>f California Regional Facility SCIENTIFIC TRACTS, DESIGNED FOR INSTRUCTION AND ENTERTAINMENT, AND ADAPTED TO SCHOOLS, LYCEUMS, AND FAMILIES. CONDUCTED BY JOSIAH HOLBROOK, AND OTHERS. VOL. I. BOSTON. PUBLISHED BY CARTER, HENDEE, AND BABCOCK. Corner of Washington and School Streets. 1831. DISTRICT OF MASSACHUSETTS, TO WIT: District Clerk's Office. BE IT BEMEMBERED, That on the seventeenth day of May, A. D. 1830, and in the fiftyfourth year of the Independence of the United States of America, Carter and Hendee, of the said district, have deposited in this office the title of a book, the right whereof they claim as proprietors, in the words following, to ' SCIENTIFIC TRACTS, designed for Instruction and Entertainment, and adapted to Schools, Lyceums, and Families. Conducted by Josiah Holbrook, and others.' In conformity to the act of the Congress of the United States, entitlefl, "An Act for the encouragement of learning, by securing the copies of maps, charts and books to the authors and proprietors of such copies during the times therein men- tioned ;" and also to an act, entitled, " An Act supplementary to an act, entitled, ' An Act for the encouragement of learning, by securing the copies of maps, charts and books to the authors and proprietors of such copies during the times therein mentioned,' and extending the benefits thereof to the arts of designing, engrav- ing and etching historical and other prints." JNO. W. DAVIS, Clerk tfthe District of Massachusetts. i SCIENTIFIC TRACTS. V. / NUMBER I. THE ATMOSPHERE. FEW subjects are more important, or less under- stood, than the atmosphere we breathe. As it surrounds the earth, and presses with great weight upon its surface, it comes in contact with everything, and bears a most interesting relation to every animal that walks upon the earth, swims in the sea, flies in the air, or creeps in the dust to every plant that is pleasant to the sight, or good for food and to every mineral that glitters in its bed, adorns a cabinet, or is used in the arts. It moves our lungs, circulates in our veins, warms us in our fires, enlivens the midnight lamp, and makes it an agreeable substitute for the light of day, fans us in the breeze, terrifies us in the tornado, conducts sound, now in the soft whisper, the voice of intelligent conversation, the flashes of the orator, or enchanting music, now in the roar of the cannon, the groans of the dying, or ter- rific thunder ; it wafts the ship, heaves the placid ocean into billows, takes the heat of the equator and carries it to the poles, and exchanges it for a cooling breeze, which it kindly returns to temper the scorching rays of the torrid sun ; or, using the language of an elegant French writer, ' In the use of the atmosphere, man is the only being who gives it all the modulations of which it is susceptible. With his voice alone, he imitates the hissing, the cries, and the melody of all animals, while he enjoys the gift of speech denied to every other. To air he also sometimes communicates sensibility ; he makes it sigh in the pipe, lament in the flute, threaten in the trumpet, and animates to the tone of his passions, even the solid brass, the boxtree, and the reed. In a word, he harnesses it to his car, and obliges it to waft him over the stormy billows of the ocean.' THE ATMOSPHERE. This faithful servant and constant friend is still neg- lected. Few appreciate its importance, none understand all its uses. While every breath furnishes us with fresh and living proof, that the atmosphere is faithful to its trust, we seldom inquire what agent is constantly moving our lungs, or by whom it was provided and fitted for its office. How then can we employ an hour more rationally or pleasantly, than in devoting it to this constant companion, and faithful, but neglected friend. Before we can understand, or at all appreciate the importance and infinitely varied relations of -this sub- stance which envelopes our globe, we must learn some- thing of its properties and operations what are its in- gredients, and how it acts. All the properties and operations of the atmosphere may be classed in two divisions, viz. chemical and me- chanical. Weight and elasticity are the properties by which nearly all the mechanical operations are con- ducted ; and although these operations are literally innu- merable and highly interesting, they must give place on the present occasion, to its not less numerous or interest- ing uses and phenomena conducted by chemical agen- cies. Among the infinite, and infinitely varied chemical operations, constantly carried on in the processes of nature and the arts, scarcely one can be found, with which the atmosphere has not some connexion and some agency. It must be understood, however, that in much the greatest number of cases, in which the atmosphere acts as a chemical agent, but one portion of it takes any considerable part. And although this portion consti- tutes but about one fifth of the whole, it fills a greater variety of offices, is more active and more efficient in giving power to other agents, in moving and calming the elements, in preserving order and promoting health and happiness, than almost any other ingredient hitherto discovered in the material universe. The name of this most efficient and useful part of the atmosphere, this almost universal agent in nature and the arts, this mover and regulator of other agents, and friend of man, is Oxigen. THE ATMOSPHERE. No substance, unless it is heat, is so universally diffu- sed through the material creation; none so extensive and varied in ks forms and combinations ; none to which artists are obliged to make such constant application for aid, or so often to consult, or, if not consulted, disap- points them with defeat, as this same oxigen, of which we are speaking. It constitutes not only the most inter- esting portion of the atmosphere, but more than seven eighths of water, nearly one half of the whole vegetable kingdom, a considerable part of soils and mountains, whatever the ingredients or the strata of rocks which compose them, enters extensively and largely into the ores of metals, and into the rare and precious minerals. But our subject is the atmosphere, and not water, vegetables, soils, mountains, metals, or rare minerals, however interesting, or worthy of attention. Oxigen, acting in the atmosphere, is the agent whose character we are HOW examining, the constant companion we are now conversing with, the neglected friend we are now consuhing. We must of course inquire what powers this agent possesses, what part it acts in the great theatre of nature ; how it gives success to the industry of the artist, and defeats his most unwearied attempts to go counter to its dictates. To explain fully all the operations carried on by the agency of oxigen, would require volumes instead of a small tract. A few of the most important can therefore be mentioned, which are the following. I. Oxigen supports life by aiding the lungs in the process of respiration. The lungs inhale air twentysix or twenty-seven times in a minute, taking in, at each inspiration, about forty cubic inches, which is something over one hundred hogs- heads a day. A chemical action is constantly carried on by the atmosphere and lungs, which the Creator of both has so wisely and wonderfully fitted to each other. By this action, nine or ten gallons of oxigen are consumed in an hour, or taken up by the blood to form another kind of an entirely different character, to be mentioned and explained in another place. Not only the human race, but the whole animal king- VOL. i. NO. i. 1 * T1IE ATMOSPHERE. dom, are constantly using and constantly consuming this vital part of the atmosphere. Fishes can no better live without air, or oxigeu, than animals upon land. This oxigen they obtain from air, which is always contained in water in sufficient quantities for their purposes. If air is entirely removed from water by distilling, or the air pump, fishes will live in it but little longer than land ani- mals when shut from the atmosphere. Not only fishes, but insects, and the humblest reptiles creeping in the dust, need a portion of oxigen, which they are constantly consuming. If any animal be confined in a vessel or tight room, from which the atmosphere is entirely excluded, he may live until the oxigen is so far consumed as to be unfit for respiration, when he will die. Many persons have lost their lives from the want of a supply of this vital air. The most remarkable instance, of such a disaster on rec- ord, occurred at Calcutta, in a prison called the ' black hole.' One hundred and fortysix English prisoners were forced into a room eighteen feet square, from which fresh air was almost wholly excluded. Very soon after they entered, a profuse perspiration commenced, followed by a high fever and raging delirium, with cries, ' air, air, water, water,' throwing out insults to their merciless cap- tors, that they might be provoked to put them out of their wretched existence. They entered this prison of death at eight o'clock in the evening, and at six in the morning, but twentythree from the whole number retained the vestiges of life. Although oxigen is essential to the support of life, in a pure state it will soon destroy it, by the sudden and powerful excitement it produces in the system. A few in- halations of pure oxigen will increase the pulse from sev- enty or eighty, to one hundred and twenty, or one hun- dred and thirty, beats a minute. Inhaling air with a very little more than the common proportion of oxigen, produces a sudden and remarkable eflect on the system. The substance known by the aame of exhilarating gas, which has a similar eflect upon the feelings with ardent spirits, or more like the vapor of spirits, or of ether, differs from the common atmosphere, THE ATMOSPHERE. only in containing thirtyseven per cent, of oxigen, instead of twentyone per cent. Aqua-fortis, or nitric acid, is composed of the same ingredients with the atmosphere, and owes its great pow- er in corroding almost every thing it touches, to the seven- tyfive per cent, of oxigen it contains. A few drops of this acid would certainly and suddenly destroy life, while the atmosphere is essential to sup- port it. II. Oxigen supports combustion. Every child knows that fire will not burn without air. But some kinds of air will instantly extinguish it. Whenever we see com- bustion going on, whether in a candle, a common fire, or in the burning of a city, we may know that oxigen is present, and is the principal agent in producing the light, the heat, or the terror. If a burning body be confined in a small portion of atmosphere, it will continue to burn until the oxigen is consumed, when it will go out. Any one may prove this by a simple and useful experiment. Place the small portion of a candle upon a piece of cork or other light body, and let them float upon water in a basin. While the candle is burning, invert a pint or quart glass, so as to enclose it, and entirely exclude the atmosphere. The flame will continue until the oxigen is consumed, when it will be extinguished, and the water rise so as to occupy about one fifth part of the glass, or the portion which was before occupied by oxigen. This ex- periment illustrates two principles ; first, that oxigen is essential to the existence of combustion ; and second, that it constitutes about one fifth of the atmosphere. It is to supply the fire with a greater quantity of oxigen, that the bellows is used. It is to exclude oxigen from burning coals, that they are covered with ashes, to pre- serve them alive when left. While enough air is admit- ted to continue the coals in a state of ignition, the quanti- ty is not sufficient to carry on the combustion with any rapidity; consequently the fire is not extinguished, nor the Coals consumed. If pure oxigen be thrown upon a burning body, it in- stantly increases the power of the combustion. If a can- dle, just extinguished, be immersed in a vessel containing THE ATMOSPHERE. this gas unmixed with any other substance, it will be in- stantly relighted. Many substances will burn in this air in a pure state, which will not in common air. If iron be heated and immersed in pure oxigen, it will instantly melt and burn, throwing its particles in every direction, with an intense and brilliant light. Consequently, if the oxigen of the atmosphere were not diluted or weakened by another kind of air, stoves, andirons, and numerous other instruments made of iron, would melt and be con- sumed the instant they were raised to a high heat. Al- most everything would be combustible, and the earth would soon present one great and general conflagration. How wise and how good, then, is the Creator of the atmos- phere, not only in furnishing it with oxigen to support life and combustion, but in diluting it with another sub- stance, to prevent its destroying every living being upon the earth, and the earth itself! III. Oxigen carries on fermentation. Without it, neither cider, beer, wine, or even yeast, or light bread, could be formed. Almost all animal and vegetable sub- stances are liable to ferment, and, consequently, to waste with decay. Wherever we witness this process, we may know that the same agent is present and employed, as conducts res- piration and combustion. Numerous substances undergo four evident and distinct changes by the power of this agent. These four changes, to which nearly all animal and vegetable matter is liable, may be familiarly and clearly represented and illustrated by one instance of common occurrence, and known to every person. A little reflection will bring to mind four changes the apple undergoes, all of which are produced by oxigen. The first is giving it a sweet taste, or forming in it a" quantity of sugar. This change takes place when apples are bruised, or ground to pomace, for the making of cider. A single apple, if broken into pieces, and exposed to the air, will show this fact. It will soon become sweet, es- pecially if the weather is warm, whatever may be its nat- ural taste. The juice of the apple, when running from the cider press, is always sweet, and contains sugar. This THE ATMOSPHERE. 9 sugar is formed by the oxigen of the air combining with the substance of. the apple. During this change, no spirit or alcohol is formed, and the liquid is of course useless to the distiller, and can do no injury by spreading intemperance. But if it be expos- ed to the air until a second portion of oxigen is added, the sugar is destroyed and alcohol formed. Alcohol, or spirits, whenever it exists, whether in beer, cider, wine, gin, brandy, rum, or any other intoxicating liquor, is formed from sugar. In the second change, or stage of fermentation, then, sugar is destroyed and alcohol formed. This change is frequently rapid and violent, especially if the liquor be moderately warm, and freely exposed to the atmosphere. When a cask of cider, recently from the press, is placed in a warm situation, and more if it be shaken, so as to bring a great number of particles in contact with the air, a commotion, sometimes violent, arises in the liquid, which is produced wholly by the action of oxigen upon it. If casks be filled with the juice of the apple as soon as it is expressed, and immersed wholly under water, or placed in any other situation so as entirely to exclude the air, the second change will be prevented, and no alcohol formed. A third portion of oxigen destroys the alcohol, and forms an acid. This change is always more gradual than the second, and sometimes produced with difficulty ; though it partially takes place, whenever cider, beer, or wine, is exposed for any length of time to a warm atmo- sphere. The change may be wholly produced in the liquids just named, or in alcohol, in almost any other form, if a large surface be exposed to the air, frequent motion given, and yeast or some other fermenter added. Not only common vinegar, but most of the stronger acids, are formed by the agency of oxigen. It has al- ready been observed, that it constitutes about three fourths of nitric acid. Sulphuric acid, or oil of vitriol, is formed by burning sulphur in oxigen, or with saltpetre, which contains it in large quantities. Phosphoric acid is formed by burning phosphorus in a similar way. For a long time it was supposed, that all acids were 10 THE ATMOSPHERE. formed by this agent, and hence its name, which signifies the former of acids. The fourth and last change, frequently witnessed in the substance of the apple, is putrefaction. If vinegar be freely exposed for a long time to a warm atmosphere, it becomes putrid. The ultimate or final effect of fer- mentation, upon all animal and vegetable matter, is, decay, or an entire decomposition. Notwithstanding every species of matter in the whole animal and vegetable kingdoms, is liable to undergo the four changes above described, certain circumstances will entirely prevent either. These circumstances are, seclu- sion from the air or moisture, and more than a moderate degree of heat or cold. By confining the flesh of quadrupeds, of fowls, or of fishes, in tin canisters, perfectly air tight, it is kept for years in a fresh state, without injury. Fresh animal food has been frequently preserved in this way during a three years' voyage at sea. Any animal or vegetable substance, if made perfectly dry, will undergo no fermentation and no decay. The same is true if kept in a frozen state, or frequently raised to a temperature near boiling heat. IV. Oxigen acts upon all the metals. By this action the rust of iron, the dross of lead, the corrosion of cop- per, brass, and silver, and the most beautiful paints, such as red and white lead, chrome, yellow and blue vitriol, is produced. Some of the metals combine with oxigen rapidly, others gradually, or even with difficulty. Iron, for example, is very liable to rust, or oxidate, as chemists say ; while gold utterly refuses to combine with oxigen, as it does with most other substances, except by an indirect pro- cess. If lead be kept in a melted state for a short time, it absorbs so much oxigen as to be changed wholly into dross, which is an oxide lead. But gold may be kept in a state of fusion and motion for months, or years, and never admit a particle of oxigen into its connexion ; so proud a substance as gold, may, however, by an indirect process, be led to combine with oxigen, and after that with many other substances, When oxigen once gets THE ATMOSPHERE. 