist in turning it. Thus it becomes a compound engine of great force, either in pressing bodies closer together ; as, in presses for paper- makers, book-binders, packers, &c. or in raising great weights. The advantage gained by this power is in pro- portion as the circumference of the circle made by the lever, or winch, is greater than the distance between the threads of the screw. A screw, acting in a wheel, is called an endless screw. You must remember, my dear Francis, that in the ap- plication of all the mechanical powers, one third must be allowed for overcoming the friction of the surfaces, and the various other obstacles to which all machines are exposed. 130 MECHANICS. The application of these powers to various experi- ments, to compound machines, to the regulation of mo- tions by fly-wheels, to the construction of mills of vari- ous kinds, to clock-work, wheeled-carriages, &c. you will find well explained in the first volume of Imison's Elements. I am, your's, &c. LETTER XVI. PNEUMATICS. STRICTLY speaking, my dear Francis, tke word Pneu- matics means the science which treats of the properties of air in general ; but it is more commonly used to ex- press the mechanical properties of elastic fluids, such as their weight, density, compressibility, and elasticity; and, in its still more restricted sense, as I shall here em- ploy it, it is considered as treating of the nature and properties of the air alone. The air which we breathe is a thin, fluid, transparent, elastic, compressible, and dilatable, body, which surrounds and covers the earth. As it is possessed of weight, in common with all other fluids, it presses upon bodies in proportion to the depth at which they are immersed in it ; and it also presses, in every direction, in common with all other fluids. It differs, however, from all other fluids in the following particulars : 1. It can be compressed into a much less space than that which it naturally possesses. It may, indeed, be condensed into 50,000 times less space than it usually oc- cupies. It is on this principle that air-guns are con- structed, which will kill birds and other animals, at the distance of 200 or 250 feet. 132 PNEUMATICS. 2. The air cannot be congealed, or fixed, as other fluids may. 3. The air is of a different density in every part up- wards from the earth's surface; decreasing in density the higher it rises, till, in the upper regions of the at- mosphere, its weight or pressure is scarcely perceptible. 4. The air is of an elastic, or expansive, nature, and the force of its spring is equal to its weight. The fluidity of air is evident, from the great ease with which bodies pass through it, as in the propagation and easy conveyance of sounds, odours, &c. These effects shew, that it is a body whose parts give way to any force, and, in yielding, are easily moved amongst them- selves. This may be considered as the definition of a fluid. The air is found to exert an equal pressure in all directions, upon whatever is immersed in it ; and its pressure is equal to that of a column of quicksilver, about 29 or 30 inches in height. Upon a common sized man, the pressure is equal to more than 50,000 ll>. weight ; because the surface of his body is about 14 square feet, or 2016 inches ; and a column of quicksilver, 29J inches high and 1 inch square, weighs 15 lb. ; there, fore, the whole pressure will be 2016+1530,240 lb. Thus, you will perceive, that the external pressure of the air would be fatal to our existence, were it not balanced by a counteracting, internal pressure. The equal pres- sure of the air, in all directions, is an effect which could not he produced, but by its perfect fluidity. Of this property, the air can never be deprived ; for whether it be kept for many years confined in glass vessels, be ex- .posed to the greatest natural or artificial cold, or be PNEUMATICS. 133 condensed by the most powerful pressure, it can never be reduced to a solid. The weight of the air is a property which belongs to it as a body ; for gravity, or weight, is a property found in all bodies. Its weight, as I have already observed, is variable : its changes and relative weights are ascer- tained by means of the barometer, an instrument which I have mentioned in my letter on Meteorology ; for as the mercury is lower, or higher, the air is lighter, or heavier. Air being 800 times lighter than water, a cu- bical foot of air is equal to 1000 ounces divided by 800, or equal to 1^ ounce ; consequently, in a room, 25 feet long, 14 feet wide, and 12 feet high, the air will weigh 25 X 14 X 12X1J 5250 ounces, equal to 328 lb. It will be serviceable for you to recollect, according to this example, that the number of cubical feet, contained in any given space, is found by multiplying the length, breadth, and height, into each other, and the product, 4200, multiplied by 1| (because each cubical foot of air weighs ij ounce) gives the weight of the air in ounces, equal to 5.250. Elasticity is the property by which air yields to the pressure of any other body, by contracting itself, and dilating and expanding itself again on the removal or diminution of the pressure. For example, on squeezing a blown bladder in your hand, you will find a sensible resistance from the inclosed air ; and, upon taking off the pressure, the compressed parts