THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA LOS ANGELES PNEUMATICS. A GLANCE AT THE SCIENCES. BOSTON: G. C. RAND WM. J. REYNOLDS & CO. GLANCE AT THE PHYSICAL SCIENCES; OR THE WONDERS OF NATURE, IN EARTH, AIR, AND SKY: BY THE AUTEOR OF PETER PARLEY'S TALES. BOSTON: PUBLISHED BY GEO. C. HAND, CORXHILL. WM. J. REYNOLDS AND COMPANY. 1852. PRESS OF GEORGE C. RA>"D & C"" QI63 CONTENTS PAGE. ASTRONOMY, 9 The Solar System, 12 The Planet Mercury, 16 The Planet Venus, 17 The Earth, 17 The Planet Mars, 25 Ceres, Pallas, Juno, and Vesta 26 The Planet Jupiter, 27 The Planet Saturn, 28 The Planet Uranus, 32 General Remarks on the Planets, . . ' . . 32 Comets, .36 The Fixed Stars, *'.* . & Meteorites, . ... . . . . 48 Aerolites, . IP Nebulous Stars, . . . ... . . 51 The Firmamental Systems, 59 PROPERTIES OF MATTER, 71 THE MECHANICAL POWERS,. 85 HYDROSTATICS, . . . . . . . . 99 HYDRAULICS, . . . . . . . .109 PNEUMATICS; OR, THE MECHANICAL PROPERTIES OF AIR, 120 OPTICS, .133 4 CONTENTS. PAGE. ACOUSTICS, "...,. 147 ELECTRICITY, 158 GALVANISM, 172 MAGNETISM, . ' . . . . ,. . . .182 ELECTRO-MAGNETISM, 192 MATHEMATICS, 195 METEOROLOGY, 210 CHEMISTRY, . 247 GEOLOGY, .......... 266 Nature of the Crust of the Earth, .... 268 Classification of Rocks, 269 Arrangement of Strata, .:.... 273 Age of Rocks, 274 Formation of Rocks and Strata, . 275 Fossil Remains, 2S1 Changes of the Earth's Surface, . . . .282 Miscellaneous Topics, 286 MINERAL KINGDOM, 289 BOTANY, 302 ZOOLOGY. . . 339 A GIAXCE AT THE SCIENCES. INTRODUCTION. NATUKAL or PHYSICAL SCIENCE is as boundless in its scope as die extent of the universe. It does not confine its researches to the narrow circle within our own observation : it is not content with the investiga- tion of objects presented to the naked eye : it goes with the telescope into the heavens, and descends with the microscope into the atom every where discovering materials for its consideration. Nor is it absorbed with observations upon the forms and hues of material ob- jects : it seeks out the hidden laws of the universe, the principles by which the Architect of the earth and heavens constructs and governs his boundless do- minions. We are apt to wrap up the true idea of scientific investigations in a bald and chilling phraseology : we call them studies of nature ; but they are, in tram, studies into the ways of God. What is natvrt, sep- arate from that active and intelligent Being to whom 1* 6 INTRODUCTION. we are indebted for life and light, that Being who gave us the Bible as well as the Sun, and is as truly the moral as he is the natural Governor of the universe ? The true mode of pursuing scientific studies is to regard them as investigations into the works of the Almighty, and every where, as well in the contem- plation of the starry firmament as in scrutinizing the more familiar objects of our own globe, to realize the presence of the Creator. In this way, science unseals the volume of Nature's revelation, to the most noble and exalting purposes. " While the telescope," says Dr. Chalmers, " enables us to see a system in every star, the microscope un- folds to us a world in every atom. The one instructs us that this mighty globe, with the whole burden of its people and its countries, is but a grain of sand in the vast field of immensity : the other, that every atom may harbor the tribes and families of a busy popula- tion. The one shows us the insignificance of the world we inhabit : the other redeems it from all its in- significance ; for it tells us that, in the leaves of every forest, in the flowers of every garden, in the waters of every rivulet, there are worlds teeming with life, and numberless as are the stars of the firmament. The one suggests to us that, above and beyond all that is visible to man, there may be regions of creation which sweep immeasurably along, and carry the impress of the Al- mighty's hand to tbo remotest scenes of the universe , INTRODUCTION. 7 the other that, within and beneath all that minute- ness which the aided eye of man has been able to explore, there may be a world of invisible beings ; and that, could we draw aside the mysterious curtain which shrouds it from our senses, we might behold a theatre of as many wonders as astronomy can unfold ; a uni- verse within the compass of a point, so small as to elude all the powers of the microscope, but where the Almighty Ruler of all things finds room for the exer- cise of His attributes, where He can raise another mechanism of worlds, and fill and animate them with all the evidence of His glory." How interesting, how instructive, is science, while we thus walk its paths in the light of God's image, and with the constant assurance that, while He thus pursues His vast operations, He is still presiding over the beat- ing of our hearts, and that not even the sparrow falls unnoticed to the ground ! How comparatively barren and desolate are the works of creation, if the Christian's God is every where invisible, and the whole phenom- ena of nature are to be resolved into an inscrutable series of causes and consequences ! In the course of the following pages, we propose only to present a rapid and distinct outline of Physical Science, as it is now exhibited in the works of learned men. Within the present century, the march of knowledge has been rapid beyond example, and at the same time, the most wonderful discoveries have been 8 INTRODUCTION. brought within the reach of every reader. Philosophy is no longer sealed up in learned languages, and kept under the lock and key of colleges and universities. In the compass of this little volume, we hope to place within the reach of our readers, not only the most im- portant results of the researches of Herschel and Laplace into the mechanism of the heavens, but of those of Lyell, Mantel, and others, into the structure of our earth ; to present the wonders of the telescope and the microscope ; in short, to open the book of natural philosophy, and take a glance at its wonderfii revelations, in respect to the stars above, and the ani mal, vegetable, and mineral kingdoms here below. ASTRONOMY. " ASTRONOMY is that department of knowledge which has for its object to investigate the motions, the magni tudes, and distances, of the heavenly bodies ; the laws by which their movements are directed, and the ends they are intended to subserve in the fabric of the uni verse. This is a science which has in all ages engaged the attention of the poet, the philosopher, and the divine, and been the subject of their study and admi- ration. Kings have descended from their thrones to render it homage, and have sometimes enriched it with their labors; and humble shepherds, while watching 10 ASTRONOMY. their flocks by night, have beheld with rapture the blue vault of heaven, with its thousand shining orbs, moving in silent grandeur, till the morning star announced the approach of day. The study of this science must have been coeval with the existence of man ; for there is nc rational being who has for the first time lifted his eyes to the nocturnal sky, and beheld the moon walking in brightness amid the planetary orbs and the host of stars, but must have been struck with admiration and wonder at the splendid scene, and excited to inquiries into the nature and destination of those far-distant orbs. Com- pared with the splendor, the amplitude, the august mo- Uons, and the ideas of infinity which the celestial vault presents, the most resplendent terrestrial scenes sink into inanity, and appear unworthy of being set in com- petition with the glories of the sky. " When, on a clear autumnal evening, after sunset, we take a .serious and attentive view of the celestial canopy ; when we behold the moon displaying her brilliant crescent in the western sky ; the evening star gilding the shades of night ; the planets moving in their several orbits ; the stars, one after another, emerging from the blue ethereal, and gradually lighting up the firmament till it appears all over spangled with a bril- liant assemblage of shining orbs ; and particularly w^en we behold one cluster of stars gradually descending oelow the western horizon, and other clusters emerging from the east, and ascending, in unison, the canopy of heaven ; when we contemplate the whole celestial vault, with all the shining orbs it contains, moving in silent grandeur, like one vast concave sphere, around this lower world and the place on which we stand such a ASTRONO5TY. 11 scene naturally leads a reflecting mind to such inquiries as these : Whence come those stars which are ascend- ing from the east ? Whither have those gone which have disappeared in the west ? What becomes,of the stars, during the day, which are seen in the night ? Is the motion which appears in the celestial vault reoZ, or does a motion in the Earth itself cause this appearance ? What are those immense numbers of shining orbs which appear in every part of the sky ? Are they mere studs, or tapers, fixed in the arch of heaven, or are they bodies of immense size and splendor ? Do they shiiie with borrowed light, or with their own native lustre ? Are they placed only a few miles above the region of the clouds, or at immense distances, beyond the range of human comprehension ? Can their dis- tance be ascertained ? Can their bulk be computed ? By what laws are their motions regulated and what purposes are they destined to subserve in the great plan of the universe ? n These, and similar questions, it is the province of Astronomy to resolve, so far as human intelligence can compass them. Vast as is the subject, and far as it may seem beyond our reach, yet in no other science have there been such gradual and constant accessions of knowledge as in this. It may at the same time be observed, that in none so much remains to be dis- covered. Laplace, who knew more than any other man of the mechanism of the heavens, said earnestly, on his deathbed, " What we know is little what we do not know is immense." The same feeling was entertained by Newton, at the moment of his im- mortal discovery of the principle of gravitation, when, 12 ASTRONOMY. with the modesty of all great minds, beside whose infinite aspirations the highest possible attainment is ever insignificant, he exclaimed, " I am but as a child standing upon the shore of the vast, undiscovered o-,ean, and playing with a little pebble, which the waters have washed to my feet." THE SOLAR SYSTEM. Comparative Size of tfie larger Planets. The Solar System is composed of a great central luminary, the Sun, whose mass is supposed to be made up of opaque matter, like the Earth, the atmosphere alone being luminous, and a number of comparatively small engirdling bodies, the planets, comets, &c., which revolve around it in various periods. The comparative ASTRONOMY. 13 size of these bodies, and their respective distances from each other, may be estimated by the following illustra- tion. On a level field, place a globe, two feet in diam- eter ; this will represent the SUN. MERCURY will be represented by a grain of mustard seed, on the circum- ference o.f a circle 164 feet in diameter ; VENUS, by a pea, on a circle 284 feet in diameter ; the EARTH, a somewhat larger pea, on a circle of 430 feet ; MARS, a large pin's head, on a circle of 654 feet ; JUNO, CERES, VESTA, and PALLAS, grains of sand, in orbits of from 1000 to 1200 feet ; JUPITER, an orange, in an orbit of nearly half a mile across ; SATURN, a small orange, in an orbit of four fifths of a mile ; and URANUS, a cherry, on the circumference of a circle more than a mile and a half in diameter. We shall now proceed to give a more particular account of these members of the solar system. THE SUN. The Sun, when viewed with a telescope, presents the appearance of an enormous globe of fire, frequently in a state of violent agitation or ebullition. Black spots, of irregular form, rarely visible to the naked eye, some- times pass over his disk, in a space of about fourteen days ; one was measured by Sir W. Herschel, in 1779, and found to be 30,000 miles in breadth. A spot, when first seen on the eastern edge, appears like a line, pro gressively extending in breadth, till it reaches the mid- dle, when it begins to contract, and ultimately disap- pears at the western edge. In some rare instances, spots reappear on the eastern side, and are even per- manent for two or three revolutions ; but they generally change their aspect in a few days, and disappear. xiii. 2 14 ASTRONOMY. Astronomers inform us, that sometimes 50 spots are seen, at once, on the Sun's surface. From 1611 to 1629, it was hardly free from spots ; while from 1650 to 1670, scarcely any were to be seen. The same irregularity has been frequently noticed. In October, 1827, 150 spots were noticed at one time. Sometimes, several small spots unite into a large one ; again, a large one separates into smaller ones, which soon vanish. These phenomena induced Herschel to suppose the Sun to be a solid, dark nucleus, surrounded by a vast atmosphere, almost always filled with lumi- nous clouds, occasionally opening and disclosing the opaque mass within. The speculations of Laplace were different ; he imagined the solar orb to be a mass of fire, and that the violent effervescences and explosions, seen on its surface, are occasioned by the eruption of elastic fluids formed in its interior ; and that the spots are enormous caverns, like the craters of our volcanoes. The theory of Herschel, however, is that most generally received by learned men. " The magnitude of this vast luminary is an object which overpowers the imagination. Its diameter is 880,000 miles ; its circumference, 2,764,600 miles; its surface contains 2,432,800,000,000 of square miles, wLk'i is twelve thousand three hundred and fifty times the srea of the terraqueous globe, and nearly fifty then? and times the extent of all the habitable parts of the I3arth. Were its centre placed over the Earth, it would fill the whole orbit of the moon, and reach 200,- 0$0 miles beyond it on every hand. Were a person to tejrjl along the surface of the Sun, so as to pass along ASTRONOMY. 15 every square mile on its surface, at the rate of thirty miles every day, it would require more than two hun- dred and twenty millions of years before the survey of this vast globe could be completed. ** It would contain within its circumference more than thirteen hundred thousand globes as large as the Earth, and a thousand globes of the size of Jupiter, which is the largest planet of the system. It is more than five hundred times larger than all the planets, satellites, and comets belonging to our system, vast and extensive as some of them are. Although its density is little more than that of water, it would weigh 3360 planets such as Saturn, 1067 planets such as Jupiter, 329,000 globes such as the Earth, and more than 2,000,000 of globes such as Mercury, although its density is nearly equal to that of lead." The most obvious apparent motion of the Sun is, that it seems to rise in the morning in the east; to traverse the heavens in a westerly direction, and at last to disappear beneath the horizon. But it is now well understood that the Sun is quiescent, and that the seem- ing motion we have described is occasioned by the daily rotation of the Earth on its axis. But although the Sun stands in the centre of the system of planets, it appears that it revolves on its axis like the other heavenly bodies, and that it completes its revolution in twenty-five days and ten hours. Every part of its equator moves at the rate of 4352 miles an hour. It is also considered probable that the Sun, attended by its troop of planets, makes a vast journey in space, but whether in a straight line, or in an immense circle, is still matter of conjec- ture. 16 ASTRONOMY. THE PLANET MERCURY. This planet is 37,000,000 miles from the Sun, and is the nearest that has yet been discovered. It is seldom seen by the naked eye ; its daily revolution is performed in 24 hours, 5 minutes, and 20 seconds. It revolves round the Sun in the space of 87 days and 23 hours. When viewed with the telescope, it presents the various phases of the moon, from a crescent to the full, round orb. Few discoveries have been made on this planet, ow- ing to the dazzling splendor of its rays. Mountains, however, have been seen ; and one of them is said to be upwards of ten miles in height, which is nearly twice the elevation of the loftiest peaks on our globe. The light upon its surface is supposed to be seven times greater than upon the Earth. If the planet be inhabited, it is obvious that the organization of the eye must be different from that of ours. It is supposed that the in- tensity of heat is not greater than with us. The diameter of Mercury is 3200 miles. Its surface contains 32,000,000 of square miles. It is about one fifteenth the size of the Earth. In its revolution round the Sun, its motion is swifter than that of any other planet, being 109,800 miles every hour, 1830 miles every minute, and more than 30 miles during each beat of the pulse. The density of matter composing Mercury is twice that of the Earth, yet it would require two millions of globes, of the same size, to make one of the size and density of the Sun ASTRONOMY. 17 THE PLAKET VESUS. With the exception of the Sun and moon, this is the most splendid of the heavenly bodies. It appears like a shining lamp amid the lesser orbs of night ; and, at particular seasons, ushers in the morning dawn and the evening twilight. But if such is its appearance to the naked eye, it becomes a still more interesting ob- ject, when viewed with the telescope of the astronomer. It passes through all the phases of the moon, from the crescent to the gibbous form ; and formerly several dark spots were noticed upon its surface. Its daily ro- tation is performed in 23 hours and 20 minutes. Sev- eral mountains have been discovered, and one of them is nearly twenty miles high, or five times the height of Chimborazo. It possesses an atmosphere supposed to be about three miles in height, and is supposed to have a satellite, or moon ; but this is not determined with certainty. The diameter of Venus is 7800 miles, being a little less than that of the Earth. It does not appear that any great quantity of water exists upon it Its quantity of light is about twice that of the Earth. It revolves in an orbit of 433,800,000 miles, in the space of 224 days and 16 hours. Its distance from the Sun is 68,000,000 miles, and from the Earth, when nearest to us, about 27,000,000 miles. Its matter is in a slight degree less dense than that ot the Earth. TEE EARTH. Although the Earth appears to be larger than all the heavenly orbs, it is, in fact, infinitely smaller, and holda 2* 18 ASTRONOMY a rank with the inferior bodies of the universe. Al though it appears to the eye of sense immovably fixed, it has a double motion one on its own axis, and one around the Sun, by which it is transported, with all its continents, and oceans, and kingdoms, at the rate of more than a thousand miles a minute. This planet, like all the other heavenly bodies, has a globular shape ; but it is not a perfect globe, it being de- pressed at the poles. The diameter, through the poles, is 34 miles less than through the equator. This curious fact was discovered by perceiving that the pendulum of a clock had 140 vibrations less in a day, at Paris, than at Cayenne, in Guiana. Further observations were made, and it was found that this variation was uniform, and that the vibrations regularly diminished in proceed- ing northward from the equator. This led to many curious investigations, which resulted in demonstrating ASTRONOMY. 