11 in possession of this metal, it readily transfers it to the surfaces of numerous substances, and extends it over a greater space than can be done by beating or any other process. By the aid of this agent, then, numerous uten- sils, and even cloth, cords, and thread, are gilded at a most trifling expense. Not only paints, but dyes are prepared, and attached to the surfaces or fabrics to which they are to be applied by this general agent. Without its aid in giving perma- nency to colors, most dyes are mere stains, and entirely removed at the first washing. Some colors it heightens or deepens after they are applied. Ink of a good quality, is sometimes pale, when first applied to paper, but after a few days, it becomes of a deep and permanent black. Indigo, when first formed from the plant, is green, as it is when first applied to cloth ; but by a short exposure to the air, or to oxigen, it becomes the beautiful and perma- nent blue so extensively witnessed in woollen, silk, and cotton goods. Oxigen is not only the most active and general agent in preparing colors and fixing them to their fabrics, but it is more generally and successfully employ- ed than any other, for destroying them. The important process of bleaching is conducted by it, whether by the old method of exposing cloth alternately to moisture and the sun, or by the modern and improved method of im- mersing it in chlorine. It seems, then, that this remarkable character not only fills numerous and important offices, but those of almost an opposite nature, and that it possesses properties, which are not only distinct, but opposite. Though oxigen is by far the most active and general agent in conducting the innumerable chemical operations of the atmosphere, another ingredient is the most abun- dant. It has already been observed, that only one fifth, or, more exactly, twenty one per cent., of the atmosphere is oxigen. The other essential ingredient is nitrogen, which constitutes seventynine parts in a hundred. This ingredient, though abundant, is almost wholly inactive. Its properties appear to be entirely of a negative character. It neither sustains nor destroys life nor combustion. It neither carries on, nor retards, fermentation in any of 12 THE ATMOSPHERE. its stages. It does not act upon metals, nor interrupt the action of its associate upon them. It takes no part in the preparation or the application of paints or dyes, nor in destroying them when formed. It neither aids the chemist, the mechanic, the housekeeper, nor the farmer, nor interferes with any of their operations. It has no agency in forming sugar, wine, cider, beer, bread, or acids, nor in corroding metals, or spreading intemperance by intoxicating liquors. Although the properties of nitrogen are almost wholly negative, and it appears to be little more than an idle spectator to the endless variety of phenomena produced by the atmosphere, it is still an essential, as well as the most abundant portion of it. Without it, the air which surrounds our globe, could not conduct those innumera- ble, constant, but silent operations, with the sole design of promoting the health and happiness of the innumerable living and active beings it envelopes. The most important office which nitrogen fills in the great theatre in which it moves, appears to be occupying the space which might otherwise be occupied by its more powerful colleague ; to dilute or weaken oxigen, which, if it occupied the whole region of the atmosphere, would possess so much power as to defeat the numerous objects so wisely designed by its creation to accomplish. While it never interferes with the vital action of oxi- gen upon the lungs, it occupies the greatest part of their contents, and by that means prevents the violence and consequent destruction which must be produced, if the large contents of those organs were wholly occupied by the more powerful ingredient in the atmosphere. While it never interrupts the beautiful and useful phenomenon of combustion in its gentler forms, as in the useful and domestic arts, it prevents the vast heap of ruins the earth must soon present, by a general conflagration, if it was enveloped in an atmosphere of pure oxigen. Not only in the extensive and constant operations of respiration and combustion, but in fermentation, the ox- idation of metals, and the minor offices of the atmosphere, it appears to be the use of nitrogen, to prevent violence, and to render the innumerable and endless movements in THE ATMOSPHERE. 13 this vast theatre of nature, gentle, uniform, and constant, at the same time, that it does not destroy or impair the power of the agent which conducts them. This abundant and harmless portion of the atmo- sphere does not, like its more enterprising neighbor, infuse itself into nearly every mass of matter in the three great kingdoms of nature, but, besides forming more than four fifths of the aerial ocean which surrounds the earth, it constitutes a considerable part of all animal substances, though less than oxigen. It is not found, in any quantities, either in the vegeta- ble or mineral kingdom, of both of which oxigen constitutes a large part. Chemists have generally considered the two substances already spoken of, as the essential ingredients of the at- mosphere. Several others are, however, always found in it, and if they are not concerned in its more important operations, they fill many minor offices in contributing to the endless and constant wants of the innumerable beings living and acting upon the earth. Such is carbonic acid, or fixed air. This gas, which is nearly twice as heavy as common air, is found to exist, at all times, in every region of the atmosphere, from the lowest ravine or cave, to the top of the highest mountain. Except in caves, wells, and some other low places, where carbonic acid settles from its great weight, it is never found in the atmosphere except in small quantities, some say a hundredth part, some a thousandth, but probably variable, existing in larger quantities, at some times, and in some places, than others. Although this substance is fortunately never found in the atmosphere except in small quantities, it frequently takes an important part in promoting the happiness, and producing the disasters of mankind. It is supposed to be an important agent in the process of vegetation ; veg- etables having the power of extracting it from the air and converting it to their own sustenance and growth. While the whole animal kingdom are constantly inhaling or consuming oxigen, and at the same time throwing off carbonic acid, the vegetable kingdom are inhaling or absorbing carbonic acid, and, a part of the time at least. VOL. i. NO. r. 2 14 THE ATMOSPHERE. are throwing off oxigen. So that these two great king- doms of nature are mutually and constantly performing these kind offices to promote the growth and prosperity of each, while they prevent the destruction of both, and of every living existence which animates, enriches, and dignifies this lower creation. For carbonic acid is not only essential to the growth of vegetables, but is certainly and instantly fatal to animals, one full inhalation of which produces death, unless a supply of oxigen is instantly provided. The moment a person enters an atmosphere of this gas, as has frequently occurred in wells, and the fermenting vats of breweries and distilleries, he drops lifeless, and past recovering, except the supply of vital air is immediate. Carbonic acid is no less fatal to combustion than to animal life. If a burning candle or coal be immersed in it, every appearance of combustion is instantly de- stroyed ; but it may be again relighted, by letting it into a vessel filled with pure oxigen. The sparkling appearance and agreeable taste of the best cider, beer, wine, and soda water, are produced by this active substance. By the loss of it, they become dead, as we say, and are not only unpleasant to the taste, but injurious to health. So that the same substance which is distressing and fatal if taken into the lungs, gratifies the taste and promotes health when received by the stomach. By three classes of operations in nature and the arts, carbonic acid is constantly formed, and oxigen destroyed. These are respiration, combustion, and fermentation. It has already been remarked, that the whole animal kingdom are constantly consuming oxigen ; they are also forming carbonic acid. The same double result is pro- duced in most instances of combustion, and in every instance of fermentation, in all its stages. It seems, then, that the vital principle of the atmosphere is constantly destroyed in vast quantities, and that a substance in- stantly fatal, both to life and combustion, is constantly forming, and yet the atmosphere continues to answer this great purpose for which it was designed, and without a sensible change in its character. THE ATMOSPHERE. 15 Chemists have not yet discovered any other process, by which this uniform and healthful state of the atmo- sphere is preserved, but the reciprocal and mutual action of the animal and vegetable kingdoms ; the former, as has already been observed, by constantly con- suming oxigen and producing carbonic acid, the latter by taking up this substance so fatal to life, and giving in exchange the ingredient which supports it. What a striking instance is this, of economy displayed by the great Architect of the universe, in the work of his material creation, and how much more striking and wonderful is its fitness to answer the purposes to pro- mote the ceaseless advancement of his intellectual and moral creation, which constitutes the worth, the dignity, and happiness of his boundless dominions. Carbonic acid is not only found in small quantities in every part of the atmosphere, but is extensively diffused through the mineral kingdom, especially in quarries and mountains of limestone, every particle of which contains a portion of it safely laid up for the use of the chemist and artist, whenever he needs its use, or wishes to prove its existence. Besides the air last mentioned as existing in the at- mosphere at all times and in all places, several others are occasionally found, and in some places they are con- stantly forming in great abundance, among which is one resembling the gas used for lighting cities. This gas is produced in large quantities in marshes, masses of stagnant water, and in large cities, where a due regard to cleanliness is not observed ; and is the cause of sickness, and perhaps of malignant fevers. By moving the earth in the bed of any pond, and even in most streams of water, bubbles will be disengaged, and rise to the surface, where they may be collected in bottles, and by applying a lighted candle, they will be found to be combustible. It is much lighter than common air, and consequently rises into the higher regions of the atmosphere, where it is frequently exploded by electricity, and is perhaps the cause of shooting stars sometimes observed in the heavens. 16 THE ATMOSPHERE. The properties and operations of the atmosphere al- ready mentioned, belong more to the several ingredients which compose it, than to the whole substance as a mass. It however possesses some properties, and performs many great and important operations, in which it is to be viewed as one body. For example, every particle of the atmosphere, even in the dryest places and seasons, con- tains a portion of moisture. By the power of absorbing and retaining this substance, it performs a most extensive and important service, in producing action, and pre- serving order, health, and life, in moving and living beings. It is constantly relieving the earth, and the numerous bodies upon its surface, from their superfluous moisture. It raises from the ocean, by the silent process of evaporation, as much water as flows into it, by the Amazon, the Mississippi, the Danube, the Ganges, the Nile, and all other rivers which it receives into its bosom. Experiments have proved that during twelve hours of a summer's day, about twenty-five hogsheads are evaporated from an acre, or sixteen thousand hogsheads from a square mile. The water thus taken from the earth, and diffused through the atmosphere, is again collected in clouds, and when the air can no longer sustain their weight, they fall in the form of rain, hail, or snow, and after enliven- ing the face of nature, or passing into the ocean, the same vehicle which before conducted it through this round of services, again takes it up to repeat the process. Not only in relieving the earth from its superfluous moisture, and in preparing materials for refreshing showers, as well as the raging storm, but evaporation is a most important and essential process for the chemist in forming his salts and powders, for the farmer in preserv- ing his hay, and for the mechanic and housekeeper in their endless and nameless operations, for preparing the comforts and the luxuries of civilized and refined society. Besides going this round of ceremonies with water, in taking it from the earth, in diffusing it far and wide, and again collecting it in clouds, and returning it to refresh the living creation, and to replenish rivers and the ocean ; the atmosphere transports it, while in clouds or vapor, from continent to continent, that it may give to the in- THE ATMOSPHERE. 17 habitants living upon the four quarters of the globe, a portion of its genial influence. Every particle of the atmosphere contains a portion of heat as well as moisture. To this substance, no less than to water, it is the general and all powerful vehicle. It is constantly transporting heat from the equator to the poles, from land to sea, and from sea to land, from the valley to the mountain, and from hill to hill. By this office, performed by the air in these great operations of nature, the heat upon the earth is in a measure equalized. The deficiency in one country is supplied from the excess of another. The intense cold of one climate is softened by tempering the scorching heat of another ; and by the equilibrium thus produced, both are rendered productive of more life and more happiness. The connexion of the atmosphere with heat is the source of every current and motion it receives, from the gentle breeze to the raging hurricane. The theory and the different kinds of winds, all of which are nothing more nor less than the atmosphere in motion, and aJl produced by the same cause, might seem to deserve a full explanation in this place. But from the space allotted to the present number, this subject must be de- ferred to another. It is difficult in this connexion, how- ever, to avoid the remark, that the expansibility of the at- mosphere by heat, and its compressibility by cold and pressure, are among its most remarkable and important features, and distinguishes this and other airs from mat- ter of every kind, either in a solid or liquid state. Air of every kind appears to be capable of expansion and com- pression to an indefinite degree. This property belongs to each gas separate, and to several combined. In the manufactory of soda water, one hundred and thirty gallons of carbonic acid is forced, by the means of a condensing pump, into a cask, of the contents of ten gallons, six of which are occupied by water. In the fine syringe, the atmosphere is suddenly compressed into so small a space, as to cause the oxigen it contains to ignite a piece of cotton, previously prepared by moisten- ing it with water highly charged with saltpetre, and then thoroughly dried. When ninetynine hundredths of the VOL. i. NO. i. 2 * 18 THE ATMOSPHERE. air contained in a receiver, is removed by an air pump, the remaining one hundredth is immediately expanded so as to occupy the space which was before occupied by the whole. If a common Florence flask, while containing nothing but air, be heated to a high degree, and then the mouth immersed in water while it is suffered to cool, the water will rise to occupy almost the whole contents of the flask. Familiar examples might be adduced, almost without number, to show the great capability airs possess, of being expanded and compressed to an unlimited degree. And every kind of air possesses this property at all temperatures. In this respect, gas differs from vapor, which in other respects resemble each other. All vapors, whether produced from liquids or solids, are, like air, highly elastic, and capable of great expansion and com- pression. But by lowering the temperature to which they are exposed, the vapor again becomes a liquid, or solid, when it loses its elasticity, and its compressibility. While air retains both at all temperatures, and in all sit- uations. This peculiar and highly interesting property of all gases, gives to the atmosphere some of its most important powers and uses. It is evidently essential to the exist- ence of winds, and to every motion of the air, unless it may be in a slight degree. It is from this property, that the atmosphere so readily gives place to all other sub- stances passing through it. Were it not for this, we should be met with a powerful resistance, whenever we attempted to move from place to place. This is the ori- gin of the trade winds, occupying about sixty degrees of the equatorial regions of the earth, blowing constantly from east to west, of the monsoons blowing six months of the year in one direction, and the other six months directly opposite, of the deadly simoom, of land and sea breezes, of the most variable and shortest currents of air, of the gentle breeze, the brisk gale, the raging tem- pest, the sweeping hurricane, the water spout, and the tornado. It is by the great expansion and compression of air, that ships are constantly moving from continent THE ATMOSPHERE. 19 to continent, that the noxious vapors of cities are removed to give place to an atmosphere more fresh and pure, and that it constantly preserves, amidst all the contaminating influences to which it is exposed, its salubrious and vital energies. In a general view of the properties, powers, and uses of the atmosphere, of the essential ingredients which compose it, and the various other substances it absorbs and wafts in its currents, the numerous odors constantly meeting us, ought not to be overlooked. This class of bodies is almost infinite in variety, which are divided into atoms so minute, that the fragrance of a flower diffuses its agreeable odor into every particle of atmosphere, to a great extent around it, and by this, adds to the beauty and sprightfulness, the sweetness of spring. Such is the power of the atmosphere, and the great divisibility of matter, that it sometimes takes up, and carries from place to place, substances of the greatest density. Even the heaviest metals, such as gold and platina, are capable of being reduced to so fine a powder, as to be supported in the atmosphere, and inhaled by the lungs. The limits to which we are confined in our view of a subject so vast and so various in its applications and uses, as the ocean of air which envelopes our globe, have re- quired us to be brief. In many cases we have been per- mitted merely to hint at a subject, whose importance seemed to require a large expansion. In none have we been able to make that full, minute, and varied application to practical and common concerns, and especially to the moral developement, which give to science its greatest utility, interest, and sublimity. Partially to answer these important purposes, to which every science and every kind of knowledge ought to be applied, this tract will be closed by a few questions relating to the subjects present- ed in the preceding pages. 1. Has the atmosphere received greater or less atten- tion in systems of instruction than its importance de- serves ? 2. Has a knowledge of the ingredients and properties THE ATMOSPHERE. of the atmosphere, or has it not, a relation to life and health ? 3. What vital process in the animal kingdom is con- ducted by the atmosphere 1 4. How many times do persons commonly respire in a minute ? 5. About how much air is received into the lungs at each inhalation 1 6. Which ingredient in the atmosphere is most abun- dant, oxigen or nitrogen ? 7. Which of the two essential ingredients in common air is most active ? 8. If a person be deprived of oxigen, how will he be affected ? 9. Which substance is essential to combustion, oxigen, or nitrogen ? 10. Does carbonic acid promote or destroy combus- tion ? 11. Why does the bellows hasten combustion ? 12. Why is fire preserved by covering it with ashes ? 13. Which ingredient in the atmosphere conducts the process of fermentation? 14. How many changes does vegetable matter undergo by the process of fermentation ? 15. What substance is produced by the first change, acid or sugar ? 16. During which change is alcohol formed, the second or fourth? 17. In what does fermentation, if continued, always result ? 18. Which change is most rapid, the second or third 1 19. How should liquids undergoing the second change be treated, excluded from the air, or exposed to it ? 20. When a liquid is undergoing the third change, or forming into vinegar, does the fermentation need retard- ing or hastening ? should it be excluded from the air or exposed to it ? frequently moved or remain at rest 1 21. Which portion of the air has the agency in cor- roding metals, and producing rust, also paints, oxigen, or nitrogen ? 22. Which metal most readily combines with oxigen, gold or iron ? THE ATMOSPHERE. 21 23. What is the chemical name of the rust of iron ? also of the dross of lead 1 24. Why does covering the surface of metals, such as iron or brass, with varnish or oil, prevent their corroding? 25. Of what substance upon the surface of the earth, except air, does oxigen constitute a part 1 26. Which is most abundant in the vegetable kingdom, oxigen or nitrogen 1 27. Where is nitrogen most abundant, in animal or vegetable matter ? 28. If the atmosphere were pure oxigen, would it be more or less favorable to respiration and combustion ? 29. What appears to be the principal use of nitrogen in the atmosphere 1 30. How is the animal system affected by inhaling pure oxigen ? 31. How is animal life affected, if full inhalations of carbonic acid be taken into the lungs ? 32. Do the lungs throw off more or less oxigen than they receive ? 33. Do they throw off more or less carbonic acid than they receive 1 34. What other operations in nature, except respira- tion, produce carbonic acid ? 