of the bladder imme- diately recover their former convexity. It is upon this property that the structure and uses of the air-pump depend. Every particle of air makes a continual effort 134; PNEUMATICS. to dilate itself: thus it acts forcibly against all its neigh- bouring particles, which exert a similar force in return ; but, if their resistance happen to cease, or be weakened, the particle immediately expands to an immense extent. This is shewn by placing thin glass bubbles, or bladders, filled with air, under the receiver of an air-pump; when, upon exhausting the receiver, or pumping out the air, they burst by the force of the air which they contain. So, also, a close flaccid bladder, containing only a small quantity of air, being put under the receiver, swells as the receiver is exhausted, and at length appears quite full. The same thing occurs on carrying a flaccid blad- der to the summit of a lofty mountain. The elasticity of the air exerts its full force in all directions, when it is at liberty; and, consequently, it assumes a spherical figure in the interstices of the fluids in which it may be lodged. The certainty of the principles respecting the nature and properties of air can only be proved by experi- ments. The air-pump, the air-gun, the barometer, and the thermometer, are the most useful instruments to be employed for this purpose. In Imison's Elements, and in Hutton's Mathematical Dictionary, you will find a clear account of their structure and uses, illustrated by plates. Supposing yon to be in possession of these instruments, and to have acquired the information alluded to, I will mention two or three little experiments, which will fur- ther instruct you in the properties of air. The smoke of a candle ascends, because it is lighter than the atmos- pheric air; but, in an exhausted receiver, it will fall to PNEUMATICS. 135 the bottom. Candles will not burn, nor can animals sus- tain life, in an exhausted receiver. Air is necessary to the propagation of sound. The sound of a bell may be beard while it is under a receiver full of air; but, exhaust the receiver, and there will no longer be any sound. The sound of a bell is much louder in condensed air than in common air. If a square glass phial be exhausted of its air, while under a receiver, and the air be suddenly let into the receiver, but so as not to obtain entrance into the phial, the phial will be dashed to pieces by the pressure of the air. Place two brass hemispheres, of three or four inches in diameter, together, the internal air being exhausted, and the pressure from without will require a force equal to 150 pounds to separate them ; but, if the external air be removed, they will separate of themselves. Place a cup, of porous wood, containing mercury, on the re- ceiver of an air-pump ; exhaust the air from beneath it ; and the pressure of the external air will force the mercury, in a shower, through the pores of the wood. Take the air from the pores of a dry piece of wood j then hold it under mercury, and admit the external air into the receiver ; and the pores will become filled'with the mercury. You are aware, that, if a light substance, as a feather, and a heavy one, as a guinea, be dropped together from the hand, the guinea will reach the ground before the feather. This shews the resistance of the air, as a body ; but, if you exhaust a tall glass receiver of its air, and then drop a guinea and a feather from the top, they will reach the bottom simultaneously, the re- sisting medium having been removed. I am,&c* LETTER XVII. HYDROSTATICS AND HYDRAULICS. THE science of Hydrostatics treats of the nature, gra- vity, pressure, equilibrium, and other mechanical pro- perties, of non-elastic fluids, particularly of water. It relates to fluids at rest. Hydraulics, on the other hand, teaches us how to estimate the velocity and force of fluids in motion ; the means of raising them in pumps, &c. Fluids are of two kinds; elastic, as air, steam, and the different gasses ; and nou-elastic, as water, oil, mercury, &c. Fluids press not only, in common with solids, per- pendicularly, but equally in all directions. The surface of different portions of water, which communicate one with another, will be perfectly level while at rest. It is on this principle that water is carried in pipes to any distance, and raised to the same height in which it stands in the reservoir. Springs will rise nearly to the height of the sources, whence they flow. The pressure of water is in proportion to the perpendicular height at which it stands, not to its quantity ; and exerts itself upwards, downwards, and sideways. From this property it is, that, if you bore a hole in the side of a vessel, filled with water, the fluid will spout out. HYDROSTATICS. 