19 the fact we have above mentioned. It is interesting to observe, that so simple a circumstance as the slower movement of clocks, in a southern latitude, should have led to so wonderful a discovery in science as the depression of the poles of the Earth. The prominent feature of the Earth's surface is its division into land and water ; the latter predominates, occupying 148,000,000 square miles, or more than two thirds of the face of the globe. It contains 296,000,000 of cubical miles of water, sufficient to cover the whole globe to the depth of more than half a mile. This superabundance of water is probably peculiar to our planet, and is conjectured to have resulted from the deluge. The surface of the Earth is further diversified by ranges of mountains, stretching across the continents and islands, and giving variety to the landscapes of every country. From these mountains flow myriads of streams, fertilizing the valleys through which they take their course, and at last losing themselves in the ocean. An atmosphere, about 100 miles in height, surrounds this terraqueous mass, which, put in motion, forms the winds, which fan the earth with gentle breezes, or heave the ocean into billows. It is the theatre where the light- nings flash, and the thunders roll ; where the meteor sweeps with its fiery train, and the Aurora Borealis dis- plays its fantastic coruscations. Were the Earth viewed from some point in the heavens as the moon, for instance it would have somewhat the same appearance as the moon does to us. The distinction between its seas, oceans, continents, 20 ASTRONOMY. and islands, would be clearly marked, and would appear like brighter or darker spots upon its disk. The conti- nents would appear bright, and the oceans of a darker hue, because water absorbs a great part of the solar rays that fall upon it. The, Earth, as it would appear from the Moon. We are quite well acquainted with the surface of the Earth, but our knowledge of its internal structure is very limited. The deepest mine does not extend more than a mile from the surface ; and this depth, compared with the diameter of the Earth, is not more than the scratch of a pin upon the surface of an artificial globe. What materials are to be found within the bowels of the Earth, will be forever beyond the power of mortals to determine. It is supposed, however, and not with- out reason, that, while the crust of the globe consists of ASTRONOMY. . 21 a framework of rocks, mingled with earth and water, the centre is occupied with a vast metallic mass in a state of fusion from heat. The density of the whole Earth, bulk for bulk, is estimated at five times the weight of water, so that it would counterpoise five globes of water of the same size. The diurnal revolution of the Earth is performed in 23 hours, 56 minutes. This gives rise to day and night ; to which arrangement of nature, the economy of the vegetable as well as of the animal world is adjusted. The annual revolution of the Earth is ac-' complished in 365 days, 5 hours, 45 minutes, and 51 seconds. From this proceed the varieties of the seasons : spring, summer, autumn, and winter, follow each other in constant succession, diversifying the scenery of nature, and marking the different periods of the year. In those countries which lie in the south- ern hemisphere of the globe, as at Buenos Ayres and the Cape of Good Hope, December, January, and February, are the summer months, while in this north- ern hemisphere, these are the winter months, when the weather is coldest and the days are shortest. The average distance of the Earth from the Sun is 95,000,000 miles. The length of the path annually travelled by the Earth in its orbit is 567,019,740 miles, or about 1000 miles a minute, or 17 miles a second. The Moon, a satellite of our own planet, is the heavenly body of which we have the most accurate knowledge. Its surface exhibits a very large number of mountains, almost uniformly of a circular or cusp- shaped form, the larger ones having, for the most part, flat bottoms within, from which rises, in the 22 ASTRONOMY. centre, a small, steep, conical hill. They offer, in its highest perfection, the true volcanic character, as it may be seen in the crater of Vesuvius. In some of the principal ones, decided marks of volcanic stratifica- tion, arising from successive deposits of ejected matter, may be clearly traced with powerful telescopes. Telescopic Views of the Moon. It is, moreover, a singular fact in the geology of the moon, that, although nothing like water can be per- ceived, yet there are large regions perfectly level, and apparently of an alluvial character. The mountains are known by their shadows, which are distinctly visible, and which are long when they are near the boundary of light and darkness, or when the sun is in the horizon, and disappear when they are 90 degrees from that boundary, or when the sun is overhead. The moon is generally believed either to have no atmosphere, or one of such tenuity as not to equal in ASTRONOMY. 23 density the contents of an exhausted receiver. From this it has been inferred that there are no fluids at the surface of the moon since, if there were, an atmos- phere must be formed by evaporation. Without air and water, it would seem that the moon cannot be inhabited ; or, if life exist there, it cannot be in any form which is exhibited in our own planet. The days and nights in the moon are each 14 days and three quarters in length: the intense heat and cold which must thus alternate would destroy human life, even on the supposition that vegetation could be maintained.* The moon, like all other heavenly bodies, appears to rise in the east and set in the western part of the horizon. Its real motion, however, is in a contrary direction that is, from west to east, or in the same direction in which all the planets move round the Sun. It is a dark body, deriving its light from the Sun, and occasionally a faint light, by reflection of the Sun's rays, from the Earth. It is about 240,000 miles from the centre of the Earth, and pursues its course around this planet at the rate of 2300 miles an hour. It * Such are the conclusions of most philosophers. Yet Dr. Dick observes, that " probably the moon is surrounded with a fluid which serves the purpose of an atmosphere, though it may be different in its nature and composition from that which surrounds the earth." He hence concludes that the moon may be inhabited, and, indeed, proceeds to assume this as the fact. Upon this, he makes a great variety of ingenious suggestions, and even supposes it to be possible to trace the operations of intelligent beings upon its surface. Dr. Olbers is also of the opinion, that the moon is inhabited by rational creatures, and that its surface is covered with vegetation not very dissimilar to that of our Earth. 24 ASTRONOMY. performs its revolution in 29 days, 12 hours, and 44 minutes. It is a curious fact, that the revolution on its axis is performed in the same time as its revolu- tion round the Earth. Accordingly, it always presents the same face to the Earth, so that we never see more than one side of it. The moon appears nearly as large as the Sun ; but it is but about one fiftieth the size of the Earth, and it would take 63,000,000 of globes, of the size of the moon, to make one of the Sun. An Eclipse of the Sun. When the Earth comes between the Sn and moon, it casts its shadow upon the latter, which is then said to be eclipsed. An eclipse of the Sun is occasioned by the moon coming between the Earth and the Sun, thus cutting off its rays. An eclipse of the moon always occurs at the time of its full ; eclipses of the Sun occur at the time of the new moon. It is one of the ASTRONOMY. 25 triumphs of science, that these sublime phenomena, formerly so fruitful a source of superstitious fear and ominous prediction, are now the subject of the most ex- act calculation, and are as much divested of every mys- terious attribute, as the common events of sunrise and sunset. THE PLANET MARS. Telescopic Appearances of Mars. The Earth is placed, in the solar system, between the orbits of Venus and Mars. The latter is 145,000,000 miles from the Sun. When nearest the Earth, its dis- tance is 50,000,000 ; when farthest, 240,000,000 miles. This fact will explain, what most persons have noticed, that this planet is at one time almost imperceptible, and at another seems to vie with Jupiter in magnitude and splendor. The diurnal revolution of Mars is performed in 24 hours, 39 minutes, 29 seconds. Its orbit is 900,000,000 miles in circumference. It performs this circuit in 1 year and 322 days. Its rate of motion is 54,649 miles every hour, which is more than a hundred times greater than the utmost velocity of a cannon-ball When viewed through a telescope, this planet pre- xiii. 3 2O ASTRONOMY. sents a variety of dark spots and belts, though of different forms and shades. Luminous spots, and zones, have also been discovered, which frequently change their appearance, and alternately disappear and return. The latter are supposed to be occasioned by snow ; the former are conjectured to be occasioned by a distribu- tion of the face of the planet into land and water. It is supposed that one third of the surface is occupied by the latter. It is probable that the diversities in the appearance of Mars, as seen through a telescope, are in part occasioned by clouds. Mars has a variety of seasons, similar to ours, and it bears a closer resemblance to the Earth than any other planet. It is 4200 miles in diameter, a little more than half that of our globe. No moon or satellite has been discovered, as attendant upon it. CERES, PALLAS, JUNO, AJND VESTA. The immense interval which lies between the orbits of Mars and Jupiter had led the astronomers to surmise that some planet, of considerable magnitude, might pos- sibly exist within this limit. But instead of one, four small orbs have been recently discovered, which bear the above names. The first, called Ceres, was discov- ered by Piazzi, in Sicily, on the first day of the present century. Pallas was discovered in March, 1802, by Olbers , Juno by Harding, in September, 1804, and Vesta by Olbers, in March, 1807. These four planets are invisible to the naked eye, and we are, therefore, indebted to the telescope for a knowledge of their existence. It is conjectured, and not without reason, that these four planets, were once ASTRONOMY. 27 united in one, and that by some mighty force they have been sundered, and thrown into their present orbits. Their diameter has not been ascertained with precision. Herschel reckons that the largest does not exceed 500 miles in circumference. In several respects, they are marked with peculiarities. The orbits of some of them cross each other, which is not the case with any other planet. They revolve in nearly the same mean distances from the Sun, that is, about 260,000,000 miles. Their annual revolutions, also, are nearly the same ; being little more than four years. They are smaller than the other planets Ceres containing but one eighth part as many solid miles as Mercury. It is probable that they are even smaller than the moons of Jupiter, Saturn, or Uranus. THE PLANET JUPITER. Tdtscopie View, of Jupiter. We now come to one of the most splendid orbs in the planetary system. Jupiter is 495,000,000 miles from the Sun, and the circumference of its orbit is 3,1 10,000,000 of miles. Around this orbit it moves in 11 years, 315 days, at the rate of about 30,000 miles 28 ASTEONOMY. an hour. Its nearest approach to the Earth is about 600,000,000 miles. A cannon-ball, flying at the rate of 500 miles an hour, would reach it in a little less than a hundred years. The daily rotation of Jupiter is performed in 9 hours, 59 minutes, 49J seconds. Its circumference is 278,600 miles. Its density is a little more than that of water, or five times less than that of the Earth. It is the largest planet in our system, being 1400 times larger than the Earth. When viewed with a powerful telescope, this planet presents a splendid appearance. Its surface, then, seems larger than the full moon to the naked eye. Its disk is diversified with darkish parallel stripes. The four satellites, revolving around the planet, generally appear in a straight line with each other. Sometimes, only two of them are visible, the other two being eclipsed either by the disk or the shadow of Jupiter ; at other times, all are seen at once. From their changing appearance, it it supposed that the dark belts of Jupiter are the body of the planet, seen through something analogous to clouds, floating in its atmos- phere at a considerable elevation above its surface. The day and night in Jupiter are nearly equal. The intensity of its solar light is 27 times less than that of the Earth. It is greatly depressed at the poles ; the diameter of the equator being 6,300 miles greater than that at the poles. THE PLANET SATURN. This planet may be considered in many respects the most magnificent and interesting body within the limits of the planetary system. Taking into view its satellites ASTRONOMY. 29 and rings, it has a greater quantity of surface than even the globe of Jupiter ; and its majestic rings con- stitute the most singular and astonishing phenomena that have yet been discovered in the sidereal universe. Its distance from the Sun is 906,000,000 of miles, which is nearly twice the distance of Jupiter, or ten times that of the Earth. The circumference of its orbit is 5,695,000,000 of miles. When nearest, it is 811,000,000 of miles from the Earth. A steam car- riage, travelling at the rate of 20 miles an hour, would not reach it in less than 4629 years. This planet revolves round the Sun in the space of about 29 years. Its motion is at the rate of 22,000 miles an hour. Its diurnal rotation is performed in 10 hours, 29 minutes, and 17 seconds. This rotation is perpendicular to the plane of its rings. Its proportion of light from the Sun is but one 90th of our own. It is 79,000 miles in diameter, and nearly a thousand times larger than the Earth. When viewed with a telescope, it exhibits belts similar to those of Jupiter, and disposed in lines parallel to the ring. These are permanent, and probably indicate a diversity of surface, either of land or water, or some substance with which we are unacquainted. Its figure is spheroidal, with considerable polar depressions. The density of Saturn is about the same as cork, or one half that of water. This is taking into view its whole bulk ; if its centre is hollow, its exterior parts may be as hard as rock. It has been said, that " while a native of earth could hardly move upon Mercury, from the strong attractive power pulling him to the ground, he could, on the planet Saturn, leap sixty feet 3* 30 ASTRONOMY. high as easily as he could here leap a yard." These suppositions are, however, unsound. The density of Mercury is double that of the Earth, and nearly that of lead ; but it must be considered that the attraction in the planets is somewhat in proportion to the masses of matter which they contain, and not in proportion to their density. Taking this principle into view, the attraction upon the surface of Saturn is a little greater than that of the Earth. It is supposed that there is no planet in the solar system, with the exception of Ju- piter, on which an inhabitant of the Earth might not move about as easily as upon our globe ; and on Jupi- ter, he would experience little more than double the weight he now feels. One of the most astonishing phenomena that have yet been discovered in the heavens, is the double ring of Saturn. As generally observed, we have a side view, in which case it presents nearly the following appearance. The outside diameter of the exterior ring is 179,000 miles ; the outside diameter of the interior ring is 152,000 miles. The breadth of the dark space be- tween the two rings is 1800 miles ; so that a body nearly as large as our moon could pass through it. The breadth of the exterior ring is 7200 miles ; of the interior, 20,000 miles. The thickness of the ring is ASTRONOMY. 