35. What substance which is fatal to life, is thrown off by burning charcoal ? 36. Is the carbonic acid in cider, beer, and soda water, favorable, or unfavorable to health ? 37. What natural agents are employed in evaporation 1 38. What finally becomes of the moisture taken up and carried off by the atmosphere ? 39. What becomes of the water which falls into the ocean, through the medium of rivers, rain, &c. 40. Which is most compressible, air or water ? 41. Does heat expand, or contract air 1 42. What is the most powerful vehicle in nature, foi transporting heat and moisture ? 43. What is the cause of winds, and all motions of the atmosphere ? 44. In what part of the earth, and over how great a space, does the wind blow in one direction through the year ? THE ATMOSPHERE. 45. Is the action of the atmosphere upon combustion, a chemical or mechanical process ? 46. Is the oxidation of metals a chemical or mechani- cal operation ? 47. Is the weight of the atmosphere a chemical or me- chanical property ? 48. In how many divisions may all the properties and operations of the atmosphere be classed 1 49. Is the elasticity of air a chemical or mechanical property ? 50. What is the principal agent in the process of bleaching ? 51. What acid is composed of three parts of oxigen, and one of nitrogen, nitric, or sulphuric ? 52. If sulphur be burned in oxigen gas, what acid is the result, sulphuric, or muriatic ? 53. How is phosphoric acid formed ? 54. What part of the vegetable kingdom is oxigen 1 55. Of what minerals does oxigen constitute a part ? 56. Is any substance more universally diffused through the -material creation than oxigen, and what ? THIS pamphlet is the first number of a series of ' tracts,' designed as instruments for operating in the great and common cause of Popular Education. They are in- tended to be brought within the comprehension, and to meet the wants of the great mass of the community ; especially of the industrious classes, who have neither timo nor opportunities to devote their lives to intellectual pursuits. It is hoped they will prove to be worthy and agreeable companions in every family circle, and that they will take some part in their conversation, and if so, that they will enliven, extend, and ennoble, this seat and source of individual and national character and happi- ness. Endeavors will be made to render these sheets welcome visitors to schools, by carrying to them useful and enter- taining knowledge, and in a measure to relieve those THE ATMOSPHERE. 23 active, sprightly, intellectual, little beings, which com- pose them, from the dull monotony of common school exercises. They are also intended to furnish profitable subjects for the exercises of Lyceums, which may be introduced, explained, and enlarged upon, in a familiar way, by those who may undertake to instruct and entertain these social assemblies. It is hoped they may be found agreeable companions, at public places of resort, such as public houses, steam- boats, reading rooms, &c. To answer the objects above proposed, it will be the earnest desire of those who conduct them, to give them the following features. 1. To have them contain useful knowledge, in the strictest sense of the word. They will deal more with facts than theories ; more with settled principles, than doubtful speculations ; more with common than rare things ; more with objects around us, than those which may or may not exist in distant parts of creation ; more with the application of the well known properties of the materials our Creator has put into our hands, and the principles he has established to fit them for our use, than to establish some favorite doctrine in a system of meta- physics. 2. They will be familiar. Technicalities and labored verbiage will as far as possible be avoided. Attempts will be made to present things, properties, principles, applications, in the simplicity of nature, and not through labyrinths of terms, and mazes of declarations. 3. They will be practical. Whatever may be the sub- jects introduced, whether the physical sciences, natural history, mathematics, political economy, agriculture, the mechanic arts, biography, or history, they will have a practical bearing upon common life and common in- terests. 4. They will be moral. It will be the constant aim to awaken and elevate moral sentiment, and to present every- thing as the gift, or under the direction of a great and wise Creator, and a constant and boundless Benefactor. APPARATUS FOR SCHOOLS, LYCEUMS AND ACADEMIES. The economy, no less than the general utility, of visible illus- trations, is no\v universally acknowledged. It has had the unanimous assent, so for as known, in not less than fifty con- ventions of teachers, *t which were present more than ten thousand persons. Apparatus is more economical than books, because one instrument is sufficient for a school, instead of an individual because it is more durable because impressions received through the eye, especially by young minds, are more clear, more rapid, more permanent, more agreeable, and, of course, more efficient, man those through the ear. The change already effected in Schools, by the geometrical diagrams and solids, is unparalleled. By them the simple elements of Geometry, from an abstruse study in a collegiate course, have liecome the most agreeable, as well as the most useful branch in Infant Schools, and to a great extent in Com- mon Schools. A manual, and three sheets of diagrams, one of which contains the manuscript letters, can be used without the solids, either in Schools or families. The price of the four articles is $0,50 ; that of the whole of the geometricals is $4. A set of common-school apparatus, embracing the geometricals, one or two instruments for Arithmetic, several for Geography, and a few for Astronomy, is $10. For Lyceums and Academies, philosophical are $15; orrery, $6; tide dial, $4; seasons, $2; whole 'of the asWtoomicals. $15: a convenient chemical set, $2r,. As some counterfeit apparatus has been made, of a defec- , tive elmrarter, and offered for sale under Mr. Holbrook's name,c and a few individuals imposed upon, purchasers will do well to lie minions of whom- they procure it. None but that made under Mr. Holbrook's direction can be used with books which are prepared and preparing to illustrate and apply it. CARTER, HENDEE AND BABCOCK have the appa- ratus above-named for sale, with a large assortment of Books designed for INFANT and PRIMARY SCHOOLS, for LYCEUMS, ACADEMIES and elementary and practical instruction generally. SCIENTIFIC TRACTS. NUMBER II. GEOLOGY. GEOLOGY is a modern science. It is but little more than a quarter of a century since it received its exist- ence, especially in our own country. Before that, it was neither understood, nor mentioned in our highest institutions of learning. Our most learned professors possessed no practical knowledge of this subject, nor did our country contain any source from which it could be obtained. They were unable to recognise or name the most common mineral in the streets. Indeed, most of the knowledge in Europe upon this subject, was drawn from conjecture, rather than facts. Within a few years past, however, Geology has made greater progress than was ever made by any other science in the same length of time. From being wholly unknown in our colleges, it has become a familiar and delightful subject in Infant Schools. Thousands of children under ten years of age, are now better practical Geologists, than any individual who could be found in the country thirty years ago. They have not collected their know- ledge from their school-rooms or their books, but from actual observation, and examination. They are ac- quainted with a certain rock or mineral from seeing it, and know its situation, by breaking it from the mass to which it was attached. Their school-rooms and their parlors bear infallible testimony both of their knowledge and their industry. Their countenances testify that their collections are the price of blows upon the rocks, rather than upon their backs. Stripes did not compel them to obtain their knowledge, but their knowledge VOL. 1. NO. II. 3 26 GEOLOGY. induced them to put on the stripes. Their exercises to obtain it were permitted, not compelled. Their sub- ject is understood rather than committed, known rather than imagined. The progress' of this subject has not only been un- paralleled as a science, but its application to Agricul- ture, to Civil Engineering, and to many of the arts, has already added to the wealth of our country, to a vast amount. It has brought to view some of the finest specimens of marble upon the earth, which had been used by farmers for common enclosures for one hundred and fifty years without being known. It has discovered valuable quarries of building materials, within a few rods of walls which were brought from a distance of as many miles. It has discovered the material from which coperas is made, and led to the art of manufacturing it in such perfection, and at so cheap a rate, as to put an end to the importation of that article so indispensable, and so extensively used in the arts. It has brought to view inexhaustible deposits of the material for the manufactory of the beautiful pigment under the name of chrome yellow, and has reduced the price of that useful substance, from sixteen dollars to fifty cents a pound. It has led to the establishment of a manufactory of epsom salts, where seven or eight hundred tons are made in a year, and of a better quality than can be procured from any establishment across the Atlantic. The numerous and abundant sources of industry and of wealth which it has opened to our country, have in- creased the treasures of wealth, no less than those of knowledge; the lovers of science and of filthy lucre, have in one instance been gratified by drinking at the same fountain. If there are yet those who need to ask what is the object of this practical, interesting and sublime science, they can be informed that it means a descriptiojytf the earth; and is hence nearly allied to geography. Both v sciences have not however the same province. They do not describe the earth in the same points. Geogra- phy, not only describes the great divisions and natural features of the earth, but the political and civil divisions, GEOLOGY. I 27 together with the changes and improvements made upon its surface by the hands of men. It not only gives an account of oceans, continents, islands and mountains, but of towns, republics, kingdoms and empires. It is not the object of Geology to give the number, names or situation of continents, islands, or mountains, but of the ingredients of which they are composed, and of the position and arrangement in which they are placed. It takes no notice of the changes which have been pro- duced upon the earth by the industry or the ravages of men, but describes the more sublime changes it has suffered, by the agency of earthquakes and volcanoes; and by the gradual but irresistible hand of time. The object of Geology is to give us a history, not of the inhabitants which have risen and fallen upon the earth, but of the earth itself. It describes its original formation and present structure, with the gradual and tremendous changes it has undergone, since it came from the hand of its Maker. It must be acknowledged that our means of nforma- tion upon this subject are comparatively scanty. Neither history, nor the present appearance of the earth, informs us with any degree of minuteness, what was its state, when * it was without form and void,' or how great or general were its changes, when the ' windows of heaven opened, and the fountains of the great deep broken up.' Nor can we penetrate beyond a few feet into the bowels of the earth, to ascertain what are its hidden treasures, or the order in which they are stored. Neither the ledges of mountains, the channels of rivers, ravines, caves, wells or any other excavation, either natural or artificial, give us any opportunity to examine the mate- rials, structure or arrangement of our globe, but a few feet below its surface. Although most of the incidents in the history of our planet, which curiosity, ever upon the alert, would fain unfold, are surrounded and deeply buried in the gloom of ignorance, a few of these incidents are still within our reach, and are too interesting and too important to be withheld from any one who has a heart to feel, or a mind to perceive. A few facts, which can be well established, 28 GEOLOGY. upon a subject so important as the creation and history of a planet, and of that on which we ourselves are placed, though but a speck in creation, can hardly fail to light up the curiosity of any mind, which has a spark remaining. To state these facts, as they are learned from history and observation, is the principal object of the present number. CHAOTIC OCEAN. The first well established fact worthy of notice re- specting the history of our planet, is that there was a time when it was one vast ocean; without a continent, an island, a mountain, a rock, a metal, or a particle of solid matter upon its surface. It contained, indeed, the elements of all solid substances, which now appear so beautiful, so rich, and so various upon its surface; but they were in a liquid state they were dissolved by heat or water, or more probably by both. Whatever might have been the agent or agents, which dissolved and held in solution the rocks, islands, moun- tains and continents, now so firm and so lofty upon our globe, the fact is denied or doubted by no one, who has resorted for information to either of the two great vol- umes, the book of nature, or the book of revelation. The sublime and interesting account found in the first chapter of Genesis of the creation of our earth, is grounded upon the fact, that it was once a vast and gen- eral ocean. Such it must have been when it was without form and void, and darkness was upon the face of the deep, and the spirit of God moved upon the face of the waters, and commanded dry land to appear. These statements imply, with a clearness little short of a direct declaration, that there was a time, v/hen our earth was a vast deep one great body of water when dryland had not appeared. This interesting fact, so clearly implied in the book of revelation, is fully corroborated in the older volume, the book of nature. The ocean now holding in solution many, perhaps most of the ingredients which constitute the solid and rocky masses volcanoes which dissolve the body of mountains and pour them from their GEOLOGY. 29 heights in a liquid form, so as to lay in ruins the fairest plains and cities below, ledges beautifully studded with crystals, and mountains intersected by seams of copper, tin, silver and gold, bear constant and infallible testimony, not merely to the possibility, but the certainty, that the most solid substances were once in a state of solution, and that our planet has been shaken to its centre by the war of its elements. CONSTANT CHANGES. History of the past, and observation of the present, unite their testimony to the fact, that the changes the earth has undergone since it came from the hand of its Maker, have been constant, and to a great extent gradual. Whatever construction shall be put upon the word day, as used in the only history we have of the creation of our earth, or however great might have been the changes it suffered during the six days there mentioned, no one can deny or doubt, that great and constant changes have taken place upon its surface since the period there referred to. The changes to which it is daily subject at the present time, must be visible to the most careless observer. The gradual but powerful and irresistible hand of time, even in the short space of ' threescore years and ten,' sometimes gives to extensive districts a new aspect and a new character. In many situations, rocks are constantly forming, in others they are in a state of decomposition; in one case the land is daily encroaching upon the sea, in another, it is carried into the ocean, and gives place to a bay or harbor. Volcanoes are pouring forth from the depths of mountains melted lava, which by a sluggish but a pow- erful and awful momentum, carry destruction in their course, and bury flourishing cities in ruins, giving no warning to their inhabitants to flee from their danger. While in one case mountains are throwing from their summits their own contents into the valleys beneath, in another the ocean is throwing up islands from its depths. In some instances, islands have arisen out of the sea in a night. VOL. i. NO. n. 3* 30 ORDER OF CREATION. The general order of time in which the earth with its furniture and its inhabitants came to its present form, is sufficiently manifest from the only authentic history we have of its creation, from reason, and from observation. The first step which was taken to change the original chaos into a convenient dwelling-place for living, acting, and intelligent beings, was the formation of dry land. That was necessary to provide for the accommodation of animal and vegetable life. When provision was made for the existence and support of the vegetable kingdom, ( the earth brought forth grass and herb yielding seed after their kind, and the tree yielding fruit after his kind, whose seed was in itself after his kind.' The creation, and continued production of the vege- table kingdom, made provision for the animal. Then the earth brought forth cattle that walk upon the earth, fowls that fly in the firmament of heaven, reptiles that creep in the dust, and fishes that move in the waters i and each after his kind. But the tenant for whom the earth, with all its pro- ductions of animal and vegetable life, and so richly provided with furniture of a thousand kinds, was not yet created. His creation was to close this august work of the great Architect of the universe. Man was not formed and placed upon the earth, until the earth was fitted for his reception, his convenience, and his happi- ness until two great lights were formed, one to rule the day, and the other to rule the night, and the stars also until the waters which were under the firmament were divided from those above the firmament, and gathered together in one place, and dry land appeared until grass, herbs, and trees yielded seed and fruit after their kind, and cattle, the fowls of heaven, every creeping tiling, and every living creature which moves in the waters, were, formed, and made to produce others after their kind, ajid put in subjection to the lord of this lower creation. Such is the general order in the work of creation, as GEOI.OGT, 31 learned from the Bible, from reason and from observa- tion; and yet we have the strongest evidence, that this order was not strictly and minutely pursued through the whole process of bringing the earth into the state in which it is now presented to our view. The whole of the mineral kingdom, all rocks and metals, soils and mountains, were not completed before the creation of the vegetable and animal kingdoms were commenced. So far from it, rocks, soils, and metals, are daily form- ing at the present time. In many instances, vegeta- bles and animals are deposited in solid rocks far below the surface of the earth. Nay, whole mountains of a great height, and hundreds of miles in extent, are com- posed of little else than the relics of animals. The greater part of these animals were evidently different kinds of shell fish. But fishes, of the kind that swim, are also found inclosed in solid rocks. In one instance, the relics of one fish were found in the mouth of another, apparently in the act of struggling for his freedom, when both captive and captor were suddenly arrested, and confined, where they closed their struggles and their lives together; and were afterwards converted into stone. In another instance, one hundred and sixteen different kinds offish were foond petrified within a short distance. It has been remarked, that fishes had probably met in general assembly, and were taken when in the act of legislating. In excavating the section of the Erie canal at Lock- port, after descending twenty feet into solid rock, several rattlesnakes were found with the whole form, though in the state of stone, almost precisely retained. At the same place and nearly the same depth, a toad was taken from the solid rock, which when