137 It is by weighing bodies in water, that their specific gravities are found. Every body that is lighter than water will swim in it, and will displace exactly as much of the water as is equal to its own weight. Thus, you must have perceived that a ship, floating on the water, displa- ces as much of the fluid as is equal to the ship and her lading in weight, and that, if the weight be increased, the ship will sink deeper in the water. Every body that is heavier than water, or that sinks therein, displaces so much water as is equal to the bulk of the body. By specific gravity is understood the relative weight which equal bulks of different bodies have to each other. One body is said to be specifically heavier than another, when, under the same bulk, itcontainsa greater weight. Thus, of two spheres of equal diameter, one of lead, the other of wood, that of lead is specifically heavier than the one of wood. A body suspended in water loses as much of its weight, as is equal to the weight of an equal bulk of water. For example : if a piece of glass weigh, in the open air, 2 lb, and, when suspended in water, it weigh only ] lb, it shews that a quantity of water, equal to the bulk of the glass, weighs 1 lb also. The specific gravities of bodies are inversely as the weights which they lose by immersion in water ; that is, the body which is heaviest, will lose least in water. The specific gravity of any solid body, which sinks in water, may be found, first, by weighing the body in the air, and then in water, and, dividing the first weight by what it has lost in the water, the quotient presents the specific gravity. Thus, if a piece of brass weigh one ounce, or 240 grains, suspend it in water, and you 138 HYDROSTATICS will find it weigh only 210 grains : divide the 240 by 30, the number of grains which it has lost in weight, and the dividend is 8 ; consequently, the specific gravity of the brass is 8 times heavier than water. Pure rain water, in all parts of the globe, is of the same weight : a cubi- cal foot of rain water weighs one thousand ounces, avoir- dnpoise. If the specific gravities are known, their real weights are readily obtained. Thus, if a cubical foot of water weigh a thousand ounces, a cubical foot of stand- ard gold will weigh 17.793 ounces, and a cubical foot of brass will weigh .8000. If the same body, as a piece of ivory or glass, be weighed in different fluids, the specitic gravities of the fluids will be lost. For example, if a piece of glass suspended in water lose 803 grains, in milk it will lose 831 grains ; and in spirit of wine it will lose 699 grains ; consequently, the specific gravities of water, milk, and spirit of wine, are 803, 831, and 699. From various experiments, it has been ascertained, that the specific gravity of a living man is .891 ; and that if the absolute weight of a middle-sized man be 135 Ib, 4 Ib of cork will keep him from sinking, so that he may re- main with his head completely above water. As I have already stated, the loss of weight sustained by a piece of glass suspended in water and milk is respectively 803 and 831 grains ; therefore, the specific gravity of water is to that of milk, as 803: 831 that is, as 1000: 1034, or a cubical foot of the milk, weighs 1035 Ib, or 35 Ib more than the same quantity of water. The following ta- ble shews the specific gravities of the respective bodies mentioned : AND HYDRAULICS. Hydrogen gas 000.105 Common air 001.12 Cork ~ 240 Fir 550 Elm 600 Beech 700 Ash 800 Living Men 891 Brandy 920 Ice 908 Oil 920 Rain water 1.000 Boxwood 1.030 Milk .. . 1.034 Mahogany 1.063 Coal 1.250 Chalk 1.793 Flint 2.570 Glass 3.000 Marble 2J05 Diamond 3.517 Iron 7-645 Copper 9.000 Fine silver 11.091 Lead 11.325 Mercury 14.019 Finegold 19.640 Platina .. ... 23.000 The instruments, most used in Hydrostatics, are the hydrostatic balance, and the hydrometer. The balance, for finding the specific gravities of bodies, differs very little from the common balance, having a hook at the bottom of one of the scales, on which the substances to be examined may be suspended by horse-hairs. The hy- drometer is composed of a ball of ivory or glass, to which is attached a graduated stem ; and, as it siuks less or more in the fluid, the fluid is heavier or lighter. It is on the principle of the science of Hydraulics, that va- rious engines, used in the mechanic arts, many sorts of mills, pumps, and artificial fountains, are constructed. Water can be set in motion only by its own gravity ; as when it is allowed to descend from a higher to a lower level ; or by an increased pressure of the, air. In the former case, it will seek the lowest situation ; and, by increasing the pressure of the air, it may be forced to 140 HYDROSTATICS almost any height, even above its natural level. This is the principle of the syphon, which is a bent tube, one leg of which is longer than the other. The shorter leg is immersed in the liquor which is to be drawn off, and the pressure of the air being taken from that part of the surface of the liquor within the tube, the liquor will rise above its natural level in the vessel, and will flow off through the longer leg. The mode of using a syphon is to fill it with water, wine, or whatever liquid may be thought proper; then, to stop both ends with the fin- gers, and in that state to immerse it in the vessel. On removing the fingers, the water will flow out of the longer leg, by its own gravity, and afterwards by the pressure of the external air on the fluid in the vessel. Many springs are derived from natural syphons, existing in the sides of mountains, &c. at various depths, and to various extents. Under common circumstances, a co- lumn of air ascending to the top of our atmosphere cor- responds in weight with a column of water 33 feet in height, allowing the bases of the respective columns to be equal. Thus, where the air is withdrawn, by suckers, pistons, valves, &c. from within a pipe of which the lower end is immersed in the water contained in a well, the fluid will rise to the height of 33 fVet within the pipe, supplying the place of the air thus withdrawn. This is caused by the pressure of the atmosphere on the surface of the water. The velocity of water, proceeding through a hole in the side of a vessel, is in proportion to the square root of the distance from the aperture below the surface of the water. In consequence, however, of the decrease of that space, as (he water is let out, the pres- AND HYDRAULICS. 