31 not supposed to be over 100 miles. When it is presented edgewise to the earth, it can only be seen with a powerful glass. This ring is not exactly cir- cular, but slightly elliptical. It is ascertained to have a swift rotation around Saturn, which is completed in about 10 hours and a half. The outer edge of the ring is 550,000 miles in circumference, and moves at the rate of more than 1000 miles a minute. This double ring is a compact, solid substance, for its shadow is distinctly seen on the planet which it encloses. It is not certain that both parts of the ring have exactly the same periods of rotation. It is about 30,000 miles from the surface of the planet, always keeps the same relative position, and attends it in all its movements. One side of it contains 146 times the surface of the whole of our globe ! These rings will appear, to the inhabitants in the firmament of Saturn, like large luminous circles or semicircles of light, stretching across the heavens from east to west, and occupying one fourth part of the sky. As they are brighter than the body of the planet, it is probable that they are of some substance which is fitted to reflect the solar light with peculiar splendor. How glorious, and diversified, must be the celestial scenery thus presented ! Saturn has seven satellites, all revolving beyond its ring. The nearest is 18,000 miles beyond its exterior edges; the most distant is 2,297,000 miles from the planet, and performs its circuit in about 79J days. The largest is supposed to be about the size of Mars, or 4200 miles in diameter. These satellites must afford a splendid appearance from the planet, as some of them must seem nine times larger than our moon. 32 ASTRONOMY. If we take this into view, in connection with the sub- lime splendor of the rings, it might almost seem that Saturn is fitted up to be the abode of some favored beings, upon whom the Creator has lavished the won- ders of his creative power. THE PLANET JJRANUS. This planet, also frequently called after its discov- erer, was made known to us by Herschel, who first saw it in March, 1781. Its distance from the Sun is 1,800,000,000 miles ; and when nearest the Earth, it is nearly the same distance from us. It moves through its orbit in about 84 years. It is the slowest-moving planet in the system, yet pursues its course at the rate of 1500 miles an hour. It. is 110,000 miles in circum- ference, and 81 times larger than the Earth. Its solar light is 360 times less than ours ; yet it is not darker than frequently happens with us in a cloudy day. Its den- sity is nearly equal to that of water. Six satellites are supposed to be connected with this planet ; but their periods and other phenomena have not yet been accurately ascertained. GENERAL REMARKS ON THE PLANETS. The planets all move from west to east, and nearly in the same plane. They are all opaque bodies, deriving their light from the Sun ; 'they are all spheroidal, ap- proaching the form of an exact globe, with slight unevenness of surface. They have all two motions ; one diurnal, around their axes, and one annual, around the Sun. They all present every part of their surface toward the Sun, and they have the alternate change of day and night. They are all connected with the Sun ASTEOXOMY. 33 by the same principle of gravitation. As we know that our Earth is inhabited by thousands of sentient beings, and was created for their accommodation, we may justly conclude that other worlds, associated in the same system, fitted up in nearly the same manner, and acting in obedience to the same great laws, have a similar design, and are, therefore, the abodes of myriads of intelligences not essentially differing from the races on this Earth. The stupendous scale upon which planets are formed their mighty masses their amazing circuits per- formed in the regions of space their almost incon- ceivable velocities still sink into insignificance, when compared with the enormous bulk of the great central luminary around which they revolve. In order to aid the imagination in its efforts to compass this subject, Dr. Dick makes the following suggestion : " There is no point on the surface of the globe that unites so many awful and sublime objects as the top of Etna, and no imagination has dared to form an idea of so glorious and magnificent a scene. The body of the Sun is seen rising from the ocean, immense tracts both of sea and land intervening ; the islands of Pinari, Alicudi, Lipari, Stromboli, and Volcano, with their smoking summits, appear under your feet, and you look down on the whole of Sicily as on a map, and can trace every river through all its windings from its source to its mouth. The view is absolutely boundless on every side, so that the sight is every where lost in the immensity. " Yet this glorious and expansive prospect is com- prised within a circle about 240 miles in diameter, and 34 ASTRONOMY. 754 in circumference, containing 45,240 square miles, which is only girrTVB'TTF P ar ' : ^ the surface of the Sun ; so that fifty-three millions, seven hundred and seventy- six thousand landscapes, such as beheld from Mount Etna, must pass before us before we could contem- plate a surface as expansive as that of the Sun ; and if every such landscape were to occupy two hours in the contemplation, as supposed above, it would require 24,554 years before the whole surface of this immense globe could be in this manner surveyed." The same writer here quoted, and to whom we are largely indebted in the preparation of this article, says, that " it is owing to the existence of the Sun that our globe is a habitable world, and productive of enjoyment. Almost all the benign agencies which are going for- ward in the atmosphere, the waters, and the earth, derive their origin from its powerful and perpetual influence. Its light diffuses itself over every region, and produces all that diversity of coloring which en- livens and adorns the landscape of the world, and without which we should be unable to distinguish one object from another. By its vivifying action, vege- tables are elaborated from inorganic matter, the sap ascends through their myriads of vessels, the flowers glow with the richest hues, the fruits of autumn are matured, and become, in their turn, the support of animals and of man. " By its heat, the waters of the rivers and the ocean are attenuated, and carried to the higher regions of the atmosphere, where they circulate in the form of vapor, till they again descend in showers, to supply the sources of the rivers, and to fertilize the soil. By the same qgency all winds are produced, which purify the atmos- ASTRONOMY. 35 phere by keeping it in perpetual motion, which propel our ships across the ocean, dispel noxious vapors, pre- vent p< stilential effluvia, and rid our habitations of a thousand nuisances. By its attractive energy, the tides of the ocean are modified and regulated, the Earth conducted in its annual course, and the moon sustained and directed in her motions. Its influence descends even to the mineral kingdom, and is felt in the chem- ical compositions and decompositions of the elements of nature. " The disturbances in the electric equilibrium of the atmosphere, which produce the phenomena of thunder, lightning, and rain, and the varieties of terrestrial mag- netism ; the slow degradation of the solid constituents of the globe, and their diffusion among the waters of the ocean, may all be traced, either directly or indirectly, to the agency of the Sun. It illuminates and cheers all the inhabitants of the Earth, from the polar regions to the torrid zone. When its rays gild the eastern hori- zon, after the darkness of the night, something like a new creation appears. The landscape is adorned with a thousand shades and colors; millions of insects awake and bask in its rays ; the birds start from their slumbers, and fill the groves with their melody ; the flocks and herds express their joy in hoarser acclama- tions ; * man goeth forth to his work and to his labor ; * all nature smiles, and ' the hills rejoice on every side.' Without the influence of this august luminary, a uni- versal gloom would ensue, and surrounding worlds, with all their trains of satellites, would be shrouded in perpetual darkness. This Earth would become a life- less mass, a dreary waste, a rude lump of inactive matter, without beauty or order.'' ASTRONOMY. COMETS. None of the heavenly bodie*s have been regarded with more interest than the comets, those wandering and mysterious bodies which, in remote ages, were beheld with superstitious terror. They have been imagined to portend war, pestilence, famine, and the death of monarchs ; ' to be the vehicles in which de- parted souls, released from the care of guardian angels, were transported to heaven ; to have been the cause of the deluge ; to reenforce the light and heat of the sun ; to break up large planets into smaller ones ; to change the climate of countries ; to introduce epidemic disorders ; and, finally, to threaten our globe with total destruction. . ASTRONOMY. 37 A great comet is indeed an object well calculated to impress every beholder with astonishment and awe. Comets have been known with tails extending from the zenith to the horizon, while the disk of the body itself was equal in size to the full moon. The belief which prevailed for a long time with regard to the nature of these bodies, was, that they were meteors of temporary duration, engendered in die atmosphere of the Earth. Some circumstances, certainly, gave a degree of plausi- bility to this supposition ; the suddenness, in many cases, of their appearance and disappearance, the transpa- rency of their tails, and the apparently small density of their bodies. But accurate observations showed that they were far beyond the region of the moon, render- ing it clear that they could not be vapors generated in our atmosphere, and giving a strong probability, to the opinion maintained of old by the Chaldeans, and sup- ported by Seneca, that they were bodies permanent as the planets of our system, and reappearing at certain intervals, depending on their peculiar orbits. It is probable that comets are nothing but bodies of gas or vapor, without any solid matter whatever. Stars have been repeatedly seen through their thick- est parts. The mechanical effect, therefore, to the Earth, from its collision with a comet, would be no greater than that of a mountain when in contact with a cloud : the result of such a collision would be the mix- ture of the gaseous matter with the Earth's atmosphere ; a permanent rise, perhaps, in the mean height of the barometer ; and, if the gaseous matter should condense sufficiently to descend to the lower regions of our at- mosphere, some effect upon animal or vegetable exist- xni. 4 38 ASTRONOMY. ence, good or bad. The Earth may actually have been many times in the tail of a comet, without having any strong marks of such an accident. The bodies of comets have varied from 30 to 3000 miles in diameter ; some of them have been entirely destitute of tails, and others have exhibited them 100,000,000 of miles in length. They move in nar- row, elliptical orbits, travelling to an immense distance out of our system, and at their return approaching, ^n most cases, much nearer to the Sun than any of the planets. Of three of them the periodical revolution has been ascertained. Encke's comet revolves in Encke's Comet. three years and a half; Biela's in six and three quar- ters; and Hal ley's in seventy-five years and a half ; the last of these made its appearance in 1835. A comet with a tail of uncommon magnitude, but with a nucleus scarcely perceptible, visited us in 1843. The great comet of 1680, when at its perihelion, or point nearest the Sun, was only at the distance of one sixth of his diameter from that great body of fire ; it conse- ASTRONOMY. 39 quently was exposed to a heat 27,500 times greater than that received by the Earth a degree so intense as to convert into vapor every terrestrial substance with which we are acquainted. One hundred and forty comets have appeared within the Earth's orbit during the last century, which have not again been seen. If a thousand years be allowed as the average period of each, it may be computed, by the theory of probabil- ities, that the whole number ranging within the Earth's orbit must be 1400. But Uranus being twenty times more distant, there may be no less than 11,200,000 comets that come within the known extent of our system. The trams of comets are always thrown off in a direction opposite to the Sun. No satisfactory solution of the nature and cause of these has been assigned. The effect is the same as if the nucleus of the comet were a globe of water, and the Sun, in shining through it, cast its refracted rays to a distance beyond. THE FIXED STARS. Such is a brief description of the solar system, which, down to the beginning of the present century, com- orised within its limits almost the whole of astronom- >cal science. Before this period, the planetary orbits .seemed to encircle all the space accessible to the human eye ; they had effectively established limits to systematic inquiry ; for astronomers had never pushed their researches into remoter depths, having, like the uninstructed multitude, gazed at the farther heavens with vague and incurious glances, content to admire their beauty and confess their mystery. This period, 40 ASTRONOMY. however, was distinguished by two events which could not have existed in combination without leading to important results. The telescope, formerly of very limited range, suddenly assumed a capability of sound- ing immense profundities of space ; and the man in whose hands it attained this ne\v power was possessed of a genius adequate to improve the highest opportu- nities. The life of Sir William Herschel marks the first and greatest epoch of modern astronomy. He was a discoverer of the first rank : mingling boldness with a just modesty, a thirst after large and general views with a habit of scrupulous obedience to the intima- tions of existing analogies, he was precisely the man to quit paths which, through familiarity, were common and safe, and to guide us into regions dim and remote, where the mind must be a lamp to itself. Herschel communicated to the world the first proof that there existed in the universe organized systems besides our own ; while his magnificent speculations on the Milky Way, and the constitution of the Nebulse, first opened the road to the conception that what was called the universe might be, and in all probability is, but a de- tached and minute portion of that interminable series of similar formations which ought to bear the same name. But before we pursue this topic farther, it will be necessary to give an account of the FIXED STARS, or that stellar firmament to which the solar system be- longs. About 2000 of these stars are visible to the naked eye; but when we view the heavens with a telescope, their number seems to be limited only by the imperfection of the instrument. In one hour Sir William Herschel estimated that 50,000 stars passed ASTROXOMT. Find Stars. through the field of his telescope in a zone of the heavens two degrees in breadth. It has been calcu- lated that the whole expanse of the heavens must exhibit about 100,000,000 of fixed stars, within the reach of telescopic vision. These .stars are classed According to their apparent brightness ; and the places of the most remarkable of those visible to the naked eye, are ascertained with great precision and formed into a catalogue. Tb whole number of stars regis- tered amounts to about 200,000. The distance of the fixed stars is too great to admit of their exhibit- ing a perceptible disk. With a fine telescope, they appear like mere luminous points. Their twinkling arises from sudden changes in the refractive power of the air, which would not be sensible to the eye if they had disks, like the planets. Thus we can learn nothing of the relative distances of the fixed stars from us, and from one another, by their apparent diameters ; 4* 42 ASTRONOMY. but as they do not appear to change their position during the passage of the Earth from one extremity of its orbit to the other, it is evident that we must be more than 200,000,000 miles distant from the nearest. Many of them, however, must be vastly more remote ; for, of two stars that appear close together, one may be far beyond the other in the depth of space. The light of Sirius, according to the observation of Sir John Herschel, is 324 times greater than that of a star of the sixth magnitude. Nothing is known of the absolute size of the fixed stars ; but the quantity of light emitted by many of them shows that they must be much greater than the Sun. Sirius is nearly four times larger, and many stars must be infinitely larger than Sirius. Sometimes stars have been known to vanish from the heavens, and never appear afterwards ; the lost Pleiad of classical mythology is one of these. The last disappearance of a star, noted by astronomers, was in 1828. Sometimes stars have all at once appeared, shone with a bright light, and vanished. A remarkable instance of this occurred in the year 125, which is said to have in- duced Hipparchus to form the first catalogue of stars. Another star appeared near the constellation of the Eagle in 389, and vanished, after remaining for three weeks as bright as Venus. On the 10th of October, 1604, a brilliant star burst forth in the constellation of Serpentarius, which continued visible for a year. A more recent case occurred in 1670, when a new star was discovered in the head of the Swan, which, after becoming invisible, reappeared, and having undergone many variations of light, vanished after two years, and has never since been seen. ASTRONOMY. 43 .n 1572, a star was discovered in Cassiopeia, which rapidly increased in brightness till it even surpassed that of Jupiter; it then gradually diminished in splen- dor, and having exhibited all the variety of tints that indicate the changes of combustion, vanished sixteen months after its discovery, without altering its position. It is impossible to imagine any thing more tremendous than a conflagration that could be visible at such a distance. It is, however, suspected that this star may be periodical, and identical with those which appeared in 945 and 1264. There are, probably, many stars which alternately vanish and reappear, among the innumerable multitudes that spangle the heavens ; the periods of thirteen have already been pretty well ascertained. Of these the most remarkable is in the constellation of the Whale. It appears about twelve times in eleven years, and is of variable brightness, sometimes seem- ing like a star of the second magnitude ; but it does not always attain the same lustre, nor increase and di- minish by the same degrees ; it goes through a com- plete revolution of brightness and obscurity in little less than three days. The cause of the variations in most of the periodical stars is unknown, but it is con- jectured that they may be occasioned by the revolution of some opaque body coming between us and them. Sir John Herschel is struck with the high degree of 4 activity evinced by these changes, in regions where " but for such evidences we might conclude all to be lifeless." Many thousands of stars seem to be only brilliant points ; but, when carefully examined, are found to be, 44 ASTRONOMY. in reality, systems of two or more suns, revolving round each other, or round a common centre. These double and multiple stars are very remote, requiring the most powerful telescopes to show them separately. The motions of revolution of many of these stars round a common centre have been ascertained, and their periods determined with considerable accuracy. Some have accomplished a whole revolution, since their discovery. One of these stars revolves round the other in 1600 years, another in 58. It sometimes happens that the edge of the orbit of a star is presented towards the Earth ; it then seems to move in a straight line, and to oscillate on each side of its primary. There are also quadruple stars, and even assemblages of five and six, revolving round each other. Besides revolutions around one another, some of the binary systems are carried forward in space, by a mo- tion common to both stars, toward some unknown point in the firmament. Two stars in the Swan, which are nearly equal, and have remained at the same distance from each other for above fifty years, have changed their place in the heavens, during that period, between four and five minutes, with a motion which for ages must appear uniform and rectilinear, because, even if the path be curved, so small a portion of it must appear a straight line to us. The single stars, also, have proper motions ; our own Sun is supposed to be moving towards a certain point in the heavens. Though the absolute distance of the fixed stars is still unknown, a limit has been found within which, probably, some of them come. It was natural to sup- pose that, in general, the large stars are nearer to the ASTRONOMY. 