found was in a torpid state, which he had retained perhaps for thousands of years, but when exposed to air and heat soon gave indications of life, and after a short time gained strength enough to hop, but after a few hops closed his existence forever. Not many years since, in the vicinity of Paris, there was found imbedded in solid rock, and forty feet below its surface, a board several feet long and eight or nine inches wide. At the same place a hammer was found, 92 GEOLOGY. the handle of which, with the board was petrified, but the hammer being of iron, retained its natural state. These are a few instances, among thousands, which might be mentioned, to prove that the changes our earth has undergone, have been gradual and constant, and that minerals, rocks and soils, and even mountains have been formed since the creation both of the vegetable and animal kingdoms commenced, and even after man was formed, and had made some advances in the arts of civilization. Indeed no one can doubt for a moment, who has paid the least attention to the subject, that our globe has been subject to constant and important changes from the time that the materials of which it is composed were formed out of nothing, until the present moment. And these changes which come within our knowledge are so great, as to afford strong evidence that the earth could not have existed for a much longer period than that assigned by Moses. AGES OF ROCKS. From views and facts already presented, it must be concluded that rocks and mountains have different ages. Some have existed for six thousand years, while others are at this moment in a process of formation. And there is good reason to believe, that every moment during the whole of this period, these formations have been going on. We not only know that rocks have different ages, but we know which are oldest. All geologists unite in the opinion, that granite was the first solid substance formed from the great chaotic ocean ; and that the coarsest masses of this rock are older than those of a finer texture. Next in age to granite, is gneiss, consisting of the same ingredients, but of a finer texture and a more slaty character. Mica slate is considered by most geologists as the third rock in age. Lime has be^n forming in all ages of the world. Some deposits of limestone are older than the most re- cent granite, while others are forming at the present moment. The oldest specimens are coarse and of a 33 crystaline structure; the most recent is fine or com- pact in its texture, and destitute of every appearance of crystalization. A bed of the most ancient limestone is found in Bolton, Massachusetts. In the western part of New York, deposits of the same rock arc constantly forming at the present time. ELEMENTS OF ROCKS. Notwithstanding the rich and endless variety in the external appearance of rocks, their elements are few and simple; and this apparent and beautiful variety is owing more to the proportion and arrangement of the ingredients which compose them, than to their number or variety. Nine simple minerals have been supposed, by many geologists, to be the elementary substances of which all rocks are composed. And it is well known, that four or five of these, constitute by far the greatest part of rocky and mountain masses, and that more than half both of rocks and soils, are formed from two of them. The names of these simple minerals, sometimes called the geological alphabet, are quartz, felspar, mica, horn- blende, lime, argillite, (common slate,) gypsum, talc, and chlorite. The two first are the most common and most abundant materials which compose the solid mass of our earth. Of the highest and most extensive moun- tains upon our globe, they are the principal, and to some extent, the only ingredients. They are also, the essen- tial elements of soils, and upon the proper mixture of quartz and felspar, or of silex and alumine, (sand and clay,) the ultimate principles found in these two mine- rals, the fertility of soils depends. These two abundant and important minerals in many instances, very nearly resemble each other, tfiough a little experience will enable any one to distinguish them. Quartz is harder than felspar, and much more various in its appearance. It is of every shade of color from nearly black to milk white. The white pebbles so common in the streets and by tUft; way-side, frequently known by the name of flint-stone>, ate a common species of quartz. Gun-flint is another. Sometimes it is 34 GEOLOGY. transparent and perfectly crystalized, when it is im- properly called diamond. Diamond rocks and hills are known in many towns in almost every section of our country. The diamond is found but in two or three places upon the earth. Crystalized quartz is sometimes of a purple color, when it is called amethyst. Jasper, carneleon, calcedony , opal, and several other precious stones, are ranked in the family of quartz. Felspar is generally white or of a light color, some- times yellowish, light red, or green, seldom of a dark color. Its fracture differs from that of quartz, as it breaks in small even surfaces or plates, somewhat re- sembling steps. A strong light thrown upon a recent fracture, gives it a peculiar indescribable lustre, by which it can always be distinguished from quartz. The two minerals are not only useful as constituting the greater part of soils, rocks, and mountains, but for an important purpose to which each is applied in the arts. Quartz is the essential, and almost only ingredi- ent used in the manufactory of glass, whether for win- dows, decanters, tumblers, bottles, or any other purpose. Felspar is always used in the manufactory of porcelain or china ware. The substance known by the name of kaolin, or porcelain clay, used both in China and this country in the manufactory of porcelain, is decomposed felspar. Mica, frequently but improperly called isinglass, is extensively associated with the two simple minerals already described in the structure of rocks. This min- eral is sometimes found in plates two feet in diameter, but much more commonly in fine scales but little larger than the head of a pin. It is commonly white, but sometimes black, and always more or less transparent. In some places, especially in Muscovy, mica is used for the windows of houses, and is hence called Muscovy glass. It is also used for lanterns, and some purposes aboard of ships, where glass would be liable to break. STRATA OF ROCKS. Into the oldest and most common rocks upon the earth, no other minerals enter in considerable quantities GEOLOGY. but the three just described. They are the essential, and almost only ingredients in granite, gneiss, and mica slate. In the oldest specimens of granite, usually of a coarse texture, the three ingredients being in large masses, the felspar is most abundant. The mica, fre- quently in large plates, is dispersed through the mass in every possible direction. As the process of forma- tion continued, the felspar became less abundant, and the mica more regular in its arrangement. The rock hence passed from coarse to fine granite, and from fine granite into gneiss. The last is slaty granite. Both contain the same ingredients, and their principal differ- ence is in the proportion, arrangement, and texture of their ingredients. As the formation of gneiss con- tinued, the felspar still continued to diminish, until it wholly disappeared. When the rock is formed, it is com- posed of quartz and mica finely mixed, and of a slaty structure, and bears the name of mica slate. This rock differs from gneiss, not only in being desti- tute of felspar, but in possessing a finer 'texture, and a smooth, but frequently an undulating surface. As these three strata of rocks are more common than any other upon the surface of the earth, they are used for a greater variety of common purposes, ly practical men. They are extensively used by farmers for en- closing their fields, and by civil engineers in the con- struction of roads, bridges, wharves, dams, canals, railways, the walls of buildings, Sec, &c. Nearly allied to granite, and frequently associated with it is sienite. In this rock, hornblende takes the place of mica in granite ; the mass is of course com- posed of quartz, felspar, and hornblende. The simple mineral last named, sometimes resembles black mica in its external appearance, but is much harder and can be readily distinguished from it, by its resisting the point of a knife, while mica is readily separated into thin scales, by the application of any pointed instrument. Sienite is found in great abundance in the vicinity of Boston, where it is at present the most common mate- rial for the walls of houses and other purposes in architecture. In Quincy, and two or three other towns 36 fiF.OLOGV. in the same vicinity, are deposits of this useful and beautiful rock, in sufficient quantities for building a thousand cities of the size of Boston. In these deposits, the sienite uniformly consists of three ingredients, though in others, it is formed of two, the quartz being wanting. The three ingredients in this useful material, will be readily seen by a glance at the front of the Tremont House, where three colors are distinctly visible at the distance of several rods. The red and most abundant ingredient is felspar; the white, quartz; and the black and least abundant is hornblende. They are equally distinct in numerous other buildings in almost every part of the city of Boston. Greenstone, or trap rock, is composed of hornblende and felspar. The former always predominates, and commonly constitutes almost the whole mass. This rock contains a large portion of iron, and is hence heavy, hard, and more difficult to break than any other stratum. It is, however, intersected by numerous scams, by which it is separated into convenient masses for erecting the walls of houses, for which it is exten- sively used in New Haven, Edinburgh, and many other places, where it is found in abundance. Two extensive ranges of mountains in New England, commence with the East and West Rocks, about two miles from the city of New Haven, both of which are greenstone. One continues in the eastern range as far as Greenfield, a part of which are Mount Holyoke and Mount Tom. The other range passes farther west, and extends to the Green Mountains in Vermont. The Giant's Causeway is a hornblende rock, called basalt. Sandstone js associated with greenstone, and is always placed beneath it, when they occur together. This rock, as its name denotes, is composed of sand, or grains of quartz and felspar, with fine scales of mica sometimes dispersed through the mass, and cemented with clay and the oxide of iron. Many thousand tons of this rock have been car- ried to Boston from Chatham, opposite to Middletown, on the Connecticut river, and more or less is transported GEOLOGT. 37 from the same place into every seaport from New Or- leans to Eastport. It is used for the underpinning of houses, door steps, hearths, jambs, &c, &c. Graywacke, in some of its deposits, resembles sand- stone. But besides embracing recks composed of grains, it extends to those consisting of large pebbles, sometimes a foot in diameter, and frequently so loosely cemented, as to fall to pieces from the effects of the weather, and other gradual operations of time. An extensive and most singular deposit of graywacke is found nearly the whole distance from Boston to Provi- dence. A circumstance respecting this deposit of graywacke coincides with that in another on the heights of Catskill mountains. The circumstance is, that the highest points of elevation in both cases, consist of the coarsest pebbles. In the descent, both rocks become finer and more compact in their texture, until they finally pass into slate. Another most singular and unaccountable fact, which is particularly striking in the New England deposit is, that ledges are intersected by numerous seams, which cut, not only the largest pebbles, but the finest grains, leaving a surface almost as smooth as if it were polished. This fact may be witnessed in the numerous walls built of this material in Dorchester and Roxbury, which by the aid of these seams present an even surface in front, notwithstanding the general fracture of the rock is un- commonly ragged and uneven. The fact is stated merely, any may explain it who are able. Some of the finest slates, and even the material of which hones are composed, are classed by geologists under the stratum of graywacke. Argillite, or common slate, used in schools and for the roofs of houses, is an abundant rock, but less com- mon than most of those already described. Extensive ranges of it exist in Vermont, which furnished during the last war, large quantities of a good quality for the roofs of houses. But as it is less expensive transporting it across the Atlantic, than down the Connecticut, the VOL. i. NO. ii. 4 38 GEOLOGT. greater part now used both for schools and roofs, is r ought from Wales. This rock is composed principally of clay, and hence soils, where it abounds, are of the same character. If argillite is put in a road where it will be finely pulve- rized by wheels, it forms a mass differing but little from a bed of clay. This rock is neither the most ancient, nor the most recent. It was evidently formed after the primitive limestone, but previously to the secondary deposits of the same rock. Lime has already been mentioned, both as an ancient and a modern rock. A few ancient deposits are found in New England. Numerous and almost boundless deposits of modern or secondary limestone, are found in New York, and States still more west and south. The masses found in New England are generally of a coarse crystalline structure, those at the west and south, more compact, and they frequently contain relics of animals and vegetables ; indeed, in some instances, the whole mass of a mountain appears to be little else than an aggregation of animal relics. Of no rock is there probably a greater variety than of limestone. It is said there are nearly two hundred varieties of marble, all of which are limestone. The deposits which do not bear the name of marble, probably present an equal variety. Chalk is, strictly speaking, limestone, as it is composed of the same ingredients, and in the same proportion with the quarries wrought for marble, and for the more common purposes of lime. The Housatonic range of limestone is perhaps the most extensive in New England. It commences in Milford, near the mouth of the river, and extends with some intermissions quite to its head, and even beyond to the St Lawrence river, if not into Canada. The Stockbridge and Middlebury marble, are in this range. In Bolton, and Boxborough, the town north, are da- posits of moderate extent. In Smithfield, Rhode Island, is a quarry, from which lime of the finest quality is pro- cured in great quantities. In Stoneham, twelve miles north of Boston, is a small deposit. This resembles GEOLOtiY. 39 the statuary marble. From Thomastown, Maine, both marble and common lime are procured for Boston mar- ket in abundance. In the western part of New Eng- land, and the eastern part of New York, is an extensive range, less ancient, than those already mentioned, and less recent than that which constitutes the principal rock in the western part of New York, and the country between that and the Mississippi. Gypsum, (plaster of Paris,) is one of those rocks which are found in great abundance in a few places upon the earth, but are not common. Nova Scotia, the western part of New York, the vicinity of Paris, France, and a few other places, contain inexhaustible deposits of it. None of consequence has yet been discovered in New England. Several varieties of this rock are found in abundance, some of which are transparent, crystallized, and beauti- ful. A common and abundant variety of crystallized gypsum is called selenite, which is in transparent plates or laminae, and resembles mica, and with that has been called isinglass. Their resemblance, however, is mere- ly in their external appearance, their elements being entirely different. Mica is also highly elastic, while selenite is not so, and cannot be bent without breaking. Gypsum has numerous uses, the most important of which is in agriculture. It has the power of entirely changing the character of some soils, and rendering the most barren, gravelly plains highly fertile. It is more favorable to the growth of clover than any other plant, but promotes the growth of potatoes, Indian corn, and on some soils, of winter grain. It is also used for vari- ous ornamental purposes as a plaster. The ingredients of which gypsum is composed, are lime, sulphuric acid, and water. Soapstone is composed of talc, with a small mixture of quartz. Both talc, and the rock which it composes are soft, and easily wrought into any shape required by their numerous uses. This rock is hewn into blocks by an axe, separated into slabs by a common saw-mill, or a hand-saw, turned into cylinders or other circular forms by a lathe, and smoothed by a plane. 40 GEOLOGY. The ease with which soapstone is wrought, the hand- some polish it is capable of receiving, and its power of resisting the effects of heat upon most other rocks, ren- der it extensively useful in the arts. Besides its vari- ous uses in connexion with heat, it is preferable to any other material yet discovered for cylinders, used in manufactories for dressing yarn to fit it for the loom. The powder of this stone when mixed with oil, has recently been applied to great advantage to the gudgeons of wheels, axletrees of carriages, and to various other purposes of a similar kind. Vermont and New Hampshire furnish soapstone in great abundance, and of various qualities. A quarry in Francestown, in the southern part of New Hamp- ahire, has furnished more for Boston market and the various manufactories in New England, than any other deposit. Orford, New Hampshire, contains a beautiful variety. Several towns in Vermont, near the Connecti- cut river, contain it in great abundance, and of a good quality. When a rock of a homogeneous base has crystals of a certain character dispersed through the mass, it is called poi'phyry. The base is more commonly some variety of hornblende rock, but sometimes . pable of separate motion, have a motion separate from that of the whole body of the earth. These waters, and this atmosphere rise towards the sun and the moon on VOL. I NO. IV. 7* 78 GRAVITATION. those parts of the earth which are towards those bodies. This conclusively proves that the gravitation of the earth towards the sun and moon is not simply the gravitation of the earth as a whole or of some nucleus in the centre, but of the various parts of the earth, as separate and dis- tinct portions of matter. There is another phenomenon, which was observed long before it was understood, and which is decisive proof of the same point. The earth is not exactly spheri- cal but is fuller in its shape around the equator. Now if the attraction of the earth to the sun is owing to some peculiar substance in its centre, it is plain that this sur- plus about the equatorial regions will have no effect upon its motions. If, however, this surplus gravitates towards the sun, as all the rest of the matter of which the earth is composed, it is plain that in some peculiar circumstan- ces it may cause a modification of its motions. This last is found to be the fact ; and it proves that the solid parts of the earth as well as the liquid and gaseous, have sepa- rately a tendency towards other material bodies which come into their vicinity. 3. GRAVITATION BETWEEN THE EARTH AND THE LOOSE MASSES UPON ITS SURFACE. It is scarcely necessary to remark that gravitation is manifested in this case by the falling of bodies, and by their weight. That there is no one downward direction, into which all bodies tend, is evident from the fact that upon different sides of the earth, bodies fall in different directions towards it. A stone in America, and another in Asia, falling to the ground, will move directly towards each other. All these motions are evidently the result of a tendency of all bodies to move towards the great mass of matter constituting the earth. The air gravitates ; that is, is attracted by the earth, and rests with weight upon it. That portion of the air which is near the surface is loaded with the burden of all that is above, and is compressed by it in a much smaller space than it would naturally occupy. This pressure produces GRAVITATION. 