141 sure gradually becomes less ; consequently, the mean dis- tance between the surface and the aperture, will be found in general a correct guide.-The pressure of water against the side of a vessel is as the square of the depth. For example, if a vessel be 2 feet deep, the pressure upon the first foot will be 1, and at the bottom 2 X 2 4. It was from their ignorance of the principle, that ' water finds its own level," that the antieuts constructed vast aqueducts, for the conveyance of water across valleys : the same effect is now accomplished by means of stone, iron, leaden, or wooden, pipes. Thus, fluids may, by their own pressure, be conveyed over hills and valleys, in bent pipes, to any height, which is not greater than the level of the spring or reservoir whence they flow. Ar- tificial fountains are constructed on the same principle. The common or sucking pump consists of a pipe which' is open at both ends, supplied with a move-able piston, exactly fitted to the bore. The forcing pump is compo- sed of a pipe, or barrel, a plunger, and two fixed air-tight valves, so disposed as to allow the water to rise freely, but to prevent its return. It is on the principle of the forcing pump, that the engines used for watering of gar- dens, and extinguishing of fires, are constructed. The steam-engine, which has of late years been ap- plied with eminent success to various purposes in the abridgement of labour, consists of a large cylinder or barrel, in which is fitted a solid piston, like that of the forcing pump. The steam, supplied from a large adja- cent boiler, in forcing up the piston, opens a valve, through which cold water rushes, on the principle of the common pump, condenses the steam, and causes the pis- 142 HYDROSTATICS, &C. ton to descend by Us own weight. The action of the piston moves np and down a large beam, which commu- nicatesjo other machinery the power of any number of horses, from one to two or three hundred. The princi- ple of the steam-engine was discovered by the Marquis of Worcester, who published it, in his " Century of In- ventions", in the reign of Charles II. It was improved by Captain Savary, who obtained a patent for it ; and it has been since successively improved by Newcomen, Bolton and Watt, of Birmingham, and others. The power of one of the engines constructed by Bolton and Watt has been thus described, from actual experiment : " An engine, having a cylinder of 31 inches in dia- meter, and making 1? double strokes per minute, per- forms the work of 40 horses working day and night, (for which three relays of 120 horses must be kept) and burns 11,000 pounds of Staffordshire coals per day. A cylinder of 19 inches, making 25 strokes of four feet each per minute, performs the work of twelve horses working constantly, and burns 3,700 pounds per day. A cylinder of 24 inches, making 22 strokes of five feet, burns 5,500 pounds of coals, and is equivalent to the work of 20 horses." Packets, and other vessels, iinpel- ed by the power of steam, have recently been brought into very extensive use, particularly in rivers and canals. The Americans have even constructed some large ships of war, to be impelled by steam. I am, your's, &c. LETTER XVIII. OPTICS. "The eye, in its elevated station, commands the most enlarged prospects. Consisting only of fluids enclosed within coats, it shews us all the graces and glories of nature. How wonderful, that an image of the hugest mountains, and of the widest landscapes, should enter the small pupil I that the rays of light should paint, on the optic nerve ; paint, in an instant of time ; paint, in their truest colours and exactest lineaments ; every species of external ob- jects I" Ferguson. MY DEAR FRANCIS, OF all the branches of mathematical science, astrono- my, perhaps, excepted, that of optics is the most de- lightful, the most calculated, by its disclosure of beauti- ful wonders, to raise the mind of man to the sublimest admiration of his Maker. In offering to your consider- ation this very slight sketch, my only regret is, that I cannot render you sensible of a thousandth part of its interest or utility. Optics explain the manner in which vision is effected ; they assign the causes of the several alterations which the rays of light undergo in the eye; and they shew for what purposes objects appear, at dif- ferent times, greater or smaller, more distinct or con- fused, nearer or remote. 144 OPTICS. This science is generally divided into two, and some- times into three, parts : Dioptrics, derived from >, through, and vtcrofj.au, I see; under which is included whatever relates to the appearance of bodies seen through transparent substances, as fish in water; Ca- t opt rics, from x