45 Earth than the small ones ; but there is now reason to believe that some stars, though by no means so brilliant, are nearer to us than others which shine with greater splendor. This is inferred from the comparative ve- locity of their movements. In consequence of the pro- gressive motion of our Sun, and its planets, all the fixed stars have an apparent motion, which tends ultimately to mix the stars of the different constellations ; but none that we know of moves so rapidly as No. 61 of the Swan ; and on that account it is reckoned to be nearer to us than any other, for an object which we are passing by seems to move more quickly, the nearer we are to it. This circumstance induced Messrs. Arago and Mat- thieu to endeavor to determine its annual parallax; that is, to ascertain what magnitude the diameter of the Earth's orbit would have, as seen from the star. They found, by observation, that the orbit's diameter of 190 millions of miles would be seen from the star under an angle of only half a second ; whence this star must be at the distance of 420 millions of times 190 millions of miles from the earth! a distance which light, flying at the rate of 190,000 miles in a second, would not pass over in less than six years. This is the smallest distance at which the star can be : how much greater its real distance is, it is impossible to say. The apparent motion of five seconds annually, which this star has, seems to us extremely small ; but at that distance an angle of one second corresponds to 24 millions of millions of miles; consequently, the annual motion of this star is 120 millions of millions of miles ; and yet, as M. Arago observes, we call it a fxed star! The doable stars are of various hoes, tat they most frequently exhibit the contrasted colors. The large star is generally yellow, orange, or red : and die snail star Uoe, parpfe, or green. Sometimes a white star is combined with a bine or purple one, and more rarely a red and white one are united. In many cases, these appearances arise from the influence of contrast on oar judgment of colors. For example, in observing a double star, when the large one is a full ruby red, or almost blood color, and the small one a fine green, the latter loses its color when the former is hidden by the cross-wires of the telescope. But there is a vast num- ber of instances where the colors are too strongly to be merely imaginary. Sir John Herschel a very remarkable fact, that, although red stars are common enough, no example of a solitary Woe, green, or purple one has been produced. The stars are Tery irregolariy scattered over the firmament. In some places, they are crowded to- gether, in others thinly dispersed. A few groups, more closely condensed, form very beautiful objects even to the naked eye, of which the Pleiades, and the constel- lation Berenice's Hair, are the most striking examples. Bat the greater number of these clusters of stars appear, to iinaiiiili il roan, like thin white clouds, or vapor ; such B the Milky Way, which, as Sir William Herschel has proved, derives its laig|jiiM from the diffused fight of the myriads of stars that form it. Most of these stars appear to be extremely snail, on BLLUSB* of their enormous distances. This wignbr portion of the heavens, consntnting part of oar firmament, consists of an extensive mass ASTRONOMY. of stars, the thickness of which is small compared to its length and breadth ; the Earth is placed at the point where it divides into two branches, and it appears to be much more splendid in the southern hemisphere than in the northern. Sir John Herschel says, " The gen- eral aspect of the southern circumpolar regions, inc.ud- ing in that expression 60 or 70 degrees of south polar distance, is in a high degree rich and magnificent, owing to the superior brilliancy and large development of the Milky May, which, from the constellation of Orion to that of Antinous, is a blaze of light, strangely interrupted, however, with vacant and entirely starless patches, especially in Scorpio, near Alpha Centauri and the Cross ; while to the north it fades away pale and dim, and is, in comparison, hardly traceable. I tliink it impossible to view this splendid zone, with the aston- ishingly rich and evenly-distributed fringe of stars of the third and fourth magnitude, which forms a broad skirt to its southern border, like a vast curtain, without an impression, amounting almost to conviction, that the Milky Way is not a mere stratum, but annular ; or, at least, that our system is placed within one of the poorer or almost vacant parts of its general mass ! The clus- ter of which our Sun is a member, and which includes the Milky Way and all the stars that adorn our sky, must be of enormous extent, since the Sun is more than 20 millions of millions of miles from the nearest of them ; and the other stars, though apparently so close together, are probably separated from one another by distances equally great." 48 ASTRONOMY. METEORITES. If such remote bodies as the fixed stars shone by reflected light, we should be unconscious of their exist- ence. Each star must then be a sun, and may be presumed to have its system of planets, satellites, and comets, like our own ; and for aught we know, myriads of bodies may be wandering in space unseen by us, of whose nature we can form no idea, and still less of the part they perform in the economy of the universe. Even in our own system, at its farthest limits, minute bodies may be revolving like the new planets, which are so small that their masses have hitherto been inap- preciable, and there may be many still smaller. Nor is this an unwarranted supposition ; many such do come within the sphere of the Earth's attraction, are ignited by the velocity with which they pass through the atmosphere, and are precipitated with great violence on the Earth. The fall of meteoric stones is much more frequent than is generally believed. Hardly a year passes without some instances occurring; and if it be considered that only a small part of the Earth is inhabited, it may be presumed that numbers fall in the ocean, or on the unoccupied part of the land, unseen by man. They are sometimes of great mag- nitude ; the bulk of several has exceeded that of the planet Ceres, which is about 70 miles in diameter. One, which passed within 25 miles of the Earth, was estimated to weigh about 600,000 tons, and to move with a velocity of about 20 miles in a second ; a fragment of it alone reached the ground. The obliquity of the descent of meteorites, the peculiar substances of which ASTRONOMY. 49 they are composed, and the explosion accompanying their fall, show that they are foreign to our system. Luminous spots have occasionally appeared on the dark part of the moon. These have been ascribed to the light arising from the eruption of volcanoes ; whence it has been supposed that meteorites have been pro- jected from the moon by the force of volcanic erup- tion. It has even been computed that if a stone were projected from the moon in a vertical line, with an initial velocity of 10,992 feet in a second, (more than fo^ir times the velocity of a cannon-ball,) instead of falling back to the moon by the attraction of gravity, it would come within the sphere of the Earth's attraction, and revolve about it like a satellite. These bodies, impelled either by the direction of the primitive impulse or by the disturbing action of the Sun, might ultimately pene- trate the Earth's atmosphere and arrive at its surface ; but it is much more probable that they are asteroids revolving about the Sun, and diverted from their course by some disturbing force ; at all events, they must have a common origin, from the uniformity of their chemical composition. AEROLITES. Shooting stars and meteors differ from aerolites in several respects. Aerolites burst from the clear azure sky, and, darting along the heavens, are extinguished without leaving any residuum except a vapor-like smoke, and generally without noise. Calculations have shown them to be very high in the atmosphere some- times even beyond its supposed limit ; and the direction of their motion is, for the most part, opposite to the D XIII. 5 50 ASTRONOMY. motion of the earth in its orbit. The astonishing mul- titudes of shooting stars and fire-balls that have appeared within these few years, at stated periods, over the American continent, and other parts of the globe, war- rant the conclusion that there is either a nebula, or that there are myriads of -bodies revolving round the Sun, which become visible only when inflamed by entering our atmosphere. One of these nebulae, or groups, seems to approach cl{>se to the Earth, in its annual revolution, on the 12th or 13th of Novembet. On the morning of the 12th of November, 1799, thousands of shooting stars, mixed with large meteors, illuminated the heavens, for many hours, over the whole continent of America, from Bra- zil to Labrador ; they were observed even in Green- land and Germany. Meteoric showers were seen off the coast of Spain, and in Ohio, on the morning of the 13lh of November, 1831. In 1832, during many hours of the morning of the 13th of November, prodigious multitudes of shooting stars and meteors fell at Mocha, on the Red Sea, in the Atlantic, in Switzerland. and England. But by far the most splendid meteoric shower on record was in 1833. It began at nine o'clock in the evening of the 12th of November, and continued till sunrise the next morning. It extended from the great lakes of Canada, southward, to Jamaica, and from the 61st degree of longitude in the Atlantic, westerly, to the 100th degree in Central Mexico. Shooting stars and meteors, of the apparent size of Venus, Jupiter, and even the full moon, darted in myriads towards the horizon, as if all the stars in the heavens had started ASTRONOMY. 51 from their spheres. Those who witnessed this grand spectacle were surprised to see that every one of these luminous bodies, without exception, moved in lines which converged to one point in the heavens. None of them started from that point ; but their paths, when traced backward, met in it like rays hi a focus, and the manner of their fall showed that they descended from it in nearly parallel straight lines. The most extraor- dinary part of the phenomenon is, that this radiating point was observed to remain stationary, in the constel- lation Leo, for more than two hours and a half, whick proves the source of the meteoric shower to be alto- gether independent of the Earth's rotation. Other ob- servations showed it to be far above the atmosphere. As all the circumstances of the phenomenon were similar, on the same day, and during the same hours, hi 1832, and as extraordinary flights of shooting stars were seen at many places, both in Europe and Amer- ica, on the 13th of November, 1834, and the two fol- lowing years, proceeding also from a fixed point in the constellation Leo, it has been conjectured, with much apparent probability, that this nebula, or group of bodies, performs its revolution round the Sun in a period of about 182 days, in an elliptical orbit, and that its great- est distance from the Sun is about 95,000,000 of miles, which brings it in contact with the Earth's atmosphere. NEBULOUS STARS. We are now about to introduce to the reader's notice the most wonderful discovery ever made in the science of astronomy, namely, a planetary system in the pro- cess of formation, or a chaos of matter gradually gath- 52 ASTRONOMY. ering into the shape of suns with their attendant worlds ! Certain dim spots, or diffused luminous patches, in the heavens, have long been known to astronomers by the name of nebula ; but their phenomena were looked upon as inexplicable, and regarded as barren marvels, until Sir William Herscnel completely surveyed them all, studied their curious relations, and formally pre- sented his views concerning their probable nature. These nebulae are of two sorts, planetary and stellar. In the former, we behold a starlike body, surrounded with a luminous atmosphere, which the strongest tel- escopes are unable to resolve into separate stars, but which, under every magnifying power, still continue to present the appearance of a vague film. Sir John Herschel says of one of them, in Orion's sword, " I know not how to describe it better than by comparing it with the cu/dling of a liquid, or to a sur- face strewed over with flocks of wool, or to the break- ing up of a mackerel sky, when the clouds begin to assume a linear appearance. It is not very unlike the ASTRONOMY. 53 mottling of the sun's disk, only the grain is much coarser and the intervals darker, and \\\ejlocculi, instead of being round, are drawn into little wisps. They pre- sent, however, no appearance of being composed of stars, and their aspect is altogether different from those of resolvable nebulae. In these we fancy, by glimpses, that we see stars, or that, could we strain our sight a little more, we should see them ; but the former sug- gest no idea of stars, but rather of something quite dis- tinct from them. " In reference to the great nebula in the girdle of Andromeda, there are grounds for a similar conclusion. So that we have this novel and most singular matter not only surrounding stars, and enveloping them as an immense chevelure, but existing also isolated, and in various conditions, from the shape of perfect diffusion, to that where, as in Andromeda, it shows a central nipple, or an apparent point of condensation. It is, perhaps, in its separate and independent form that it fills us with most astonishment. The profusion with \vhich it is distributed, in this form, in both hemispheres, and, indeed, through all the heavens, would imply that it fulfils, or is pressing to fulfil, some important func- tion in the material economy." This strange fluid, a self-luminous, phosphorescent, material substance, exists in a great variety of forms, but generally in a globular shape, and in all varieties of density. Some of the masses are only a thin milky patch, of equal tenuity in every part ; in others, there is a slight condensation toward the centre : this con- densation augments, till, at length, we behold a distinctly- formed star, surrounded by a nebulous atmosphere. 5* M ASTRONOMY. The inference is irresistible, that they are masses of chaotic matter, in a highly diluted or gaseous state, gradually subsiding, by the mutual gravitation of their particles, into stars and sidereal systems. This is the hypothesis of Laplace with regard to the origin of the solar system, which he conceived to be formed by the successive condensations of a nebula whose primeval rotation is still maintained in the rotation and revolution of the Sun, and all the bodies of the solar system, in the same direction. Even at this day, there is presumptive evidence, in the structure and internal heat of the Earth, of its having been at one period in a gaseous state, from an intensely high temperature. The question will naturally occur here, How can such stars as we see come out of these nebulous masses ? and can any star, thus produced, resemble in character the known individuals of our heavens ? To a certain extent this inquiry has been answered, in- geniously and satisfactorily. It is manifest that the orbs arising out of a nebula would be subject to a motion of rotation on an axis, as the Sun is, and, in all probability, the fixed stars are. The confluence of particles toward a centre of attraction would, in gen- eral, if not universally, produce a whirlpool, of which an illustration is extant in the confluence of almost all differently-flowing streams. A rotary motion once communicated, its velocity would increase with the process of condensation. The resulting orbs, then, would rotate ; and as the circumstances of their origin would vary, they would rotate in varying times. The phenomenon of the double stars is also explained here. The whirlpool motion of the original nebula would in- ASTRONOMY. 