79 a great many curious effects.* If a vacancy is anywhere produced, the surrounding air is forced by this pressure violently into it. If the air is removed from a bladder, the sides are forced together, and no effort can separate them so as to leave within an empty space. When the piston or box of a pump rises, it brings up with it the air within the pump ; the load of air upon the water around it forces the liquid up into the space thus left. When the air is exhausted by a suitable apparatus from a thin glass vessel, its sides will often be crushed inwards by the pressure of the surrounding air. It was well known that the atmosphere would rush with violence into any vacant space long before the facts were referred to the right cause, and there was a long and obstinate controversy among the philosophers, wheth- er the phenomena in question were really owing to the weight and pressure of the air, or to What one party called Nature's abhorrence of a vacuum. This controversy was at last settled by experiments made upon a certain moun- tain in the south of France, by which it appeared that the tendency of the air to rush into the vacant space was decidedly less upon the elevation, than at the ordinary level of the ground. Now, as in ascending an eminence, we pass above a considerable portion of the atmosphere, it was natural that what remained above the summit, should press less heavily than the whole. The difference of the effects was therefore very easily accounted for, on the supposition that they were both owing to the pressure of the air ; and as it was absurd to suppose that Nature's abhorrence ef a vacuum would be less upon a mountain, than in a valley, the advocates of this latter theory gave up the point.f The gravitation of the air may also be proved, as it often has been, by a very simple experiment. A vessel, filled as usual with air, is weighed. The air is then removed by an air pump, and the vessel, now empty, is weighed again. The difference, which is very sensible, shows the weight of the air which had been removed. It may be at first imagined that there are some excep- * See No. II., p. 54. t See No. III., p. 41. GRAVITATION tions to the remark that all bodies on or near the surface of the earth, tend to move towards it. Smoke ascends ; vapors rise ; clouds float gently in the sky ; and bal- loons, filled with peculiar gases, soar into the air, bear- ing with them heavy burdens. These, however, far from being exceptions to the rule, are only instances of its perfect operation. A block of wood, though it has in itself a tendency to fall, will rise to the surface of the water, into which it is plunged. The water having a stronger tendency to move towards the earth, presses down under it, and it rises by the very power of gravita- tion itself. The gravitating air, in the same manner, forces up the vapor, the smoke and the balloon, all of which would fall with the rapidity of a stone, if nothing resisted their motion. 4. GRAVITATION BETWEEN SMALL BODIES AND PARTS OP THE EARTH. The facts which have been mentioned under the pre- ceding heads, show the existence and the power of grav- itation in cases so numerous and diversified as to render it highly probable that this force is a universal property of matter. There remains, however, much to be added to the evidence ; and one very interesting class of exper- iments are those which show the attracting power of mountains. It was suggested by Newton, that if the law which he discovered were a universal law, a plumb line suspended by the side of a mountain, would deviate from a perpendicular by the attraction of the mountain. The experiment has since been made in two instances, and with complete success. It was first tried in South America, by the side of the celebrated Mountain, Chim- borazo, by some French Mathematicians. It was ascer- tained by very accurate experiments that a plumb line suspended near the mountain was drawn out of its per- pendicular towards the mountain eight seconds. This, it will be perceived, is a very small angle, and the deviation would be expected to be small, if we consider how small is the mountain compared with the whole bulk of the earth. There is every reason to place confidence in the correctness of the result. GRAVITATION. 81 Not many years after, a similar experiment was tried in Scotland, upon Mt. Schehallien, with a similar result ; and the influence of the mountain in drawing the weight from a true perpendicular was established beyond a doubt.* The particular object of the experiment with Schehal- lien, was not to prove the reality of the attraction exert- ed by the mountain, for this was considered as previously settled, but, to prove from the amount of the attraction accurately ascertained, the iveigkt of the, earth. The size of the mountain was ascertained, and also its at- tracting force, and these were compared with thxxse of the earth, and the result showed that the earth attracted more in proportion to its size than the mountain. The philosophers inferred from this that it was composed of heavier, i. e. denser materials, in the proportion of about two to one. 5. GRAVITATION OF SMALL BODIES ON THE EARTH'S SURFACE TOWARDS EACH OTHER. The preceding facts and statements, going as far as they do towards establishing the fact that every portion of matter attracts and is attracted by a',1 other matter, will very naturally suggest the inquiry whether this power is perceptible between small bodies on the earth's surface. It will be said that if this tendency of matter to approach matter is universal, two balls placed upon a level table would have a tendency to roll together. This tendency might exist, and yet not be easily manifested ; for an attractive power, which, in so large a mass as the earth, might have power to move rocks and avalanches * The reader may perhaps have the curiosity to inquire how the deviation of the plumb line from the perpendicular, and particularly its exact amount could be sustained. A telescope was fixed in a perpendicular position by a plumb line, and then moved from its position until it pointed towards a certain fixed star. The dis- tance to which it was moved was noted. The apparatus was then taken to the opposite side of the mountain, and the observation re- peated. It was found that the distance to which the telescope was moved, was different at the two stations. This would not have been the fact, if the plumb lines had been parallel. For the fixed star is at so great a distance that its apparent direction would be in both cases the same. 82 GRAVITATION. with prodigious violence, might exist in bodies so small, as the balls upon the table, and yet not be sufficient to overcome the difficulties which impede their motion. The little inequalities of the table, too small to be perceived by the senses ; the resistance of the air which must be removed from between them if they come together, and the want of perfect regularity in their form, would, per- haps, be sufficient to prevent the motion taking place, when there was a real tendency to it. If the two balls are suspended by strings, and are brought nearly into contact, the obstruction to motion would be less than before ; still the balls in moving towards each other must evidently rise slightly, on account of the nature of their suspension. Consequently their tendency to come to- gether must be sufficient partially to lift them, or its ef- fects would not be visible. Various ingenious plans have been devised for sus- pending bodies in such a manner as to render sensible their gravitation towards each other. An instrument called, from the name of its maker, Cavendish's machine, accomplished the object. Two leaden balls suspended by an apparatus so contrived as to diminish as much as possible resistance and friction, gravitated sensibly to- wards each other. The method adopted was very similar in principle to that employed by Mr Coulomb, for render- ing sensible other weak attractions. This method, very simple and easily imitated, he applied not only to demon- strating gravitation between small bodies, but also to ren- dering sensible, and to measuring very many other weak forces. His instrument was substantially a bar connecting two heavy leaden balls, a and 6, and suspended by the string d, c, attached to the centre of the bar. If now a heavy mass be brought near the ball b, the lat- ter may move towards it with- out being lifted at all, for it may move round horizontally only twisting the string cd. If it move but little, the twist or (~~\_ slight force, and as"this is all torsion of the string is a very O GRAVITATION. 83 which is to be overcome, the whole apparatus is called the torsion balance. When a similar contrivance is re- sorted to to detect and measure weak forces in Electri- city or Magnetism, little circles of paper in the former, and small magnetic bars in the latter case, are substitut- ed for the leaden balls. For these purposes, a single silk worm's thread, is generally used for the line to which the bar is suspended.* By these and similar experiments, the last remaining link is furnished to that beautiful chain of inductive rea- soning, by which universal gravitation is established; and the whole series is abundantly sufficient to satisfy any mind by which it is attentively considered, that the great Creator has made it an unvarying and universal law, that every particle of matter draws towards itself every other, and is itself reciprocally and equally drawn. How simple is the principle; how immense the variety and greatness of its effects ! If this single principle, con- sidered in connexion with the unconceivable multiplicity of useful effects which result from it, were the only proofs of design which the creation afforded, the Atheist would be compelled as he is now, to abandon reason and argu- ment, and rest his cause on the bad passions and propen- sities of the human heart. THE CAUSE OF GRAVITATION. The question has very often arisen, what is the nature of the connexion between one particle of matter and another by which this tendency to approach is produced ? Jupiter and Saturn go out of their respective paths to approach each other. Why do they do it 1 How is the effect produced ? When the stem decays, the apple rapidly goes to the earth. Is there any intervening sub- stance which communicates an effect from one to the other? Can an explanation of the kind like the first described in the introductory remarks, be given of this * The writer has a torsion halance, constructed to be used as an electrometer, in which the line suspending the bar of shelleac, is of glass, a very attennuatcd thread, spun from window glass by a blow pipe and a lamp. 84 GRAVITATION. phenomenon 1 No explanation ever has been, probably none can be given, none is needed. The true state of the case is in all probability this : The Creator has determined that any two portions of matter placed at any distance from each other, shall tend to approach ; and by his own direct agency he carries continually this determination into effect. This is all. Not only nothing more has been discovered, but probably there is nothing more to discover. In regard to gravitation, we probably know the whole. It is one of the few cases when the human mind has finished its work. It has reduced the various and complicated phenomena to one single and most simple principle, and the operation it is most philo- sophical to refer to the Being ' who upholdeth all things by the word of his power.'* LAWS OP GRAVITATION. It remains to point out some simple particulars of the manner in which the principle of gravitation operates, and one who has not much considered the ultimate sim- plicity which reigns in nature will be surprised at their statement. The gravitating influence, then, which any portion of matter exerts, may be considered in the following par- ticulars : 1. It cannot be interrupted or changed. 2. It is the same upon every species of matter. 3. It is the same at every distance. It may seem singular to present seriously and formally such propositions as these, which only state the unchange- * We use the word probable, frequently in these remarks, because it would be rash to say positively that no intervening link can be discovered between the cause and the effect in this case. But when we consider that if such a link could be discovered, its own connexions must be sustained by the agency of the Supreme, and when we consider the simplicity and uniformity of the operation of this law, we may, until clear evidence to the contrary is presented, safely consider ourselves as having in this instance arrived at Primordial Law of JYature. GRAVITATION. 85 ableness of this principle in its operation. Some very important consequences however result from them, and all the effects produced by gravitation depend upon them. They consequently deserve, each, a separate considera- tion. 1, It cannot be interrupted or changed. A stone falls towards the earth as rapidly when some substance is below it, intervening between it and the earth, as without such intervention. In other words, nothing can cut off the communication between one body and another, so as to interrupt the gravitating force which tends to bring them together. If, in the case of the tor- sion balance, a plate of brass, or of any other substance, is brought between the two bodies with which the experi- ment is tried, it will not in the least degree interfere with their action. It may perhaps at first view, appear that no one would have expected such an interference. We should naturally have supposed, it may be said, that the interposition of a foreign substance would not cut off the communication. But this impression results from long familiarity with the fact. Young persons are always surprised to see a penknife attracting a magnetic needle through the glass or the wood of the case ; and an ex- cited electric causing the leaves of the electrometer to diverge, when the instrument is enclosed in an air-tight glass case, is often exhibited to a class in Philosophy as a wonderful phenomenon. No reason can be assigned, why the attraction of gravitation should act through an intervening substance, more than that of electricity and magnetism, or why the mind, which is surprised at it in the former case, should consider it a matter of course in the latter.* As the gravitating power of matter cannot be cut off, * The attraction of magnetism is cut off by the interposition of au iron plate. A curious subject, of a moment's reverie, may be fur- nished by reflecting on the complete revolution in the arts and the business of life, which would be produced by the discovery of a sub- stance, which would in the same manner interrupt gravitation, an4 thus enable man to destroy weight at his pleasure. VOL. I. NO. IV. 8 86 GRAVITATION. so it cannot be changed or taken away. Heat a mag- netic needle, and it loses its power. Touch an excited electric, and it will attract no longer. But a bullet or a stone can by no human skill be deprived of its weight. The Creator has made this principle the inseparable and immutable property of every material object which he has formed. 2. It is the same witJi every species of matter. This may at the first sight appear untrue. For it will at once occur to the reader, that since gravitation is the cause of weight, and some bodies are much more heavy than others of the same magnitude, there must be a dif- ference in the gravitating power. For example, since a ball of lead falls much more rapidly and with much greater force than a ball of cork, it might be inferred that the gravitation of the former is much greater than that of the latter. But there is another way to account for the rapidity and force with which lead descends; i.e. by supposing that there is more lead than cork in balls of equal size. The struc- ture of the cork may be such that the particles are not compact together, so that they may be three times as many particles of lead in the same space. If this were true, the whole mass of lead would fall with three times the force, if every particle, whether of lead or of cork, were attracted alike. Now as the force with which the ball of cork would fall, must be less than that of the lead, its velocity must be less too, for it has the same quantity of air to remove from its path, and less power with which to remove it. It would therefore be more retarded. If now the atmosphere is removed, and the two bodies are allowed to fall through empty space, they will fall together, if both kinds of matter are attracted equally. For if each is drawn in proportion to the quantity in each, it is plain that they will be equally affected. This experiment has often been tried ; it is called the guinea and feather, experiment, because a guinea and a feather have been often used as the light and heavy bodies. A tall glass vessel is fitted to the air pump, with the GRAVITATION. 87 guinea and feather so fixed at the top, that they can be dropped by touching a wire. They go exactly together to the bottom; which shows that the air is the only cause why the feather usually falls more slowly. It will be very evident that the constitution of nature might easily have been such, that some species of matter would have been more strongly attracted than others. It may be a subject of interesting reflection, to consider, what would have been the effects of such an arrangement, and by what phenomena such a fact would be made manifest. 