55 evitably cause an orbitual revolution of binary and more complex systems. A diffused nebulosity is sometimes seen broken up into two or more round nebulae, yet hardly separated. If these individual masses rotate, or are like whirlpools, they must act on each other as wheels ; the result may be illustrated by a very fa- miliar example. AValk along the side of a river, and notice the little moving eddies caused in such multi- tudes by the interference of currents from the unequal sides of the stream ; follow these small eddies for a moment, and observe how, on being whirled down the stream, they come into contact or proximity to each other ; that instant they form, a system, the one revolv- ing round the other, or rather both revolving round some intermediate point. The Sun, and, probably, the other orbs, are attended by planets ; and it is, perhaps, the most interesting part of the whole speculation, to follow Laplace in his ac- count of the gradual formation of these minute circum- stellar bodies from the bosom of the condensing nebula. In any given state of the rotating mass, the outer por- tion, or ring, is in the condition of having its centrif- ugal force exactly balanced by its gravity. The rota- tion increasing in rapidity in consequence of the pro- gressing condensation, the mass of the nebula will abandon this outer ring of 'matter, which may after- wards continue to circulate about the star. Imagina- tion may conceive several zones of vapor thus succes- sively abandoned, and moving, with velocities corre- sponding to their position, around the Sun, or central nebulous mass. The particles of such rings might condense into a solid or liquid substance ; but, unless the 56 ASTRONOMY. formations were originally uniform in all their parts, an iVnprobable hypothesis, they would not condense as rings. We have, in fact, only one example of such a circumstance in the rings of Saturn, a phenomenon altogether invaluable in illustration of the primary con- dition of our system. In most cases, these zones would divide, and form several masses, circulating around the Sun. The same process, in the mean time, would be going on with regard to the planets, in the formation of their satellites. Distinct evidences of the originally nebulous state of the solar system are not wanting. There is a phe- nomenon called the zodiacal light, which may be seen in the twilight of morning and evening, in the neigh- borhood of the Sun, in the shape of a pyramid, or cone, rising above the horizon, and considerably inclined on one side. It appears to extend beyond the orbit of Venus, and is regarded as a portion of the original nebular mass of our system not yet condensed. The comets, moreover, are evidently nebulous bodies, and most of them are strangers to our system, or rather, fortuitous visitants. This fact merely indicates that we must seek their origin in the external spaces, and we find it in those masses of nebulous fluid with which they are intimately connected by constitution, and whose formerly questionable existence they render visible and almost tangible. How interesting the change which passes over the whole aspect of these wandering bodies, when viewed in their true position, not as anomalies, not as monstrous and disturbing intruders into a system with which they are not connected by any harmonizing ties, but as outposts of a mighty sys- ASTRONOMY. 57 tem t which vastly extend our notions of that amount of formless matter existing among the stellar intervals, and which are themselves in progress toward a more perfect organization ! In illustration of the process of the formation of stars and systems from nebulse, the following cut speaks to the eye, and is more valuable than pages of descrip- tion. Each figure in this plate is the representation, Stars and Systems forming from J^'elndte. not of an individual, but of an extensive class; and it would seem that a series so well marked, so striking in its aspects, must indicate the presence and influence of a great law. From absolute vagueness to distinct structure, and then on to the formation of a defined central nucleus, the nebula seems growing under our eye ! " We look," says Laplace, " among these ob- jects as among the trees* of a forest; their change, in the duration of a glance, is undiscoverable : yet we perceive that these are plants in all different stages ; we see that these stages are probably related to each other in the order of time, and we are irresistibly led 58 ASTRONOMY. to the conclusion that the vegetable world, in the one case, and the sidereal world in the other, exhibit, at one instant, a succession of changes requiring time, which the life of man, or the duration of the solar system, may not be sufficient to trace out in individual instances." There is a creature called the ephemeron, whose life is limited within a mere point of time ; in a single day it dances out its existence in the sunbeam. That creature lives in the presence of all the phenomena of vegetable growth ; it may see trees, it may see flow- ers ; but how could it, or its generations, actually ob- serve their progressive development ? In relation to the nebulse, man is but an ephemeron. Fifty lives succeeding each other, and of a length to which indi- viduals often attain, would reach backward beyond the recorded commencement of his race ; and, in the muta- bility of things, fifty more may close its career. Thus no more than what one hundred ephemera can see of the progress upward of the majestic pine, will man, perhaps, ever actually behold of the changes of the nebula?. Yet, after all, where is the intrinsic difference be- *ween the formation of a system of worlds, and the growth and progress of the humblest leaf from its seed to its intricate and most beautiful organization ? That which bewilders us is not any intrinsic difficulty or disparity, but a consideration springing from our own fleeting condition. We are not rendered incredulous by the nature, but overwhelmed by the magnitude, of these creations ; our minds will not stretch out to em- brace the periods of this stupendous change. But time is illimitable, and we are speaking of the operations, and tracing the footsteps, of a Being who is above all time ; we are contemplating the energies of that almighty ASTRONOMY. 59 mind, to whose infinite capacity a day is as a thousand years, and the lifetime of the entire human race but as the moment which dies with the tick of the clock that marks it which is heard and strafghtway passes. THE FIRMAMENTAL SYSTEMS. Notwithstanding the amazing extent of the worlds, and systems of worlds, we have described, they do not constitute the whole universe, but only a very small part of it. Countless firmaments, or clusters of stars, distinct from ours, and at an immense distance from it, exist, sustaining an independent position, as individual constituents of creation. We have already carried our researches into what seemed the infinity of space ; but we must now go forth into far deeper infinity among these firmaments, and ascertain their character. In the intervals between the stars of our own system, and at an immense distance beyond them in the depths of space, many clusters of stars may be seen, like white clouds, or round, comets without tails. When examined with proper instruments, they convey the idea of a globular space, insulated in the heavens, and filled full of stars, constituting a family, or society, apart from the rest, subject only to its own internal laws. The number of these masses is very great. In the northern hemisphere, after making all allowances, those whose places are fixed cannot be fewer than 1000 or 2000 ; and we may form some idea how plentifully they are distributed, by recollecting that this is at least equal to the whole number of stars which the naked eye beholds at once on any ordinary night. 60 ASTRO NOMF. Various Forms of Nebula. To attempt to count the stars in one of these clus- ters, would be a vain task ; they are to be reckoned not by hundreds, but by thousands. On a rough computa- tion, it appears that many of them must contain 10 or 20,000 stars, compacted and wedged together in a globular space, whose area is not more than a tenth part of that covered by the moon; so that its centre, ASTRONOMY. 61 where the stars are seen condensed, is one blaze of light. If, as we have every reason to believe, each of these stars be a sun, and if they be separated by inter- vals equal to that which separates our Sun from the nearest fixed star, the distance which renders the whole cluster barely visible to the naked. eye, must be so great, that the existence of this splendid assemblage can only be known to us by light which must have left it a thousand years ago ! These clusters have a variety of shapes some of them most singular and fantastic. In many of them, individual stars are distinctly defined. As they become more remote, the intervals between the stars dimmish, and the light grows fainter. In their faintest stellar aspect, they may be compared to a handful of fine, sparkling sand, or, as it is aptly termed, star-dust Beyond this we see no stars, but only a streak, or patch, of milky light. Vast multitudes of these are so faint as to be with difficulty discerned at all, till they have been for some time in the field of the telescope, or are just about to quit it. Occasionally, they are so vague, that the eye is conscious of something, without being able to define what it is ; but the unchangeableness of its position proves that it is a real object. The central cluster of stars, in the preceding cut, is a good specimen-object, as it is a representative, or type, of a very large class. Notwithstanding the partial irregularity of its outline, it seems almost a spherical mass, in which, with a degree of greater compression toward the centre, the stars are pretty equally and regularly diffused, so that, to the inhabitants 01 worlds near its central regions, its sky would spangle xra. 6 62 ASTRONOMY. uniformly all around, and present no phenomenon like the Milky Way, in ours. Others of the spherical clus- ters show a much greater compression about the cen- tre a circumstance which would manifestly augment the proportionate number of orbs of the first magni- tude in view of those living within the compressed portion, and thus render their visible heavens incon- ceivably brilliant. Firmaments, however, are by no means confined to the spherical form, as we have already remarked. In the southern hemisphere, a phenomenon, known by the name of the Magellanic Clouds, long excited the wonder of all beholders These clouds have been found to be immense nebulae, or firmaments, of a singular shape. The following is a representation of one of them. This nebula, according to the description of Sir John Herschel, who spent some time at the Cape of Good Hope, in astronomical researches, " is a congeries of clusters of irregular form, globular clusters, and nebulse of various magnitudes and degrees of condensation, ASTRONOMY. 63 among which is interspersed a large portion of irresol- vable nebulae, which may be, and probably is, star-dust, but which the powers of the twenty-feet telescope show only as a general illumination of the field of view, form- ing a bright ground, on which the other objects are scattered. Some of the objects in it are of very singu- lar and incomprehensible forms the chief one espe- cially, which consists of a number of loops, united in a kind of unclear centre or knot, like a bunch of ribbons disposed in what is called a true-love knot There is no part of the heavens where so many nebulae and clusters are crowded into so small a space as this cloud!" But it is when we arrive among the almost bewil- dering multitudes of unresolved systems, that we are most forcibly struck by the variations of their fantastic shapes. The unresolved clusters being at depths much profounder than the sites of the others, the sphere ap- propriated to them is, of course, of larger radius, and far more capacious, so that there is room for greater numbers, and also a more wonderful display of variety. The accompanying sketch exhibits a few of these 64 ASTRONOMY. curious shapes. The annular form sometimes occurs ; one fine instance of it is in the constellation of the Lyre. The oblong sharp hoop, represented in the preceding cut, is probably likewise a large ring, but appearing sharp in consequence of its oblique position with regard to us. How utterly different from ours must be the aspects of the sky to the inhabitants of such a firma- ment ! The space within the ring is nearly a blank, but not perfectly so, a very thin mass of light spreading over it ; so that, to the eye of a spectator placed within the space, the sides will appear nearly an utter blank, while the other part of the heavens will be engirdled with a zone of the most dazzling lustre. One of the most singularly-shaped clusters is the large object in the preceding page. It has the shape of an hour-glass, or dumb-bell ; the two connected hemispheres, as well as the connecting isthmus, being bright and beautiful, manifesting a dense collection of stars in those regions, while the oval is completed by two spaces, which do not transmit a greater quantity of light, nor indicate the presence of a larger number of stars, than the comparatively vacant interior of the ring above described. We are lost in mute astonishment at these endless diversities of character and form. But in the apparent aim of the things near and around us, we may perhaps discern some purpose which such variety may serve. It seems the object, or result, of known material arrangements, to produce every variety of creature; and perhaps it is one end of this wonderful evolution of firmaments of all orders, magnitudes, and forms, that there, too, the law of variety may prevail, ASTK030MT. 65 and room be found for unfolding the whole riches of die Almighty. Of all these wonderful exhibitions, there is no one more singular than what we are about to describe. Although the telescope has not yet enabled us to lay out the plan of our own cluster from interior surveys, it exhibits what seems to be its very picture hung up in external space. The accompanying cut represents a nebula resting near the outermost range of telescopic observation, which is the fac simile of the system to which we belong ! A double representation is given, one of them showing it in a broadside, and the other in an edgewise view. It has its surrounding ring, oftbe^^ precise form which we have been inclined to attribute to our Mflky Way. It adds much to the interest with which we contemplate this cluster, that the inhabitants there must see our system precisely as we see them? namely, sideways ; so that we behold objects of the same aspect when we look at each other. Singular affinity of forms ! What link, what far-reaching sympathy, E 6* 66 ASTRONOMY. can connect these twin masses, that unfathomed firmament and ours ! What virtue is there in a shape so fantastic, that it should be thus repeated ? or what is the august law, exerting its force at the opposite ex- tremities of space, which has caused these correspond- ing shapes to come into being ? Struck with an absorbing and most natural astonish- ment, we soon start the inquiry, Wkat are these clus- ters doing 1 What is their internal condition ? What are their mechanisms ? And what the nature and affections of the bodies which compose them ? Here we approach the region of clouds and doubt ; the solid ground of fact and observation begins to fail us. Yet we are not without warrant in pronouncing that these vast masses are not grouped together by chance, or at random, but that every such union of stars indicates law and system. The only light we find, among these immense spaces, is a welcome gleam of evidence that nature there is also uniform, since the simpler firma- ments manifest, by their shapes, the prevalence of an internal attractive power. Notwithstanding the fan- tastic forms which sometimes occur, the round or glob- ular structure is the general or favorite one ; and in most of these round clusters there is also a strongly- marked increase of light towards the centre, much more than would arise from the circumstance of the eye then looking through the deepest part of the group, and thereby seeing, at once, the greatest number of its stars. This phenomenon decidedly indicates compres- sion, in a greater or less degree ; nor is it confined to masses having the perfectly spherical figure. " There are besides," says Sir William Herschel, " additional ASTRONOMY. 67 circumstances, in the appearance of extended clusters and nebulae, which very much favor the idea of a power lodged in the brightest part. Although the form of these be not globular, it is plainly to be seen that there is a tendency to sphericity, by the swell of the dimen- sions the nearer we draw towards the most luminous place denoting, as it were, a course, or tide, of stars, setting towards a centre. And if allegorical expres- sions may be allowed, it should seem as if the stars, thus flocking towards the seat of power, were stemmed by the crowd of those already assembled, and that while some of them are successful in forcing their pred- ecessors sideways out of their places, others are them- selves obliged to take up lateral situations, while all of them seem eagerly to strive for a place in the central swelling and generating spherical figure." Here another grand field for contemplation is opened. Even the heavens are not stable ! These globular masses, at least, are in process of growth, are ripening ; they are congregating together toward that nucleus round which a new order of things is slowly growing up, and where, perhaps, a mighty orb, whose dimen- sions almost affright the imagination, is preparing for its birth. And this process is, after all, only the pro- longation of the condensing of a simple nebula. Al- ready, some few of its particles have come together and formed its secondary stage ; and now that secondary stage, which we term a firmament, is passing into a third, where all the dispersed atoms will be gathered together, and lodged at the centre of the mass ! Our own firmament presents appearances which not only sustain the foregoing conclusions, through a strong 68 ASTRONOMY. analogy, but point the way to still bolder thoughts. The Milky Way has been already described as a ring, for the most part isolated, in which the stars are very dense, and where the aggregating power has drawn them from the general mass, and, by some curious op- eration, compressed them into a crowded girdle. But neither is this girdle uniform. It is divided into groups, chiefly inclining to the spherical form, and separated from each other by dark spaces, like wrinkles of age. Sir William Herschel counted no less than 225 such groups, or subordinate clusters, within the portion of it which he examined ; and as all these were of a kind to mark the action of gravity, he inferred the existence of a clustering power, drawing the stars of it into sepa- rate groups, a power which had broken up the uni- formity of the zone, and to the irresistible force of which it was still exposed. " Hence," says he, in one of those bold moments in which he fearlessly traversed the infinities alike of past and future, " may we be certain that the stars will be gradually compressed through successive stages of accumulation till they come up to what may be called the ripening period of the globular cluster, and total insulation ; from which it is evident that the Milky Way must forcibly be broken up, and cease to be a stratum of scattered stars. We may also draw an important additional conclusion from the gradual dissolution of the Milky Way ; for the state into which the incessant action of the clustering power has brought it, is a kind of chi'onometer, that may be used to measure the time of its past and present existence. And although we do not know the rate arid going of this mysterious chronometer, it is, nevertheless, ASTRONOMY. 