3. It is the same at every distance. This statement will excite surprise also until it is ex- plained. Let A be any body of matter, ^-"""* and cde, and fgh, represent concentric spheres around it. Now, what we mean to say is, that the force of attraction which A exerts, at the distance cde, in every direction from A, that is, in the whole sphere, will be tho same which it will exert in the whole sphere at fgh. In other words, the whole amount offeree exerted in every direction, by a body, at any distance, is the same with the whole amount in every direction, at any other distance. It will be evident from this principle, that, since the whole amount is at every distance the same, and as the sphere of influence increases the farther we go from the body, the force at any one point must beless. In other words, as the power of gravitation extends in every direction, the farther we go from the body, it is, as it were, diffused over a greater space, and consequently will be, in any one point, weakened by this diffusion. For 88 GRAVITATION. example, let A be the attracting body, and b another body which it attracts; ^ now from what was f== stated before, it will readily appear, that the force of gravitation exerted by a, between the points c and d, is the same with that between b and e. But at d, the body is under the influence of only half this force, for all that part which is between c and e does not act upon the body, whereas at be it feels the whole. It would appear therefore from this diagram, that if we double the distance of any attracted body, we diminish the amount of force which acts upon it by one half. This however is not strictly true, for the preced. ing diagram represents only a part of the increase of space, in receding from a point. In the adjoining fig- ure, where c represents the centre, and oc, od, &,c, lines diverging from it, and 6 and a corresponding spaces at dif- ferent distances, it will be seen that b is more than twice as large as a, for it doubles in length, and also in breadth; it is consequently four times as large. If b were three times as far from o, as a is, it would accordingly be nine times as large. Consequently the force of gravitation, in order to be equal at every distance, must be diffused, as it were, in proportion to the square of the distance ; and as the power over any particular body will be inversely as the diffusion of the force at the distance of the body, it follows that this power will be inversely as the square of the distance. This result, it is necessary for the reader fully to understand, and to fix in his memory. It is considered a fundamental law. Expressed in general terms it is as follows : The force of gravitation exerted by one body upon another, is inversely as the square of the distance of the bodies* * This is usually stated at once as the fundamental law of gravi- tation, and iiot deduced, as we have done, from the simpler state- GRAVITATION. This will be easily understood and remembered, if the reader is careful to notice the distinction which is made between the whole force exerted in every direction, by any central mass, which whole amount is, at every distance, the same, and that particular part of this force, which operates upon an attracted body, when placed at differ- ent distances ; this particular part being inversely as the square of the distance. How simple and beautiful are these laws. They are not exceptions to the general principle, nor even modifi- cations. They are, on the contrary, simple assertions of its unvarying uniformity. From these, however few, and simple, and negative in character as they are, every one of the complicated and powerful effects of this principle can be mathematically deduced. The motions of the heavenly bodies in their orbits, the exact curves which they describe, every deviation from their regular paths ; the tides, both aqueous and aerial, with all their fluctuations; the path of every cannon ball; the ve- locity of every falling stone, and the rapidity of the vi- brations of every pendulum, can all be accurately cal- culated and ascertained, from this simple principle of an attractive power, exerted by every particle, which, under all circumstances, and at every distance, remains invari- ably the same. EFFECTS PRODUCED BY GRAVITATION. It is not possible to consider within the limits of the present number, the various and complicated effects pro- duced by gravitation. Their variety and complication arise from the operation of this principle in combination with others. Strictly speaking, gravitation never pro- duces but one effect, and that is a tendency of one body to approach another, in the inverse ratio of the square of the distance. This simple effect is, however, modified ment, that gravitation is the same at every distance. When it is so stated, the young scholar is often surprised that Providence should have adopted that exact mathematical ratio of decrease. This sur- prise is removed by considering the subject in the light in which we have presented it. VOL. I. NO. IV. 8* GRAVITATION. by the presence, and contemporaneous action of various other principles. There is only one class of these effects, which proper- ly comes within the scope of our present design ; and that is the case of bodies falling freely. This subject we shall proceed briefly to investigate. The result to which we shall come, and which is called the general law of falling bodies is this The spaces passed over by falling bodies are as the squares of the times. That is, if one body is falling one minute, and another two minutes, the spaces which they will respectively de- scribe, will be as the squares of those numbers, so that the second will not fall simply twice as far as the first, but four times as far. For as the square of 1 is 1, and of 2, 4, the spaces will be as 1 to 4. In the same manner, if one body fall 2 seconds, and another 3, the number of feet of the respective descents will be as 4 to 9, for 4 is the square of 2, and 9 of 3. We cannot here give the mathematical demonstration of this principle, but to illustrate it a little, let the reader imagine a mass of rock loosed from the edge of an over- hanging cliff. No one doubts that the impetuosity of its descent will be increased by the distance it has to fall, but every one has not a distinct idea of the nature and cause of this acceleration. In order to understand this, let us think of its condition after it has fallen through the first ten feet of its descent. Suppose that, at this point, the force of gravitation should suddenly cease, would the rock stop in its descent 1 By no means ; it would go for- ward with the velocity which it had previously acquired, precisely as the cannon ball continues to move swiftly through the air, after the explosive force of gunpowder ceases to operate upon it. The impulse given in this latter case is over in an instant, but the motion continues until the resistance of the air extinguishes it. In the same manner, if the force of gravity were to cease at the end of the first ten feet of the fall of the rock, the mass would move on, with the velocity which it had already acquired, to the earth, excepting that it might lose a little by the resistance of the air. But gravitation does not GRAVITATION. 91 cease. It gives new impulses every moment, which come in to increase the last acquired velocity. Now it is this constant and regular acceleration, which gives to the motion of falling bodies their chief peculiarity, and from this it results, that at the end of 1, 2, 3 and 4 seconds, the spaces through which the body will fall, will be as .1, 4, 9 and 16, which are the squares of the numbers repre- senting the time. It has been found by experiment, that a body falling freely, passes through 16 feet and 1 inch in one second ; in 2 seconds it will pass through 4 times that distance ; in 3 seconds 9 times, and so on. From this, a little arithmetical ingenuity will easily calculate the distance, through which a heavy body will fall, in any number of seconds. The height of a precipice, or the depth of a well may be, by the assistance of a stop watch, measured on this principle, though the result may not be very ac- curate. DISCOVERY OF THE PRINCIPLE OP GRAVITATION : SIB ISAAC NEWTON. The subject of gravitation suggests to the mind of every reader, the name of the great English Philosopher, Sir Isaac Newton. We cannot more appropriately close this treatise, than by describing some of the leading in- cidents in his life. He was born about three hundred years ago, in Lin- colnshire, England, at a place called Woolsthorpe. He was so small, and feeble in his early infancy, that little hope was entertained of his life. This has been the case with many individuals, who afterwards attained to high intellectual eminence. His father died before his birth, but his mother did all in her power to provide for him the means of education. At one of his schools, he dis- played a very singular capacity for mechanical contri- vances. ' By means of little saws, hatchets, hammers, and all sorts of tools, he made models of wood, when his com- panions were at play ; and such was his dexterity, that he constructed a wooden clock, and a good model of a windmill, which was erected about that time in his neigh' borhood. Into this model he sometimes put a mouse, GRAVITATION. which he called his miller, and by means of whose action he could turn the mill round when he chose. He exe- cuted also a water clock, about four feet high, with a dial plate at the top for indicating the hours. The index was turned by a piece of wood, which either rose or fell by the dropping of water. The passion for these me- chanical occupations, often withdrew his attention from his regular studies ; and in consequence of this, the other boys gained places above him, till he was roused to outstrip them all by a little extraordinary exertion. The intermission of his mechanical pursuits, which was thus rendered necessary, rather increased than abated his ardor for them. He introduced the use of paper kites among his school-fellows. He made paper lanterns, by the light of which he went to school in the winter morn- ings; and he frightened the country people by tying them to the tails of his kites, in a dark night. He watched too the motions of the sun with great diligence ; and by means of pegs placed in the wall of the house where he lived, and marks for the hours and half hours, the time of the day was shown to every person, on what went by the name of Isaac's dial. He had also a great turn for drawing; and, according to the account of Mrs Vincent, who was niece to the wife of Sir Isaac's land- lord, at Grentham, he frequently made little tables and cupboards for her and her play-fellows. She mentions also his having made a cart with four wheels, in which he could drive himself by turning a windlass.' Young Newton was, before a long time, recalled from school, to assist his mother in managing the farm. But he did not succeed very well in this employment. His taste for mechanical employments continued, and he felt very little interest in the labors of the plough and the hoe. Sometimes he went, with another person who was employed upon the farm, to a neighboring town to mar- ket, to sell their produce. In such cases he frequently left his charge, and spent his time at an apothecary's, where he found books which interested him, leaving his business in the hands of his attendant. He was not al- ways faithful to his duties at home. ' The study of a book, the execution of a model, or the superintendence GRAVITATION. 03 of a water-wheel of his own construction, often occupied his attention when the sheep were astray, and the corn was treading down by the cattle.' This was wrong. No interest in science, or desire for intellectual improve* ment, can excuse neglect of duty, and especially any un- faithfulness to a trust reposed by a parent. Not long after this, Newton, at the age of eighteen, en- tered the University at Cambridge. Here he soon became distinguished for his interest and progress in mathematical science, and soon after receiving his degree, he turned his attention to some experiments upon light and colors^ which, however, we must not stop to describe. The plague soon after broke out at Cambridge, and to avoid it, he retired to his former home. Here he spent two years, and during this period of retirement and seclusion, he made the discoveries in regard to gravitation, which have immortalized his name. One day, when he wa seated alone in a garden, the fall of an apple arrest- ed his attention. Why should it go towards the earth? was the question. A common mind would have been satisfied with saying, that the support of the stem was removed, and that it fell of course. This, however, did not satisfy our philosopher. He reflected on the subject long. It was not that the fall of the apple appeared new to him. It was only the occasion, which led his mind to reflect on this universal tendency towards the earth, with which, as a fact, he had long been familiar. He consid- ered, that this tendency towards the earth, was the same in all parts of the earth, and at all places. It was not sensibly increased or diminished, in deep valleys or on lofty mountains. It occurred to him that the power might perhaps extend far from the earth, into the regions of the air. It might possibly affect the moon, and if so, if it should prove that the moon was constaatly under the influence of an attraction towards the earth, the nature of her motion in an orbit would be at once explain- ed, and all, the cumbrous perplexities of former philoso- phers, to account for the celestial revolutions would be ended at once. ' I will make the calculation,' thought he, ' and ascertain whether the motions of the moon cor- respond with such a supposition.' 94 GRAVITATION. * Now, although the force of gravity might not be sen- sibly less, at the tops of the highest mountains, than at the ordinary level of the earth's surface, he conceived it to be very possible, that at so great a distance as that of the moon, it might be considerably different. To make an estimate of what might be the degree of diminution, he considered that, if the moon be retained in her orbit by the force of gravity, no doubt the primary planets are carried round the sun by a like power ; and by compar- ing the periods of the several planets with their distances from the sun, he found that if any power like gravity kept them in their orbits, its strength must decrease inj>ro- portion as the squares of the distances increase.' Supposing therefore, that the force of gravity decreas- ed, i. c. so far as its operation upon particular bodies is concerned, in the above named ratio, he proceeded with the laborious calculation. The result disappointed him. It did not correspond with the fact. The hopes which he had cherished of throwing new light on this subject were blasted, and he gave up the consideration of it. The reason of this failure was, however, an error gen- erally prevalent at that time in regard to the size of the earth. He supposed it smaller than it really was. This affected the calculation so as to produce the wrong result which had discouraged him ; and it was not until several years afterwards that he discovered this cause of error. When he did discover it, he resumed the calculation. As his work, on this second attempt drew towards the close, he foresaw the successful result. The joy of attaining success after a previous failure, the magnificence of the expected discovery the changes, which he could easily foresee would be produced in the opinions of man- kind on this subject, the rapid advances which astron- omy might now be expected to make, all burst upon his mind, bringing with them so many agitating emotions, that he could not complete his work. He was obliged to call in the assistance of a friend by whom the result was obtained. We cannot follow this great philosopher through the remaining incidents of his life. He pursued with much diligence and success his mathematical and philosophical GRAVITATION. 95 studies, arid became sometimes involved in controversies in defence of his opinions. These he much regretted, for he was of a mild and peaceable disposition. His character was marked with almost all that is excellent. There were no eccentricities cherished, no singulari- ties in manners or opinions ; he was kind to others, modest and unassuming in regard to himself ; he de- pended on patient, persevering effort for his success in all his efforts, and by his constant fidelity, in the discharge of the duties of social life and religion, he seemed to aim more at happiness, than at fame. QUESTIONS. What is the law of gravitation 1 Is the truth of this to be proved by theoretical reason- ing, or by the observation of facts ? What is the first class of facts named 1 Has it always been known that the revolutions of the planets were caused by the attraction of the central body ? Is there any other evidence of attraction between ce- lestial bodies, besides the attraction of the sun for the planets 1 What effect is produced by the attraction of the plan- ets for each other 1 Are these disturbances of the regular motions nume- rous ? Are they great? What is the second case in which the operation of gravitation is pointed out? What parts of the earth are capable of a separate mo- tion towards the sun and moon 1 What is the third class of facts named 1 How is the attractive force of a mountain made evi- dent? Upon what mountain was the experiment first tried ? Was the effect perceptible? Upon what other mountain has the experiment been made! Were the effects produced very great? How much did the plumb line deviate? Is there any other way in which gravitation has been made apparent? 96 GRAVITATION. Can you name any of the general laws of gravitation ? What is the general character of these laws ? Can the force of gravitation be in any way intercept- ed 1 Can it be weakened 1 Is the whole amount of attractive force exerted by any body the same on different distances 1 Is that part which is exerted at different distances, upon the same body, the same 1 Can any reason be given why bodies should attract each other 1 Mention some of the leading particulars in Newton'3 Life? ADVERTISEMENT. CARTER, HENDEE & BABCOCK, BOSTON, HAVE LATELT PUBLISHED AN ELEMENTARY TREATISE' ON GEOMETRY, simplified for beginners not versed in Algebra. Part 1, containing PLANE GEOMETRY, with its application to the Solution of Problems. By Francis J. Grund. Second Edition. Extracts from the Preface. Popular Education and the increased study of Mathematics, as the proper foundation of all useful knowledge, seem to call espe- cially for elementary treatises on Geometry, as has been evinced in the favorable reception of the first edition of this work wirhin a few months of the date of its publication. A few changes have been made in the present edition, which, it is hoped, will contribute to the usefulness of the work as a book of elementary instruction. 'As regards the use of it in schools and seminaries, the teacher will find sufficient directions in the remarks inserted in the body of the work. At a late meeting of the School Committee of the city of Boston, Mr Grund's Geometry was recommended as a suitable book to be used in the Public Schools. Similar testimonials of the merits and usefulness of the work have been received from Teachers and School Committees, in various parts of New England. C. H. & B. have in press and will publish in a few weeks, ' A Treatise on Solid Geometry, by Francis J. Grund : intended as a Se- quel to the ' First Lessons in Plane Geometry.' Apparatus has been prepared by Mr Josiah Holbrook, calculated to illustrate the Problems contained in the abore work. SCIENTIFIC TRACTS. NUMBER V. ANIMAL MECHANISM. THE EYE. BY JEROME V. C. SMITH, M. D. A VARIETY of professional works are already before the public on the anatomy of the eye, but it is questionable whether any of them are sufficiently divested of technical language, to be of utility to that class of readers who are only interested in the beauties of science. Without making pretensions to originality, the writer of the following pages will endeavor to simplify a subject, too generally considered abstruse, that it may be under- stood by those who have neither patience, time, nor in- clination to pursue it under the guidance of a public instructer. Well acquainted, as anatomists are, with the minute organization of the eye, no one has been able to explain how or why we see. Although the visual organs are constructed with such exact reference'to the laws of light, that telescopes and microscopes, made upon truly philo- sophical principles, are but imitations or modifications of the apparatus of the human eye, there is still a differ- ence between the animate and inanimate, the most wonderful and astonishing. The first is a. perceiving in- strument ; the second, a receiving. The eye can only perform its destined functions, in connexion with a living system, regulated bj an existing harmony of all its complex machinety, consisting of nerves, blood vessels and brain, However perfect in its several tunics the eye may be, or* transparent in its fluids, VOL. i. NO. v. 9 93 ANIMAL MECHANISM. if the sensorium become disordered by disease, it no longer recognises the images or impressions transmitted to it through the visual nerve. Thus it will be under- stood, that the eye may labor, receive, and transmit a miniature picture of all it perceives to the soul ; but, if there is a derangement of that mass of mysteriously constructed matter, filling the whole skull, which all experience de- monstrates to be the seat of thought, no idea is excited. On the other hand, when individuals suddenly lose their sight, without materially injuring the optic nerve, they sometimes dream of seeing. In this case, imagination excites the nerve in such a peculiar and inexplicable manner, as to call up the idea of vision. This nerve being formed with exclusive reference to that function, in the economy of animal life, any impression upon it will excite a corresponding impression in the brain, and no other. All creatures, from man downward, living on land, have their eyes very similar in structure. The same quantity of light that enables a man to see distinctly, will also answer for a horse, an ox, and, indeed, most of the domestic and graminivorous animals. A natural inference would be, then, were it not otherwise known by dissection, that all the parts entering into the composition of their eyes, in order to produce the same effect as in man, were of the same materials. In carnivorous animals, the original principle of vision is preserved, but most curiously modified, according to their habits and characters. Those that feed on herbage, are commonly of social dispositions, feeding in companies through the day, and quietly ruminating or sleeping through the night. Those, on the contrary, that live by violence, preying on'those they have slain, are generally solitary: they lie in ambush, alone, watching for their victim ; and it is so ordered, by the immutable laws of nature, that they slay such as are more timid and helpless than themselves. In order to accomplish this, with the greatest certainty, carnivorous animals have the power of seeing in the dark. Fishes, by a further modification of the original appa- ANIMAL MECHANISM. 