69 certain, that, since a breaking up of the parts of the Milky Way affords a proof that it cannot last forever, it equally bears witness that its past duration cannot be admitted to be infinite." Here is a vision of unfathom- able changes of the solemn march of the majestic heavens from phase to phase, obediently fulfilling their awful destiny. If the aggregation of the stars in the Milky Way still goes on, as it prognosticates, for ages, the clusters which now, with some intermission, form its ring, will become isolated, and appear in the character of separate sys- tems. But'if this may happen in future time, may not something similar have happened in time past ? The aspect of the heavens affords much to countenance this supposition. We can point out, for instance, a cluster of a remarkably irregular form, very narrow in one direction, and surprisingly ragged in the edges. Can it be possible that masses of stars have been torn away from it in certain directions, so that its thinness may simply indicate that, through the action of some irresist- ible cause, parts of it had there ripened sooner ? Sin- gular to relate, it is precisely towards these thin sides, and almost immediately beyond them, that the vast mass of neighboring isolated clusters is found clus- ters all spherical, and grouping together in extraordi- nary proximity. But these operations are, perhaps, only types of what may have occurred on a far more majestic scale. The separate firmaments which our telescopes have dis- covered show, even more emphatically than the groups in the Milky Way, the efficacy and progress of a clus- tering power. May not they all have come originally 70 ASTRONOMY. from one homogeneous stratum, or mass of stars, so that their present isolation, their separation, and vari- ous grouping, are only the measured movements of the clock, the gigantic steps of the hand, by which Time records the days of the years of the existing mechanism of the universe ? Stupendous the conception, that these great heavens the heavens which we have deemed a synonyme of the Infinite and Eternal are nothing else, after all, than one aspect in which matter is des- tined to present itself, and that their history is like the birth, life, death, and dissolution, of the fragile plant ! If this, indeed, be true, and the idea can be supported by many probabilities, how immense the sphere of real existence ! How little can we ever know of it ! at least, how much must be referred to that higher state of existence, an expected eternity of sublime contem- plation ! NOTE. In the preceding pages, under the heads of " Nebular Stars," and " Firmamental Systems," we .have given the state of astronomical science, as generally received, at the present time. But some late observations, made by a gigantic telescope, executed for Lord Rosse, seem to render it probable that the diffused, un- formed nebulae, noticed by Herschel and others, are in fact only groups of stars, too remote to be separately distinguished by the telescopes they used. It must now be considered questionable, whether there are in space any masses of matter differing from the solid bodies which compose planetary systems. PROPERTIES OF MATTER. MATTER is the general name which has been given to every species of substance, or thing, which is capable of occupying space, or which has the qualities of length, breadth, and thickness ; consequently, every thing which can be seen or felt, is said to be matter. In describing the properties of matter, it must be under- stood that they do not apply to the masses, or substances, commonly met with, but to the uncompounded or prim- itive materials of which such substances are formed. These original component parts, of which all substances 72 PROPERTIES OF MATTER. are made up, are styled simple matter, elementary principles, or, simply, elements. The ancients, as is well known, supposed that there were but four ele- ments, or simple substances Fire, Air, Earth, and Water ; and out of these, or certain combinations of them, all the substances in nature were formed. But modern chemistry, as we shall show hereafter, has dis- covered that these elements are by no means simple, but capable of being decomposed. Every solid body, or dense mass, possesses what is called a centre of gravity, which is the point upon or about which the body balances itself, and remains in a state of rest, or equilibrium, in any position. The centre of gravity may be described as a point in solids which always seeks its lowest level. In round, square, and all regularly-shaped bodies, of uniform density in all their parts, the centre of gravity is the centre of these bodies. When a body is shaped irregularly, the centre of gravity is the point upon which the body will balance itself, and remain in a state of rest. The line of direction is an ideal line drawn from the centre of gravity of any body, and passing to the ground in a direction perpendicular to the earth's sur- face. When this line falls within the base of the body, or the part upon which it stands, the body will keep its position ; but if the line falls without the base, the body will fall, or overturn. By keeping this principle in view, stability and safety will generally be secured in the erection of works of art, such as houses, monu- mental edifices, spires, steeples, as well as in the lading of wagons, and carts, and other vehicles. In every instance, the base ought to be sufficiently large to PROPERTIES OF MATTER. 73 admit of the line of direction falling within it. Through ignorance of this principle, and from want of expe- rience, we often see stage-coaches and wagons laden in such a manner that their centre of gravity is liable to too great a change of position, and that they are overturned, to the personal injury, and even loss of life, of the passengers. In the annexed cut, a loaded vehicle is represented as crossing the side of a hill, which has raised one wheel above the level of the other wheel, so as to incline the body of the vehicle very consider- ably from the horizontal. The centre of gravity is represented in two different positions ; a lower one with the line of direction L C, and a higher one with the line of direction U C. If there had been no load upon the vehicle, the line of direction would have remained at L C ; and as it falls within the wheel, or base, the vehicle would have maintained its balance. But if the wagon had been laden, the centre of gravity would have been raised, and, the line of direction U C conse- quently falling without the wheel, the vehicle must overturn. An exception to this rule occurs in the case of xiii. 7 74 PROPERTIES OF MATTER. skaters, in making their circular turns on the ice, in which they bend, or lean, greatly beyond the perpen- dicular position, without falling. This is owing to the contrary effects of centrifugal force, a notice of which will next engage our attention. All bodies, in flying round a centre, have a tendency to proceed in a straight line ; and this principle of motion is termed centrifugal force. Examples of this tendency are very familiar to our observation. When we whirl rapidly a string with an apple at one end of it, and suddenly allow the apple to fly off, it proceeds at first in a straight line, but gradually falls to the earth. We see many applications of this principle every day ; great use is made of it, also, in manufactures and ma- chinery. In the grinding of corn, and in the making of pottery and glass, it saves much trouble and expense. If a skater or equestrian should stand perfectly upright while turning corners and describing circles, he would inevitably fall on his side, being overturned by the cen- trifugal force. But by leaning inwards, the centrifugal force is counteracted by gravity, and this forms a sup- port to his overhanging body. Thus, centrifugal force is the tendency to fly off in a straight line from motion round a centre ; and the power which prevents bodies from thus flying off, is called the centripetal, or centre-seeking force. In the case of the apple, the centrifugal force is the impetus given to the apple, which would make it fly away, if the string were to break. The centripetal force is the string, which prevents it from flying away, and gives a circular direction to its motion. It is upon the mutual action of these two forces that PROPERTIES OF MATTER. 75 the stability of the solar system depends. If the ten- dency of the earth and planets to gravitate towards the sun were removed, they would fly off from it in perfectly straight lines, and never return ; and if it were not for the centrifugal force, which is a result of their circular motion, they would rush to the very body of the sun ; and, in either case, the harmony of the solar system would be entirely overturned. Bodies, on being projected by any impulsive force, are called projectiles, and are observed to pursue a curvilinear 'and bent line of direction in their motion. The bending from the straight line is produced by the force of gravity, and " the ctiange is proportional to the impressed force ." A ball fired from a cannon, a stone thrown from the hand, and water spouted from a con- fined vessel, furnish familiar examples of curvilinear motion. The investigation of the paths which bodies describe when thrown, and of many things relating to their motion, results in certain definite rules, called the laws of projectiles. Skilful generals, in bombarding towns, and attacking vessels, at safe distances, take great ad- vantage of their knowledge of these laws. There are many very interesting circumstances ecu- nected with this subject, which our space will not allow us to notice. Notwithstanding the various substances which nature offers to our observation may differ essentially in touch, weight, and appearance, yet the elements of which they are composed all possess the common, mechanical properties of matter, which properties are five in num- ber namely, 1. The particles of matter are solid, and 76 PROPERTIES OF MATTER. occupy space. 2. They are infinitely divisible. 3. They are impenetrable. 4. They possess mobil- ity, but are inert. 5. They universally attract and are attracted. The first of these properties needs no proof; for the definition already given of matter is, that it has length, breadth, and thickness ; and nothing can have these properties without occupying space, and being solid. These characteristics exist in all matter, although at first they may be invisible : thus air, which cannot be seen, is matter ; for if a glass tube, open at both ends, have its upper end closed by the finger while its lower one is immersed in a jar of water, it will be seen that the air is material, and occupies its own space in the tube, for it will not permit the water to enter it till the finger is removed, when the air will escape, and the water will rise to the same level inside, as outside, of the tube. The second -property of matter is, that it is infinitely divisible ; or, in other words, that the original compo- nent parts, or elementary particles, of which all things are formed, are small beyond conception. Thus, if marble, or any other brittle substance, be reduced to the finest powder which human art can produce, its original particles will not be bruised or affected since, i this powder be examined by a microscope, each grain will bs found to be a solid stone, similar in appearance to the block from which it was broken, and of course, if we possessed suitable implements, would admit of being again subdivided, or reduced to a still finer powder. If a single grain of copper be dissolved in about fifty drops of nitric acid, and the solution be afterwards diluted with about an ounce of water, it is evident that PROPERTIES OF MATTES. 77 a single drop of it must contain an almost immeasu- rably small portion of copper. Yet, so soon as this comes in contact with a piece of polished iron, or steel, that metal will become covered with a perfect coat of copper, which shows how infinitely the copper can be divided without any alteration in its texture. Gold be- comes so attenuated under the hammer, in forming it into gold leaf, that the 500,000th part of a grain is visible to the naked eye, or the 5,000,000th part through a microscope magnifying but ten times. It has been calculated that a pound of gold would gild a silver wire 24,000 miles in length, or capable of en- compassing the globe. But the wonders of art sink into nothing when compared to those of nature. Lee- wenhoek, the celebrated microscopic observer, affirms, that he has counted two millions of animalculae in a portion of the roe of a codfish no larger than a com mon grain of sand. That matter is infinitely divisible, admits also of demonstration on mathematical principles ; for if a par- ticle of matter, however small, be laid on a plane sui face, it must necessarily have an upper and an under part, or a part which touches, and a part which does not touch, the plane. The third property of matter its impenetrability seems to have been adopted by Nature, that her works might be everlasting, and incapable of wearing out ; for, although matter, in many instances, seems to disappear, as in the cases of burning and evaporation, yet chem- istry distinctly proves that it is incapable of annihila- tion, and that the original particles, in all cases, still 7* 78 PROPERTIES OF MATTER. exist, though, by a change of arrangement, they are made to assume a different appearance. Mr. Olmstead, speaking of this subject, says, " In all the changes which we see going on in bodies around us, not a particle of matter is lost ; it merely changes its form ; nor is there any reason to believe that there is now a particle of matter either more or less than there was at the creation of the world. When we boil water, and it passes to the invisible state of steam, this, on cooling, returns again to the state of water, without the least loss. When we burn wood, the solid matter of which it is composed passes into different forms some into smoke, some into different kinds of airs or gases, some into steam, and some remains behind in the state of ashes. If we should collect all these various prod- ucts, and weigh them, we should find the amount of their several weights the same as that of the body from which they were produced ; so that no portion is lost Each of the substances into which the wood was re- solved, is employed, in the economy of nature, to con- struct other bodies, and may finally reappear in its original form. In the same manner, the bodies of animals, when they die, decay, and seem to perish ; but the matter of which they are composed merely passes into new forms of existence, and reappears in the structure of vegetables, or of other animals." Even substances which appear soft, such as air and water, appear hard when submitted to proper examina- tion. Thus a quantity of water, falling in an open tube, seems to exert no particular force, on account of the resistance which it meets with from the air ; but if the PROPERTIES OF MATTER. 79 air be previously removed by the air-pump, there will be no resistance, and the water will sound like the fall- ing of shot, or stones. This is called a water-hammer. Air differs from water in being elastic, but its solidity is shown by the difficulty of compressing a bladder filled with it The fourth property of matter namely, that it pos- sesses mobility, but is inert is the constant object of our observation. By mobility is meant, that it may always be moved if a sufficient force be applied to overcome its weight, or vis inertia : and by being inert, we understand that it is inactive, or indifferent to either rest or motion, yet admits of either, but always exerts a power to remain in that state in which it is found. For instance, when a person is riding on horseback, and the horse suddenly stops ; or is in a carriage, or boat, which is impeded by striking against an obstacle ; the person is thrown forward, from his insensible en- deavor to remain in the state of motion in which he then was. That this is the case with inanimate as well as animate nature, will appear by giving a sudden push to a bowl of water, when the water will flow over on the side on which the impulse was given ; but if once the bowl is put in motion, and then suddenly stopped, it will flow over on the opposite side. Num- berless other instances may be found, in the difficulty of putting heavy bodies, such as ships, loaded wagons, &c., into motion. From this property of matter, if a stone or any inanimate mass is undisturbed, it will re- main forever motionless ; and when once put into mo- tion, would continue in it, and move forever, were it not prevented by the resistance of the air, and by friction. 80 PROPERTIES OF MATTER. Attraction is the fifth property of matter, and exists in every individual particle. All matter attracts, and is attracted, in proportion to its quantity ; therefore, all things upon the earth incline, or are drawn, towards its centre, because the earth is the largest mass of matter in their immediate vicinity. There are several kinds of attraction distinguished by the names of cohe- sion and gravitation magnetic, electric, and elective attraction, or affinity. These, in their general effects, with the exception of the last, appear nearly similar, although they depend upon different circumstances. The attraction of cohesion is that power which unites the separate or individual particles of matter, and forms them into masses, or bodies. This attraction, in general, does not extend to any sensible distance from the body ; and hence, when the parts of any substance are sep- arated or broken, it is difficult to unite them. But if they can be brought into sufficiently close contact, this attraction operates, and they are joined. On this prin- ciple, two pieces of hot iron may be hammered together and united. A plate of lead, and one of tin, passed together through a flatting-mill, become combined into one plate of metal. Glues, cements, and solders, act in the same manner, upon the respective substances to which they are applied, by stopping up the pores, or interstices, and making the contact more perfect. The agency of this principle is shown by pressing two lead planes together, when they will adhere so firmly as to require considerable force to separate them ; and the increasing ratio of this attraction, as bodies approach each other, is very well shown by floating two corks on the surface of the water, when they will run together PROPERTIES OF MATTER. 81 with an accelerated motion. The power which holds all things to the earth's surface is this same attraction ; but when spoken of as applying to worlds, it is called the attraction of gravitation. As the attraction of cohesion is common to all matter, it would appear that particles of every descrip- tion must indiscriminately cohere and stick together, and form substances ; and, consequently, that an infinite variety of compounds would be found in nature, with almost an impossibility of any two of them being alike. Such would, undoubtedly be the case, were it not for that modification of attraction called affinity, or elective attraction: this, however, belongs rather to chemistry, than to the present division of our subject By this power, the general effects of cohesion are restrained, and only one particular species of matter will unite with another, unless, in some cases, by the interposition of a third or fourth material ; in conse- quence of which, only a definite number of natural substances are formed, and the same thing always appears with nearly similar characteristics. Capillary attraction is that species of attraction by which fluids are raised in small tubes, and is a modifi- cation of the attraction of cohesion. If a capillary tube, or tube of very small diameter, be immersed in fluid, that fluid will rise to a certain height in it pro- portionate to the size of its base, rising highest in the narrowest tubes. This depends on the cohesive at- traction exerted by the sides of tHe tube, and accounts for sap rising in the pores or tubes of vegetables. The increasing force of this attraction with the di- minished size of the tube, is beautifully shown by two F 82 PROPERTIES OF MATTER. square glass planes, touching at one edge, and sep- arated at the opposite one by a wedge. On immers- ing these in water, and then raising them out of it, a portion of the water will be retained in that mathe- matical curve denominated an hyperbola. Capillary attraction also causes fluids to rise in sponges, sugar, sand, and other porous bodies, as soon as they come into contact with them. The comparative density of bodies by which is meant their variation in weight while of the same dimensions most probably depends upon their original molecules, or atoms, being of such forms, and so dis- posed, as to admit of their coming into more or less close contact. Thus a greater number of particles will pack, or lie, in any given space, if their forms are regular, than could do so were they irregular. For example, it may be supposed that 1,000,000 particles of gold are contained in a cubic inch of that metal : 500,000 particles of iron might also be capable of occupying the same space, and 100,000 particles of wood. In the iron and wood there must, therefore, be many more pores, or interstices, than in the gold ; and of course the gold will be the heaviest, or most dense. This increased density and weight do not therefore arise from the individual particles of gold being heavier than those of wood, but from a greater num- ber of them being forced into the same space ; for the original particles of matter are presumed to be all of the same weight; arM thus gold, which is one of the heaviest solids, will, when dissolved, remain suspended in ether, which is the lightest of all visible fluids. It is impossible to obtain the absolute weight of bodies PROPERTIES OF MATTER. 83 which vary in density, by weighing them in the open air, for the air will buoy up that which has the least density more than that which has the greatest And thus, although a piece of cork and a piece of lead may exactly balance each other at the ends of a scale-beam, yet that balance will be destroyed as soon as they are placed in an exhausted receiver ; for then the cork, by losing the buoyant assistance of the air, will preponder- ate ; thereby proving that it contains more matter than the lead, though not in the same compass. This prin- ciple is sometimes further elucidated by the experiment of letting a guinea and a featner fall together in a glass receiver: when this is full of air, the guinea falls while the feather is floating about ; but when the air is with- drawn from the receiver, they both reach the bottom at the same instant. Since the earth is of a globular form, and the power of attraction is in proportion to the quantity of matter, so, of course, the inhabitants, and all things upon the earth's surface, will be attracted, or drawn downwards, in a direction tending to its centre; for since the longest line which can be drawn through a circle, or globe, is a diameter which must pass through its centre, so this will likewise pass through the greatest quantity of matter contained in any one directica in it, and consequently all bodies will fall in a direction pointing to the centre of the earth. Hence the use of plumb-lines for obtaining perpendiculars to the horizon, for setting the sides of buildings upright, &c. Besides the above-described five properties of mat- ter, it possesses yet another property, of great impor- tance namely, its power of arrangement, commonly 84 PROPERTIES OF MATTER. called polarity. The attraction of cohesion sufficiently accounts for the formation of masses, or substances, by drawing the original particles of matter together, and then holding them in contact ; but it is found that they are not only drawn and held together, but that the same matter always takes the same arrangement, or forma- tion. Thus a piece of iron, tin, or any other metal, or mineral, will, when broken, always exhibit the same arrangement and disposition of parts, or grain, as it is generally called. And so strictly are the laws of combination found to prevail in the union of elements, and formation of substances, that a novel and important character is given to modern chemical researches, ap- proaching almost to mathematical precision ; it being ascertained not only that the same materials will, in most cases, assume the same form, but that the ingre- dients which enter into the composition of substances do so in certain definite proportions, which cannot be changed without also changing the character of the substance they form. THE MECHANICAL POWERS, THE Mechanical Powers are certain simple arrange- ments of machinery, by means of which weights may be raised, or resistance overcome, with the exertion of less power, or strength, than is necessary without them. In a mechanic power, the weight, or resistance, to be acted upon, and the power, or strength, which acts upon it, should both move at the same time ; and any thing constitutes a mechanic power, in which the motion of the power produces a simultaneous motion in the resistance, provided less power is necessary than is due to the weight, or strength, of such resistance. From this general definition, it might appear that xiii. 8 86 THE MECHANICAL POWERS. every machine capable of generating force would be a mechanic power ; but simplicity is likewise essential, and hence the mechanical powers may be said to be the elements of machinery ; and they are, in fact, so elementary as to admit of no simplification or altera- tion. They are but six in number ; and the names by which they are distinguished are, the LEVER, the WHEEL AND AXLE, the PULLEY, the INCLINED PLANE, the WEDGE, and the SCREW. Out of the whole, or a part, of these, it will be found that every mechanical engine, or piece of machinery, is constructed. THE LEVER. This is the simplest of all the me- chanical powers, and is generally considered the first. It is an inflexible bar, or rod, of any kind or shape, so disposed as to turn on a pivot, or prop, which is always called \\sfulcrum. It has the weight, or resistance, to be overcome, attached to some one part of its length, and the power which is to overcome that resistance applied to another; and as the power, resistance, and fulcrum admit of various positions with regard to each other, so the lever is divided into three modifications, distinguished as the first, second, and third kinds of lever that portion of it which is contained between the fulcrum and the power being called the acting part, or arm, of the lever ; and that part which is between the fulcrum and the resistance, its resisting part, or arm. A beam, or rod, of any kind, resting at one part on a prop, or axis, which becomes its centre of motion, is a lever; and it has been so called, probably, because such a contrivance was first employed for lifting weights. This figure represents a lever used to move THE MECHANICAL POWERS. 87 a block of stone : a is the end to which the power, or force, is applied ; b is the prop, or fulcrum ; and c is the weight, or resistance : this is a simple crowbar, or hand- spike. According to a fundamental principle of dy- namics, the power may be as much less intense than the resistance as it is farther from the fulcrum, or moving through a greater space. A man at a, there- fore, twice as far from the prop as the centre of grav- ity of the weight, i, will be able to lift a weight twice as heavy as himself ; but he will lift it only one inch for every two that he descends ; for it is also a principle of this science that what is gained in power is lost in time. There is no limit to the difference of intensity in forces which may be placed in opposition to each other by the lever, except the length and strength of the material of which the levers must be formed. Every one has heard of the boast of Archimedes, " Give me a lever long enough, and a prop strong enough, and with my own weight I will move the world ! " But he must have moved with the velocity of a cannon-ball for millions of years, to alter the position of the earth 08 THE MECHANICAL POWERS. half an inch. In mathematical truth, this feat of Archimedes is performed by every man who leaps from the ground, for he kicks the world away from him when he rises, and attracts it again when he falls back. The common claw-hammer for drawing nails is a striking example of the power of a lever of this de- scription. A boy who cannot exert a direct force of fifty pounds may, by means of this kind of hammer, extract a nail to which half a ton may be suspended, because his hand moves eight inches, perhaps, to make the nail rise one quarter of an inch. The claw-ham- mer also proves that it is of no consequence whether the lever be straight or crooked, provided it produces the required difference of velocity between power and resistance. The part of the hammer resting on the plank is the fulcrum. Pincers, or forceps, are double levers, and so are common scissors. The steel- yard is a lever with unequal arms. The second kind of lever possesses the same degree of power with the first, and operates with the same results. The third kind cannot be called a mechanical power, for, since its resting arm is longer than the acting arm, it must lose power, though it gains time. The most familiar examples of the occurrence of this kind of lever, are in the use of common fire-tongs, and in rearing a tall ladder against a wall. But the circumstance that principally gives importance to it, is, that the limbs of men and all animals are formed of it ; for the bones are levers, the joints are the fulcra, while the muscles which give motion to the limbs, or THE MECHANICAL POWERS. 89 produce the power, are inserted and act close to the joints, causing action at the extremities. To calculate the effect of a lever in practice, we must always take into account the weight of the lever itself, and its bending. But in speaking of the theory of the lever, we usually leave these out of the question considering it as a rod without weight or flexibility. THE WHEEL AND AXLE. This power consists of two parallel wheels, pulleys, cylinders, or circles, having one axis in common. The letter d here marks the wheel, and c an axle affixed to it. We see that, in turn ing together, the wheel would take up, or throw off, as much more rope than the axle as the circumference of the wheel is greater than that of the axle. If the proportions were as four to one, one pound, at i, hang- ing from the circumference of the wheel, would balance four pounds at a, hanging from the opposite side of the axle. A common crane for raising weights con- sists of an axle, to wind up, or receive, the rope which carries the weight, and of a large wheel, at the circum- ference of which the power is applied. The power may be animal effort on the outside of the wheel, or 8* 90 THE MECHANICAL POWEES. the weight of a man, or beast, walking on the inside, and moving it as a squirrel moves the cylinder of his cage. By means of a wheel which is very large in propor- tion to its axle, force of very different intensities may be balanced, but the machine becomes of inconvenient proportions. It is found preferable, therefore, when a great difference of velocity is required, to use a com- bination of wheels, of moderate size. In the following figure, three wheels are seen thus connected. Teeth in the axle, d, of the first wheel, c, acting on six times the number of teeth in the circumference of the second wheel, g, turn it only once for every six times that c revolves. In the same manner the second wheel, by turning six times, turns the third wheel, h, once ; the first wheel therefore turns thirty-six times for one tarn of the last ; and as the diameter of the wheel c, to which the power is applied, is three times greater than that of the axle, which has the resistance, three times 36, or 108, is the difference of velocity : therefore 1 pound at b will balance 108 pounds at a. THE MECHANICAL POWERS. 91 On the principle of combined wheels, cranes are made, by which one man can lift many tons. It is even possible to make an engine, by means of which a little windmill, of a few inches hi diameter, could 'ear up the strongest oak by the roots ; but of course this would require a long rime for its work. The most familiar instances of wheel-work are in our clocks and watches. One turn of the axle on which the watch- key is fixed, is rendered equivalent, by the train of wheels, to about 400 turns, or beats, of the balance- wheel ; and 'thus the exertion, during a few seconds, of the hand which winds it up, gives motion for 24 or 30 hours. By increasing the number of wheels, time- pieces are made which go for a year; and if the ma- terial would last, they might easily be made to go for a thousand years. THE IXCLHS-ED PLAXE is described by the above cut. A force pushing a weight from C to D, only raises it through the perpendicular height, E D, by acting along the whole length of the plane, C D ; and if the plane be twice as long as it is high, one pound at B, acting over the pulley, D, would balance two pounds at A, or any where on the plane ; and so of all other quantities and proportions. A horse drawing on a road where there is a rise of one foot in twenty, is really lifting one twentieth of his load, as well as overcoming the friction and other resistance of the carriage. Hence 92 THE MECHANICAL POWERS. the importance of making roads as level as possible ; and hence the error, which has often been committed, of carrying roads directly over hills, for the sake of straightness, when, by going round the bases of the hills, the distance would scarcely have been increased, and all rising and falling would have been avoided. Hence, also, a road up a very steep hill must be made to wind, or go zigzag, all the way ; for, to reach a given height, the ease of the pull to the horses is greater, exactly as the road is made longer. An in- telligent driver, in ascending a steep hill by a broad road, winds from side to side all the way, to save his horses what little he can. Hogsheads of merchandise, which twenty men could not lift by applying their strength directly, are often seen moved out of, or into, wagons by one or two men who have the assistance of inclined planes. On some canals and railroads, the loaded boats and cars are drawn up by machinery on inclined planes. It is sup- posed that the ancient Egyptians must have used this mechanical power to assist in elevating and placing those immense masses of stone with which their pyra- mids and other gigantic piles of architecture were constructed. In our speculations upon the power of the inclined plane, we suppose the plane to be perfectly smooth, and that bodies move upon it without friction or imped- iment ; but this can never be the case in practice, even in the most perfect machines; consequently, some allowance must be made from the calculated effect, and when carriages move jipon rough or sandy roads, this allowance must be considerable. THE MECHANICAL POWERS. 93 THE PULLET. A pulley is a grooved wheel, around which a rope is passed, and is either fixed or movable. The preceding cut represents a fixed pulley, which never changes its position : a is the wheel ; I a beam, or roof, from which the wheel is suspended ; c is the power hanging at one end of the rope ; and d is the weight at the other end. In such a construction, it is evident that the weight for instance, ten pounds is equally supported by each end of the rope, and that a man holding up one end, only bears half of it, or five pounds ; but to raise the weight one foot, he must draw up two feet of rope ; therefore, with the pulley, he lifts five pounds two feet, when he would be obliged to lift ten pounds one foot without the pulley. This kind of pulley, however, possesses no mechan- ical advantage. To raise a pound weight from the ground at one end of the cord, the power of one pound must be exerted at the other. Its object, then, is not to save power, but to give convenience in pulling. For instance, by pulling downwards, a weight may be raised upwards ; or, by pulling in one direction, a load may be made to proceed in another. Thus, in drawing a bucket out of a well, it is much easier to pull down- wards, by means of a rope passing through a pulley 94 THE MECHANICAL POWERS. over the head, than upwards, by drawing directly at the bucket. Many wheels may be combined together, and in many ways, to form compound pulleys. Wherever there is but one rope 'running through the whole, as snown here, the relation of power and resistance is known by the number of folds, or turns, of the rope which supports the weight. Here are six turns, and a power of one hundred pounds would balance a resistance of six hundred. The chief use of this pulley is on board ships, where it is called a Hock. It aids so powerfully in hoisting the masts and sails, &c., that, by means of it, a small number of sailors are rendered equal to the duties of a large ship. There is no assignable limit to the power which may be exerted by means of pulleys. A machine may be constructed to raise with ease any weight which the strength of the materials will bear, provided the com THE MECHANICAL POWERS. 