99 ratus, common to all others, probably see with peculiar distinctness, in the darkest night, at unfathomable depths of the ocean. With another alteration, not unlike changing the distances between the lenses of a spy-glass, another fami- ly of animals, as seals, &,c, see alternately in two elements. Still further, ou the descending scale of creation, insects are provided with motionless eyes, giving them the fa- culty of seeing in every possible direction. And, lastly, in snails and some kinds of worms, the eyes are fixed at the extremity of a moveable feeler, adapting them to dif- ferent focal distances, or they can be drawn entirely within the head, for safe keeping, when not in use, pre- cisely on the same principle of care that we draw out the slides of an opera glass, and close them up again, when no longer needed. Were we desirous of describing the nice variations in the mechanism of the eyes of the several species of ani- mals adverted to in this preliminary, however interesting it might be to some, would, perhaps, appear tedious to others. Confining ourselves, now, to the exclusive con- sideration of the human eye, we shall proceed with an orderly description of its several parts, hoping that the few scientific terms which must necessarily be retained, will not prove to be a serious embarrassment. THE SOCKET IN WHICH THE EYE ROLLS. Several thin pieces of bone assist in the formation of the orbit, which, in a dry skull, is shaped much like a pear, with its large end turned outward. The upper plate of bone is arched, slightly resembling an arch of a bridge, having the brain resting on it above, and the eye ball moving under it below. Externally, the eyes are at con- siderable distance, but the inner termination of the coni- cal orbits, answering to the small end of the fruit, are quite near together. At their points, is a ragged hole, in each, through which the nerve of vision enters the brain. A large quantity of fat is deposited in this socket, between the bones and eye-ball, that the latter may always move with perfect freedom, and without friction, in all directions. After a long sickness, this cushion of fat is 100 ANIMAL MECHANISM. absorbed, with that deposited in the bones, to sustain the system, which accounts for the sinking in of the eye : aa the person recovers, the stomach resumes the task of taking care of the body, the fat is deposited again, and the eye becomes prominent as before. GLOBE OF THE EYE. When detached from the surrounding parts, the eye- ball does not appear exactly round : it is, in outline, more than two thirds of a large sphere, with a portion of a lesser glebe laid upon it. The use of this arrangement is obvious. If the ball had been actually round, the compass of vision would have been very limited : as it is, the smaller portion, by its short curve, protrudes so far beyond the socket, where the globe is lodged for safely, that the sphere of vision is very much enlarged. MUSCLES OF THE EYE. To move the ball, cords, called muscles, were neces- sary ; otherwise, animals would have been obliged to turn their bodies as often as an object was to be seen. Of these, four are straight, going from the sides of the ball, to be fastened jiear the hole, at the termination of the bony cavity : their office is to hold the eye firmly, in a fixed position, as in steadily contemplating a painting. Two others are given, making six in the whole, to ex- press, principally, the passions of the mind : they are denominated the oblique, in consequence of their oblique movement of the eye. One rolls it downward and out- ward, as in viewing the shoulder ; the other, going through a loop, which is so purely mechanical, that it has been the theme of admiration with philosophers in all ages, carries it upward and inward. The last action can be shown by looking at a button, held on a line with the nose, midway of the forehead. Although these oblique muscles exist in monkeys and nearly all tribes of quadru- peds, they are imperfectly developed ; showing most conclusively that they were designed for expressing the feelings and passions of man an ineffable language, ANIMAL MECHANISM. 101 which all the brute creation have the sagacity to under- stand. When one of the four straight muscles is shorter than its fellow on the opposite side, it produces the cross- eye, or squinting. FIG. 1. Explanation of Figure 1. This plan, from a careful dissection of the right eye, exhibits the muscles, viewed obliquely from its upper and outer side. a The eye-ball. 5 Part of the upper eye-lid, c Tunica Conjunctiva, or inclination of the common skin of the forehead, which turns over the edges of the lids, anil is finally carried over the front of the globe, but is perfectly transparent at this point. d The integuments of the right side of the nose. ee The optic nerve. / The four straight muscles, with the levator, or raising muscle of the upper eye-lid, together with the superior oblique muscle, embracing the optic nerve where it enters the orbit. g The levator of the lid drawn aside. VOL. i. NO. v. 9* 102 ANIMAL MECHANISM. h Levator occuli, or superior straight muscle, to roll the ball upward. i ibductor occuli, rolls the ball outward. k Adductor occuli, rolls it towards the nose. / Depressor occuli, rolls the ball downward, towards the cheek. m The superior oblique muscle, passing through a loop at n. n Called the trochlea, or pully, but, in fact, a simple loop. o Insertion of the superior oblique muscle in the eye-ball. p The inferior oblique muscle, taking its rise from a bone. q The insertion of the tendon of the inferior oblique muscle in the first coat of the ball. COATS OP THE EYE. Such is the mechanical arrangement of the different coats or coverings of the eye, answering in use, to the brass tubes of a spy-glass, that one is fitted within the other, like a nest of boxes : they are three in number. Anatomists, however, make minute subdivisions of these, of no practical benefit to themselves or others. Explanation of Figure 2. FlG - 2 - This is a plan of the coats, or as they are termed in anatomical works, tunics. Reference should be made to this after reading the text. The natural figure of the eye, in out- line, is preserved. a The Sclerotic, or first hard tunic. b The Choroid, or fleecy tunic. c The Retina, or third and in- most tunic, which is an expan- sion of the optic nerve g the certain seat of vision. d The Cornea, or prominent, transparent circle, over which the lids close, in winking, hereafter to be described. e The Crystalline lens, or little magnifying glass of the eye, about a quarter of an inch in diameter. /Is the space filled by one of the fluids of the eye, and called the anterior chamber. g The stump of the optic nerve, which is prolonged into the sub- stance of the brain. ANIMAL MECHANISM. 103 1st. The first is the Sclerotic* coat, thick, firm and possessing but little sensibility. Its hardness gives secu- rity to the delicate membranes beyond ; affords attach- ment for the muscles ; and by its elasticity, equally distends the ball, that none of the humors may suffer from pres- sure. Happily the hard coat is very rarely diseased. Fishes have a sclerotic coat strictly hard, being either cartilaginous or firm bone, graduated in this respect ac- cording to the depth to which they descend in search of food. Without this compensation, the great weight of the water above would crush in their eyes instantaneously. Through this coat, in what is called the white of the eye, the occulist plunges a needle to cure some kinds of blind- ness. 2d. Ckoroidi- is the name of the second coat, having a dark red color, and apparently slightly connected with the first. By carefully cutting off the sclerotic from a bullock's eye, with scissors, the choroid will be beautifully exhibited, sustaining the humors. Minute dissection, under a microscope, shows this tunic is a complete web of arteries and veins ; hence its reddish hue. Between this and the sclerotic, fine silvery threads are seen, which hold a control over the iris, yet to be described, deter- mining by their influence how much or how little light may safely be admitted into the eye. Fungous tumors have their origin in this coat, growing so rapidly as to burst the sclerotica, pushing their way out of their orbit down upon the cheek, incorporating the whole ball in one prodigious mass of disease. The inside of this membrane resembles closely woven wailed cloth, having a fleecy nap, similar to velvet, called tapetum.^. This tapetum is particularly interesting in a philosophical point of view, as on its shade of color, in a great measure, as will be more fully explained in the sequel, depends the power of seeing in the dark. 3d. Retina,^ so called from its resemblance to a net, completes the number, being the innermost and last. Its * Sclerotic, from a Greek word meaning hard. t Choroides, like a lamb-skin, fleecy. t Tapetum resembling cloth, called tapestry. Retina a net. 104 ANIMAL MECHANISM. color is that of gum arable, or ground glass : nothing can be more delicate, being too tender to bear its own weight. In fact, it is the expansion of the optic nerve, the imme- diate seat of vision. To see it well, an eye should be taken to pieces in a tumbler of water. Explanation of Figure 3, FIG. 3. from dissection of a human eye, the organ being represented of the proper size. a The optic nerve. bb The Sclerotic coat cut and turned outward, c A circular portion of the Scle- rotica, being a rim of the white of the eye, cut, and turned up- ward, having in its embrace the cornea. d The cornea, ee One half the iris, in its place, the other half being removed. /The Pupil, soon to be described, with the crystalline lens in its place. g The Ciliary circle, or second vertical partition, within the eye, behind the iris. h h Choroid coat, i The Ciliary processes, or ruffle like plaits of the ciliary circle, yet to be explained. A small portion of the iris is cut away to show them A- A portion of the ins cut and turned back. I The floating points of the ciliary processes, also turned back. m The middle smooth part of the retina, seen by cutting a hole through the choroid coat. n The roots of the ciliary processes, to which the black paint, secreted by the tapetum or inner surface of the choroides, adheres. o The ciliary processes inserted into the capsule, or sack which contains the crystalline lens. THE CORNEA. Anteriorly, that clear, shining wall, resembling a watch crystal, which finishes the membranous box, is called the cornea. Simple as this thin crystal appears, it is infinitely curious in structure. It is made of thin, pelliv- cid plates, one over another, held together by a spongy elastic substance. By maceration in water, a few hours, the sponge will absorb it, to such a degree, that the plate? ANIMAL MECHANISM. 105 may be distinctly felt to slide upon each other, between the thumb and finger. Little glands, like bags of oil, only to be seen by the most powerful microscope, are lodged under the first plate, which are continually oozing out their contents upon the surface, which gives the sparkling brilliancy t^ this part of the eye. As death approaches, this flu- forms a pellicle, like a dark cloud, over the lower portion of the cornea. This formation is taken to be a sure indi- cation of approaching dissolution. Many diseases are peculiar to the cornea ; such, for example, as a milky colored effusion of matter under the external plate, pre- venting a free transmission of light to the interior. See fig. 2, letter d, and fig. 3, letters c and d, for representa- tions of the cornea. By looking into a person's eye, there seems to be a vertical partition, either black, blue, or hazle, as the case may be, which prevents us from looking into the concealed regions beyond, having a round hole in its centre. Scientifically, this partition is called the iris, while its central orifice is denominated the pupil How the diam- eter of this hole is enlarged or diminished, anatomists have never been fortunate enough to explain, satisfac- torily, the apparatus is so minute, that they cannot decide upon its true character. One fact, however, is certain, that the pupil is large or small, according to the quantity of light that may be necessary to the formation of a dis- tinct picture of the object seen, and this change is effected without our being conscious of the action. Resembling other delicate membranes, in many respects, we are unwilling to confuse the subject with a descrip- tion that would distract the mind of a new beginner. From the reflection of such rays as are not admitted through the pupil, or central hole, we account for much of the lively brilliancy of the iris. On its back side it is rather fleecy, like the tapetum, but dissimilar in other respects. Over this is spread a black, blue, hazle, or tea- colored paint, which gives a permanent color to the eye It has been often remarked, that the eyes and hair ordi- 106 ANIMAL MECHANISM. narily correspond ,in color. Whenever the iris acts, as for instance, il does, in going from a dark, into a light room, the pupil is made smaller, acting uniformly in its fibres, to keep it circular. On returning to the dark apartment, the pupil enlarges again. A knowledge of this fact, will explain the reason of a painful sensation in \^'ie eye, caused by a strong and sudden light. As soon as the iris has had time to diminish the size of its pupil, we can endure the same luminous object with perfect comfort. When we leave a well lighted room, on first going into a dark street, everything appears lurid and indistinct. The iris soon begins to enlarge the pupil, to admit more light, and when that has been accomplished, although in comparative darkness, we recognise objects without an effort. Acting independently of the will, its duties are like those of a faithful sentinel, always con- sulting the safety of the splendid optical instrument con- fided to its care, with reference to its subserviency to the being for whose use it was exclusively constructed. Were it otherwise, were it left to our own care, how often it would be neglected, and indeed, totally ruined, solely for the want of undivided attention. All that complex system of machinery, on which life and existence are constantly depending, (the vital organs,) are wisely placed beyond the reach of the laws of volition. If the pulsation of the heart, the function of the lungs, or the circulation of blood in the brain, depended upon our attention, our recollection of the fact, that they must be kept in motion, or we could not live, we should be in great danger of forgetting it, and therefore die in our first slumber ! Parrots have a voluntary control over the pupil, opening and closing it at pleasure. How this is done, or why, in the constitution of that bird, it is neces'sary, we cannot determine. Cats, also, appear to have a similar power of moderating or graduating the quantity of light, admitted into their eyes, as it suits their own convenience. In carnivorous quadrupeds, the pupil is commonly oval, and oblique, permitting them to look from the bottom to the top of a tree, without much elevation of their heads. Graminivorous quadrupeds have an oblong pupil, placed horizontally, with respect to the natural position of the ANIMAL MECHANISM. 107 body. This form gives them the faculty of surveying the expanse of a field, at once. Farmers are familiar with the circumstance that the ox, without being obliged to undergo the fatigue of circuitous inarches, walks directly to the best feed in the whole lot, provided the enclosure be a plain. See fig. 3, letters ee, and k. Fig. 4, let- ters c c. CILIARY PROCESSES. Ciliary Processes. Directly behind the iris, is a second curtain, having a central hole through it, corresponding with that through the first curtain, but nearly as large as the whole diameter of the lens. All the luminous rays which are converged by the convexity of the cornea, which is, in effect, a plane convex lense, cannot enter through the pupil ; many of them strike the plane of the iris, and are reflected back, as on a looking-glass, without pene- trating its substance. If an) rays were to get through, by such an irregular process, it would produce great con- fusion, by destroying the outline and vividness of the image previously made on the visual nerve, through the natural opening. To prevent such mishaps, the paint on the back of the iris is to absorb such rays as are not reflected, and have a tendency therefore to pass onward. Nature, as though fearful that circumstances might so alter the condition of the pigment,* as that some light, notwithstanding this precaution, might penetrate, has interposed this second veil, solely it is supposed to stop all wandering rays. This ciliary curtain presents three thicknesses, and lastly has a thick coat of black paint on its back. In order to give it treble security, as it regards thickness, it is plaited like the folds of a ruffle. There are seventy folds in the human eye, of equal width, nicely laid, one over the other. A part so highly important, cannot be overlooked in studying the philosophy of vision. * Pigment paint. 108 ANIMAL MECHANISM. FIG. 4. Explanation of Figure 4. This plan presents a longitudinal section of the left eye and bony orbit. a The upper eye-lid, shut b The cornea. cc The cut edges of the iris. d The pupil or round hole through the centre of the iris, which, in the living eye, resembles a black, highly polished dot. te The cut edges of the sclerotic and choroid tunics, with the retina, before exhibited in the preceding drawings. f The crystalline Zens, as it is lodged, with reference to other parts. - gg The Ciliary processes continued from the choroid coat. The plaits are here distinctly seen. In other designs accompanying this article, they will be noticed in a front view. h The optic nerve running from the brain, through the bones, to the globe of the eye, apparently closely embraced by the recti, or straight muscles. The levator, or muscle that raises the upper eye-lid. k The upper straight muscle of the eye, called levator occult. I Inferior straight muscles, its antagonist, on the under side of the ball, called depressor occuli. m A section of the infeiior oblique muscle, called obliquus infe- rior, used in rolling the eye upward and inward, as in looking ai a button laid above the root of the nose. The superior oblique, passing through a loop, carries the eye downward and outward, as in looking at the top of the shoulder. These two muscles, by old writers, were termed rotatores and amatores, in allusion to their office of rolling the ball and expressing passions. nn A section of the blood vessels and nerves, with a large quan- tity of fat, surrounding the optic nerve. This fat lies between the muscles and betwixt the socket and globe. ANIMAL MECHANISM. 109 HUMORS OF THE EYE. By humors, medical writers mean the fluids which dis- tend the eye-ball. They are three in number, pos- sessing different densities, and varying much in quality, quantity and use. Beside fulfilling the first intention, viz., distension, they are so purely transparent, as to offer no obstruction to the free passage of light. Too much care cannot be bestowed on the anatomy of these fluids by surgeons, as they are the seats of many remarkable diseases. Those only interested in this description, as general philosophers, by close examination, will have a perfect idea of them, and will consequently understand the real nature of some of the many" causes that weaken the power of vision, or ultimately produce a total blind- ness. The gratification afforded by the examination of a bullock's eye, tracing the several parts by this paper, will be an ample compensation for the labor, because it will forever fix on the mind interesting discoveries, and lead the reader, insensibly, to a course of reflections, productive of much intellectual enjoyment. AQUEOUS HUMOR.* The aqueous humor is the first in the order of demon- stration, lying directly back of the cornea, so clear, that one unacquainted with the existence of it, would not suspect a fluid there. In volume, it is far less than the others : it keeps the cornea prominent, always at the same distance from the iris, in the early periods of life. The space occupied by the aqueous humor, is called the anterior chamber of the eye. (See fig. 2, letter/.) Pass- ing freely through the pupil, it also fills an exceedingly thin apartment, the circumference of the iris, called the posterior chamber. Thus it will be comprehended, that the iris, or in familiar language, first curtain, is actually suspended and floating in a liquor. Were it not for such a contrivance, the iris would soon become dry and shriv- elled, by the intensity of the sun, and therefore rendered totally unfit to perform its appropriate office of opening * Aqueous like water. VOL. I. NO. V. 10 110 ANIMAL MECHANISM. and closing the pupil. An opinion is current, founded undoubtedly in truth, that the aqueous humor is never suffered to remain long at a time, but, on the contrary, is constantly poured in and again drawn off by an infinite number of invisible ducts. By being stationary, it would become speedily turbid, and finally lose its transparency. A knowledge of the rapidity of the secretion, has been the means of encouraging occulists to undertake novel methods of extracting cataracts, a kind of dark mote, through the cornea, as the most certain mode of restoring sight. Twentyfour hours after drawing off the aqueous humor, by a puncture, the anterior chamber will be full again. Old age, characterized by a gradual decay in the vigor of all the individual organs, shows also its insidious ap- proach in the eye. Vessels that have toiled with untiring diligence to the meridian of life, begin to show a loss of energy. Those which have carried the new, pure liquid, forward a less quantity in a given time than for- merly, while those whose task it was to convey away the old stock, are dilatory in the performance of their work. Hence, from being kept too long in the reservoir, in consequence of a tendency to become more turbid, does not allow the light to pass with former facility to the nerve : elderly persons, therefore, have indistinct vision from this cause, similar to looking through a smoky at- mosphere. The writer has a favorite Newfoundland dog, whose eyesight is impaired in this way. Fishes have no aqueous humor at all, as it could be of no service in the element in which they swim : the water surrounding them is the aqueous humor to their organs. Kept, as the humor is, in its own capsule, gives other advantages to the apparatus of vision : it is a concavo-convex glass, ab- solutely and indispensably requisite in an instrument that will produce an image by the same laws that govern the human eye. A sensible diminution in the quantity of this fluid, is very apparent in people advanced in years : the cornea becomes flatter ; the segment of the transpa- rent cornea is so altered, that rays of light are no longer converged as in younger days. This, together with cor- responding derangements within the globe, constitutes ANIMAL MECHANISM. Ill the long-sightedness of old age, mechanically overcome by wearing convex spectacles. So gradually are the changes wrought by aye, that glasses of different focal distances are sought from time to time, to keep pace with the progress of decay. The ingenuity of man is nowhere more curiously dis- played, than in thus availing himself of his discovery of the laws of refraction, in producing artificial lenses to gratify his eye, a never failing source of enjoyment, long after nature has begun to draw the blind that will ulti- mately close between him and the world forever. CRYSTALLINE LENS.* As magnifying glasses of different refractive pow- ers give perfection to optical apparatus, so it is with re- spect to the lenses within the ball. The coats of the eye are equivalent to the tubes of such ingenious instruments. By crystalline lens, is simply meant a body like a button, resembling pure flint glass, somewhat of the shape of a common sun glass, convex on both sides. Its posterior convexity is greater than its anterior, thereby bringing the rays to a point a little distance behind it. Careful investigation shows that this lens is made of a series of plates, applied to each other like the coats of an onion : the centre is firmer than the edges or space between the nucleus and margin. As a whole, it possesses a highly refractive property, but in different degrees, according to the thickness of the lens, receding from the centre to the circumference. Over the whole, to keep it from sliding in any direction, that the centre may not get without the axis of vision, is an envelope, having connexion with all the coats, where they are united on the borders of the cornea, and where it joins the white part of the eye. Being equally trans- parent with the lens itself, it cannot be conveniently ex- hibited. One of its properties is elasticity, though not to the extent we should at first view be led to imagine from the following remarks. Cataracts, the most frequent cause of blindness, origi- * Crystalline lens, resembling crystal or glass. 1 12 ANIMAL MECHANISM. nate in the lens ; sometimes half way between the centre and margin, but ordinarily in the centre. They are either a peculiar deposition of opaque or milky matter, entirely preventing the ingress of light, or an opacity of some of the internal layers of plates, equally destructive to vision. Nothing short of the actual introduction of the couching needle within the globe, or a knife, promises any hope of recovery. Many children are born with this affection ; at all ages, they are liable to form : perhaps the habit of gazing habitually on a strongly reflecting surface, may have a tendency to generate the disease. To remove ca- taracts by extraction, the operator slides a sharp, thin knife, resembling a lancet, through the cornea, from one side to the other, cutting one half from its natural attachment leaving it, when the knife comes out, in the form of a flap, thus : FIG. 5. Explanation of Figure 5. This plan represents an eye, surrounded by its natural appen- dages with a knife passing through the anterior chamber of the eye. A dotted line indicates the lower edge of the flap, made by cutting oflfjust one half the cornea from its attachment with the sclerotica, in ordei 1 to allow the crystalline lens to escape, whenever the knife is withdrawn. As a matter of course, the aqueous humor escapes in a twinkling, at the same moment, the capsule of the lens, previously ruptured, ^designedly, by the point of the knife, as it slides along, spasmodically acts upon the lens by spon- taneous contraction, and protrudes it through the wound. Undoubtedly, the grasp which the straight muscles have on the ball, accelerates its escape. Thus, in taking away the obstruction to sight, the whole lens is extracted. Perhaps the question may arise ANIMAL MECHANISM. 113 how the eye is to answer its original design with the loss of one of its important glasses ? To couch, an operation often mentioned, and often per- formed, is to thrust a delicate needle through the white of the eye, just on its border, till the point reaches the lens, which is then depressed into the lower part of the eye, below the optic axis, so that light may, by entering the pupil, arrive at the nerve. In this last operation, fears are always entertained, that the lens may rise again to its former position, rendering a repetition of the operation indispensable. Secondary cataracts sometimes form, after couching or extraction, and arise in consequence of a thick- ening and opacity of the capsule, which is left behind. Such cases are more alarming in their progress than a disease of the lens, as no surgeon is warranted in promis- ing even a partial relief. If he attempted to tear away the membrane, he might also rend every other within the globe. A few facts of this kind which have a. practical bearing, more or less interesting to every person, may lead to cor- rect views, in relation to some of the diseases which are common to this curious, wonder-working organ. FIG. G. Explanation of Figure 6. This is a scheme showing how a bad operator, by introducing the couching needle too near the cornea, may rupture the ciliary processes, and actually divide the lens in two pieces, without moving it from the optic axis. A The vitreous humor, B The lens. CC Ciliary processes, torn by the lower part of the need's, thereby doing great violence and a permanent injury to the organ. DD The iris. E The anterior chamber of the aqueous humor. VOL. I. NO. V. 10* 114 ANIMAL MECHANISM. Explanation of Figure 1. This figure represents the mode, and, in fact, the place into which the cotiching needle is in- troduced, in the opera- tion of couching. A The pupil is seen through the transpa- rent cornea. B Thezm. C The needle, with the handle elevated so as to depress the point. D The lens and point of the needle in outline: this precisely represents the position of the lens after couching. To complete the operation, it must be carried a little back before withdrawing the needle. VITREOUS HUMOR. Beyond the two humors we have been describing, is the third, differing essentially from either of them. In the first place, in volume it far exceeds the others, qp- cupying more than two thirds of the whole interior and posterior of the ball. Its consistence is that of the white of an egg, but kept in its place by its own appropriate capsule ; it presents many interesting phenomena. When the sack is punctured with a pin, it flows out slowly, in consequence of its adhesiveness. Like the preceding humors, it is transparent, allowing the free passage of light through its substance, and also possesses the addi- tional quality of allowing the rays to separate again, as they leave the point at which they were converged, just back of the lens. Observation proves that the vitreous humor is kept in place by being lodged in cells. Per- haps a piece of sponge might give a tolerable idea of the cellular structure, admitting it to be as transparent as the water which it absorbs. On its fore part, it has a depres- sion, in which the posterior convexity of the lens is lodged, as represented in this diagram. Concave, therefore, in front, and convex behind, gives another kind of optical glass, known as the meniscus, the crescent, faintly resembling the first quarter of the new moon. If ANIMAL MECHANISM. 115 by accident, or a want of skill, the operator suffer this humor to escape, in any of his operations, the globe at once diminishes in size, and all hope of the restoration of a diseased eye is lost. The small mistake of pricking the sack containing the vitreous humor, will decide whether the patient is to live in never ending night. FIG. 8. Explanation of Figure 8. One dotted line, indicates, in this diagram, the aqueous hu- mor ; another the iris, and a third the lens, and the fourth the vitreous humor. Let it be remembered that all the space between the back side of the lens and optic nerve, is filled com- pletely, with the glairy, vitreous humor, the third fluid, and in- most of the eye. OPTIC NERVE. Any person possessing an ordinary share of curiosity, can examine the optic nerve, or nerve of 'sight, at leis- ure, in slaughter houses, fish markets, and in fowls. In the human eye, perhaps, to be clearly understood, out- side the eye, as it extends to meet the brain, it is like a cotton cord, larger than a wheat straw, of a rnealy white- ness, not far from three quarters of an inch in length. Arising from the substance of the brain, it traverses the bony canal till it reaches the back of the eye-ball ; as soon as it arrives in contact, as it were, it is suddenly divided into innumerable filaments, which wend their way into the globe, through very minute holes. From a fanciful resemblance to a sieve, this spot on the scleroti- ca, is called the cribriform plate. When the threads have emerged within, they assume another form, by ex- panding into a web, constituting a third or inmost box. Some believe the nerve is spread on a thin, unseen mem- brane, in the form of a highly organized nervous paste. Here, on this pulp, having considerable range of surface, is the sole seat of vision. A vulgar opinion presupposes pome exceedingly acute nervous point, the exquisite 116 ANIMAL MECHANISM. point of vision. Nothing, however, is more absurd; vision includes considerable surface. In the centre of the substance of the nerve, an artery penetrates the eye, accompanying the Jilaments, to nourish the humors. When the cornea has been cut away, and the iris detach- ed, this vessel may be distinguished, of a bright scarlet, spreading its hair-like branches about, like the limbs of a tree. The nerves which give sensation to the eye, connecting it with the system, may be noticed, as previ- ously remarked, lying between the two first coats. The optic nerve conveys to the mind the sensation of the ex- istence of things, as perceived by the eye, while the commands of the same mind are conveyed to it by these little threads of nerves, so insignificant, as to be often overlooked in a dissection made purposely for them. FIG. 9. Explanation of Figure 9. In this figure, the cornea is cut away, and the sclerotic dissected back. This is a beautiful and easily accomplished dissection. In a bullock's eye all these delicate nerves can be readily displayed. A pair of sharp pointed scissors and a few pins, to hold parts to a board, are the proper instruments. Even in schools, ladies could exhibit most of this beautiful optical apparatus. ANIMAL MECHANISM. 1 17 a The optic nerve. b The sclerotic coat turned back, so as to show the vessels of the choroid coat. ce The ciliary nerves, seen piercing the sclerotic coat, and pass- ing forward to be distributed to the iris. The iris, so highly organized, is not supplied by any nervous influence from the optic, but by the hair-Jike nerves, here displayed, creeping to its map- gin between the two exterior coats. d A small nerve passing from the same source to the same termi- nation, but giving off no visible branches. ee Two vena vorticosce, or whirling veins, so denominated, because they seem to fall into shapes, resembling falling jets of water ; these return the blood from the eye, sent in by its central and other arteries. / A point of the sclerotic^ through which the trunk of one of the veins has passed. A lesser vein. The point, or circular point of union, where all the coats of the eye, together with the cornea and iris, seem to be glued firmly together. t The iris. A; The straight fibres of the iris. I A circle of fibres or vessels, which divide the iris into the larger circle /,: and the lesser one m. m This letter points to the lesser circle of the iris, n The fibres of the lesser circle, G The pupil. PIGMENTUM NIGRUM.* Lastly, to complete the internal structure, and fit it for the performance of its destined office, the inside surface of the second coat, choroides, is thoroughly painted black. In the order of explanation, thjs paint is just behind the retina. When the humors have been taken out the pig- ment is readily examined. The use of it is very obvious; viz., to absorb any aberrating or unnecessary rays of light, which confuse the vision ; or destroy the intensity of the impression on the nervous expansion of the retina, and indeed, to suffocate them entirely. JUNICA CONJUNCTIVA. Posteriorly, the eye, by its long cord of optic nerve, seems to rest on one extremity of an axle ; the opposite in front, being the skin, passing over the eye, as it conies *Pigmentum Nigrum black paint 118 ANIMAL MECHANISM. down from the forehead, to join the cheek. To compre- hend, clearly, the manner in which the eye is fastened, before, observe how the skin turns over the edge of the lid, going about three quarters of an inch back, striking the ball to which it is made fast, then folded back upon itself, adhering to the whole anterior surface of the cornea, dipping down and finally mounting over the margin of the lower lid, and ultimately losing itself on the face. As we cannot recognise this, on a living eye, it will at once lead one to suppose it is as clear as glass, which is the case. Streaks of blood, when the eye is inflamed, lie covered over by the tunica conjunctiva. Now if particles of sand, or other irritating substances get under either eye-lid, they cannot possibly enter but little way, before reaching the duplication of this trans- parent skin ; there is no danger, therefore ; the offending matter cannot get so far between the socket and ball, backward, as to abridge the free motion of the organ, or do a permanent injury to the parts. This partition, or doubling over of the conjunctiva, is a curious provision, as we are thereby enabled to reach the source of irrita- tion. The principle of introducing eye-stones, to extract foreign matter, is this, and not owing, as vulgarly sup- posed, to the crawling about of a smooth piece of sulphate of lime, on some forty or fifty feet. The stone is so much larger than the extraneous body, already there, that it excites a proportionably larger quantity of tears, to wash it away : in essence, therefore, we submit to a greater temporary evil, to get rid of a lesser one. Pre- cisely on this principle, a person chewing tobacco, is constantly spitting : the vile weed is so offensive to the nerves about the region of the throat and tongue, a stimulant so unnatural and uncongenial to the constitu- tion of the body, that the saliva is poured out, with in- creasing copiousness, to wash it from the mouth. Serpents annually shed their skins, which, unaccount- able as it at first appears, are whole over the holes where the eyes were. That thin sheet, so very clear and fine in texture, is the conjunctiva, showing its origin, hence a similar origin may safely be inferred over other eyes. ANIMAL MECHANISM. 119 Every species of animal with which naturalists are con- versant, possess this defensive transparent membrane. MEMBRANA NICTITANS. A third eye-lid is given such animals as are destitute of hands, or are incapacitated, by the arrangement of their limbs, from reaching their eyes. This is called mem- brana nictitans, and a more striking piece of mechan- ism, there is not in existence. It slides from one angle of the eye to the opposite one, under the first pair of lids, and that, too, whether the others are open or shut, being totally independent of them in muscular action. Its use cannot be mistaken : it is on purpose for clearing away matter that may be irritating to the eye. Any ex- traneous substance is brushed from the cornea in an instant, by the broad sweep of the night lid. Birds that seek their food in the night, as owls, defend their irrita- ble organs, through the glare of daylight, by drawing over this singular curtain. Dogs, cats, foxes, wolves, bears, lions, tigers, &c, can each of them, by this brush, remove the minutest mote from the cornea, more expeditiously than any occulist on the globe. Perfection is everywhere observed in animal mechanics. The eye would soon become a useless instrument, not- withstanding the nice adjustment of its several parts, were it not for the external apparatus of eye-lids, glands and tears, whose combined action keeps it always in a condition to be useful. Were not the cornea frequently moistened, it would become dry and shrivelled. To ob- viate this, a sack of fluid is fixed just under the edge of the orbit, above the eye-ball, which is continually pouring out its contents by the pressure and rolling of the eye. Flowing through numberless apertures, it washes the crystal, and finally passing into grooves, on the inner margin of both eye-lids, runs to their terminations in a small pin like orifice, at the inner angles. To keep them open, a hoop is set in the mouth of this lachrymal duct. This, too, can be shown by turning the lid outward by the finger. Finally, the tears are conveyed into the nose 120 ANIMAL MECHANISM. FIG. 10. through a bony tube, answering the double purpose of softening and keeping moist the living membrane, on which the sense of smell depends. On both eye-lids, at the roots of the eye-lashes, are in each, a row of glands, equivalent to bags, smaller than pin heads, which ooze out an oily secretion, to prevent the adhesion of them to- gether, as is sometimes the case when the eyes , are much inflamed. Surely such manifest provision for contingencies, and for the preservation of this one piece of mechanism, indicates Super-human contrivance. Explanation of Figure 10. This plan exhibits the na- tural size of the passages of the t