95 bination be not so complex as to exhaust the power by the friction produced. THE WEDGE. This power acts on the principle of an inclined-plane force moving forward between resist- ances, to overcome or separate them, instead of being stationary, while the resistance is moved along its sur- face. The same rule, as to mechanical advantage, has been applied to both cases, the force acting on the wedge being considered as moving through a space equal to its length, C D, and the resistance as yielding through a space equal to its breadth, A B. But this rule is far from explaining the extraordinary power of the wedge. It appears that, during the tremor produced by the blow of the driving-hammer, the wedge insin- uates itself, and advances much more quickly than the above rule anticipates. The wedge is used for many purposes, as for splitting blocks of stone and wood ; for squeezing strongly, as in the oil-press ; for lifting great weights, as when a ship of war, in dock, is raised by driving wedges under her keel. An engineer in London, who had built a very lofty and heavy chimney for his steam engines and furnaces, found, after a time, 96 THE MECHANICAL POWERS. that it was beginning to lean on one side. By driving wedges under that side, he succeeded in restoring it to a complete perpendicular. The wedge is the least used of the simple mechan- ical powers, but the principle upon which it acts is in extensive application. Needles, awls, bodkins, and driving nails, are the most common examples. Knives, swords, razors, the axe, chisel, and other cutting instru- ments, also act on the principle of the wedge ; so like- wise does the saw, the teeth of which are small wedges, and act by being drawn along while pressed against the object operated upon. When the edge of a scythe, or razor, is examined with a microscope, it is seen to be a series of small, sharp angularities, of the nature of the teeth of a saw. THE SCREW may be considered as a winding wedge ; for it has the same relation to a straight wedge that a road, winding up a hill or town, has to a straight road of the same length and acclivity. A screw may be described as a spindle, a d, with a thread wound spirally round it, turning or working in a nut, c, which has a corresponding spiral furrov^ fitted to receive the thread. Every turn of the screw carries it forward in a fixed nut, or draws a movable nut along upion it, by exactly THE MECHANICAL POWERS. 97 .he distance between two turns of its thread ; this dis- tance, therefore, is the space described by the resist- ance, while the force moves in the circumference of the circle described by the handle of the screw, as at i, in the figure. The disparity between these lengths, or spaces, is often as a hundred, or more, to one ; hence the prodigious effects which a screw enables a small force to produce. Screws are much used in presses of all kinds ; as in those for squeezing oil and juice from vegetable bodies, as linseed, rape-seed, almonds, apples, grapes, sugar-cane, &c. They are used in the cotton-press, which reduces a great spongy bale, of which a few, comparatively, would fill a ship, to a dense package heavy enough to sink in water ; and in the common printing-press, which forces the paper strongly against the types. The screw is the great agent in the coining machinery of mints. As a screw can easily be made with a hundred turns of its thread in the space of an inch, and at perfectly equal distances from each other, it enables the mathe- matical instrument maker to mark divisions on his work with a minuteness and accuracy quite extraor- dinary. When a screw is at liberty to move equally in all directions, it is simply called a screw ; but when it is confined at its ends, so that it can merely revolve, with- out advancing or withdrawing, it is called an endless screw, and in this case it generally acts into the teeth of a wheel, either to move or be moved by mat wheel ; but its power is alike in both cases. The screw, though a mechanical power, can hardly be called a simple instrument, because, from its great friction, it always requires the assistance of a lever to turn it ; and when G mx 98 THE MECHANICAL POWERS. so turned, its power is estimated by taking its circum- ference, and dividing this by the distance between any two of its threads. Yet, after all, there seems to be no reason, except long-established usage, why the appellation of Me- chanical Powers should be restricted to the six contri- vances above explained ; for many others equally de- serve it ; and, in fact, the mightiest of all mechanical devices, the steam engine, does not derive its power from solid substances at all. HYDROSTATICS. THIS science has for its object the examination of the mechanical laws which regulate the motions, pressure, gravitation, and equilibrium, of inelastic fluids, as well as their effects upon bodies which floaKupon or are immersed in them. The construction of pumps and machines for raising and conveying water, and of machinery to be moved by it, is made a separate branch of the same inquiry, under the name of HY- DRAULICS, which will be the subject of the next chapter. The incompressibility of water had long been sus- pected, but was first fairly put to the test in the Acade- my del Cimento at Florence, La 1650. A quantity of pure water was introduced into a hollow sphere of gold, as being the most dense and compact metal, and a screw, working in a water-tight joint, was then forced into the globe among the water, by which it was compressed with great force; and it was found that the water refused to admit of this compression, but actually oozed through the pores of the metal, and appeared like dew on the outside of the globe. This process is called the Florentine experiment. Mr. Canton afterwards repeated this experiment in a very accurate manner, and with some variation of form. He enclosed a quantity of mercury in a glass 100 HYDROSTATICS. tube similar to those used for thermometers, but of greater dimensions, and he observed to what point the mercury rose when the whole was heated to 50 de- grees of Fahrenheit : after this, the mercury was made to expand, by increased heat, until the whole tube was filled, and in this state its end was hermetically sealed. The mercury, being thus relieved from the pressure of the atmosphere, did not fall down to its original situa- tion, but stood nearly a third of an inch higher than before, by which mercury was proved to be an ex- pansible, and consequently a compressible, fluid. The tube was now emptied ; and water which had been long boiled, to clear it from any air which it might contain, was substituted in the place of mercury, and treated in the same manner. The water was found to stand nearly half an inch higher, when relieved from at- mospheric pressure, than it did before ; from which it was inferred that water is slightly compressible, though to so small a degree as to be of no consequence in practice. This experiment, however, was on a very small scale, and nothing further was done towards ascertaining the degree of condensation that water would admit of, till Mr. Perkins, an American, tried some very ingenious and decisive experiments upon it. He was first led to the subject by the contemplation of a simple, but hitherto unexplained fact ; namely, that, when a bottle, completely filled with water, well corked and secured, was sunk into the deep sea by a heavy weight, k always returned again to the surface, either with the cork pushed into the inside, or protruded in a greater or less degree ; but the water in the bottle was, in all cases, HYDROSTATICS. 101 turned from fresh to salt. Mr. Perkins, therefore, tried several experiments of this sort with cylinders of brass and iron, constructed for the purpose. The result of these trials established the fact of the compressibility of water in the most satisfactory manner. 30,000 pounds pressure to the inch will lessen its bulk one twelfth. Fluids have weight, and gravitate towards the earth, according to their density, in the same way that solids do ; but, from the want of cohesion among their par- ticles, they are incapable of assuming any particular form without assistance, and, consequently, they always take the shape of the vessel which contains them. They also exert a certain force against the sides of that vessel, from their tendency to fall, which consti- tutes their lateral pressure ; for fluids not only press downwards with their whole might, in obedience to gravitation, but they press sideways, or laterally, in all directions at the same time, and from the same cause ; and consequently, no fluid can remain in a state of quiet equilibrium unless every part of its surface is equidistant from the centre of the earth, or in what is generally called a level plane, though that apparent plane is, in fact, not a plane, but partakes of the con- vexity of the earth. And it is for the purpose of es- tablishing such an equilibrium that fluids always run from a higher to a lower situation. For the purpose of explaining the manner in which the surfaces of fluids become level, it may be very fairly supposed that the particles of which they are composed are placed one upon another, so as to form what may be termed pillars or columns of particles ; and supposing all the particles to be of the same size 9* 102 HYDROSTATICS. and weight, the columns on one side of the vessel will be an exact balance to those on the other side. The cause of bodies floating upon fluids, or sinking in them, may be explained the same way ; for, whenever a solid is immersed in a fluid, it displaces a quantity of water, and consequently renders the columns of particles underneath it shorter, and, therefore, lighter, than those which surround it. But the weight of the floating body becomes a counterpoise to the greater length of the surrounding columns, and must in every case be precisely equal to the quantity of water which it dis- places. Consequently, all things which are lighter than their own bulk of water will swim, and all that are heavier must sink. A ship, therefore, of 500 tons' burden must displace 500 tons of water from the bed, or hollow, which it makes from the keel up to the water line ; and in this way the actual tonnage of a ship is estimated, although her nominal burden is fixed by another species of measurement. The truth of this position is very satisfactorily proved by putting the model of a ship into a scale, and ex- actly balancing it with water in the other scale. The ship is then removed, and placed in a small cistern quite filled with water, when a quantity of it will flow over, and, on taking the ship out, it will be found that the vacuity will be exactly filled by the water in the scale, being the weight of the floating body. Notwithstanding the above experiments seem to prove that the pressures of fluids are in consequence of a mechanical equilibrium, dependent upon the gravitation of equal quantities of matter acting against each other, yet, on more mature examination, it is HYDROSTATICS. 103 found that such pressures are regulated by perpen- dicular height, 'and the area of the surface acted upon, without any regard to quantity, or absolute gravity. For, although a pound of water can, in itself, produce no greater effect than is due to a pound, yet, from the properties of fluids, it may be so disposed as to pro- duce the effect of many hundred pounds. This has obtained the name of the hydrostatic paradox. The bottom of a vessel bears a pressure proportional to the height of the liquid ; so likewise do those parts of the sides which are contiguous to the bottom, because the pressure of fluids is equal every way. Thus the sides of a vessel must every where sustain a pressure pro- portional to their distance from the upper surface of the liquid ; whence it follows that, in a vessel full of a liquid, the sides bear the greatest stress in those parts next the bottom, and the stress upon the sides de- creases with the increase of the distance from the bottom, and in the same proportion ; so that, in vessels of considerable height, the lower parts ought to be much stronger than the upper. This has been illus- trated by a striking experiment A strong, though small, tube of tin, twenty feet high, was inserted in the bung-hole of a hogshead : water was poured in till it rose within a foot of the top of the tube ; the hogshead then burst, and the water was scattered about with incredible force. The running, or spouting of fluids, from the sides of vessels, arises, likewise, from lateral pressure, and is, consequently, influenced by the height of the column, without regard to the quantity it contains : consequently, if any given quantity of water issues in a certain time 104 HYDROSTATICS. from a hole in a cask, or reservoir, double that quanti- ty will issue from another hole, of precisely the same dimensions, if it be situated four times as deep as the first, below the surface of the fluid. A similar hole, nine times as deep, will deliver three times as much fluid in the same time. The discharge is, therefore, as the square root of the depth beneath the surface ; which law is of great importance in the practical con- struction and arrangement of water-works, and, if not attended to, may occasion a great waste of power. From the principles already advanced, it follows that a stream will always rise as high as its fountain-head : that is, if a tube, twenty miles long, and rising and descending among the inequalities of the land, were nearly filled with water, and could have its ends brought together for comparison, it would exhibit two liquid surfaces, having precisely the same level, and, on either end being raised, the fluid would sink in it, to rise in the other. If there were two lakes, on adjoining hills, of different heights, a pipe of communication descend- ing across the valley, and connecting them, would soon bring them to the same level ; or,- if one were much lower than the other, it would empty the latter into the former. The ancient method of supplying cities with water was by means of aqueducts, or bridges, built over the valleys, and supporting either pipes, or a con- duit, or channel. These stupendous and costly erec- tions, the remains of which still adorn the ruins of ancient cities, are supposed to have owed their origin to an ignorance of the above principle of hydrostatics ; but it is quite as probable that the ancients were com- pelled to erect these structures by the practical difficulty HYDROSTATICS. 105 of uniting a long range of pipes in such a manner as to remain perfectly water-tight against the pressure of a heavy column of water. This is not easy even in the present improved state of the mechanic arts, and with all the advantage of cast-iron and the most durable materials, instead of stone and earthenware, which appear to have been chiefly used for pipes in the construction of the older water-works. Even at the present day, it is found more convenient to conduct water to cities, from long distances, by open aqueducts than by pipes, as has been done at New York. Here the purest water is conveyed from the River Croton, which is forty-one miles from the city, to a reservoir which will hold 150,000,000 gallons. From this reservoir, it is carried by pipes to all parts of the city, in sufficient quantities to supply every demand for it, for domestic uses, for watering streets, and extinguishing fires. What has been said upon water-works equally ap- plies to fountains ; for a jet can be produced only by the effort of water to rise to its level, or by its being under the influence of condensed air, or some other force. Thus, if an elevated cistern, or reservoir, be kept sup- plied with water, and a tube descends from its lower part, ending in a small orifice pointing upwards, the water will spout from it, and form a jet nearly equal in height to that of the water from which it is supplied ; but, for want of that support which the fluid derives from the sides of a tube, or close vessel, and from its being in constant and rapid motion through the resisting air, it will never gain the full height of the column of supply. Since the weight which a body loses, when immersed 106 HYDROSTATICS. in a fluid, is always the weight of as much of that fluid as is equal in bulk to itself, it follows that the weight lost by the body cannot at all depend either on the depth of the fluid itself, or the depth to which the body is immersed. An anchor loses no more of its weight when it is at the bottom, than when it is just below the surface ; for in both cases it loses the weight of as much water as is equal in bulk to itself. It is not easier to swim in deep than in shallow water ; for whatever is the depth, a man loses the weight of as much water as is equal in bulk to his own body ; for which reason, shal- low water will buoy him up with as great force as deep water. Indeed, it is easier to swim in the sea than in a river, because salt water is specifically heavier than fresh. In the Dead Sea, the water of which is more deeply saturated with salt than any other body of water in the world, this principle is strikingly illustrated. In the Travels of Mr. Stephens is the following account of his attempting to swim in this lake : " I know, in refer- ence to my own specific gravity, that, in the Atlantic or Mediterranean, I cannot float without some little move- ment of the hands, and even then, my body is almost totally submerged; but here, when I threw myself upon my back, my body was half out of water. It was an exertion even for my lank Arabs to keep them- selves under. When I struck out, in swimming, it was exceedingly awkward ; for my legs were constantly ris- ing to the surface, and even above the water. I could have lain there and read, with perfect ease. In fact, I could have slept." There are few, if any, animals that are specifically heavier than common water. The substances, indeed, HYDROSTATICS. 107 of both animals and vegetables, are specifically heavier ; the floating of either is, therefore, to be attributed to the cells, or receptacles, interspersed within them, which are filled with air, oil, and substances lighter ; so that, taken together, they form a mass specifically lighter than common water. Thus the bulk of the body is increased by distending the chest in inspira- tion ; this has been tried by an experiment on a fat man, of an ordinary size, by finding what weight he could support, so as to have the top of the head just above water. When his lungs were full of air, Ihe was found to rise with fourteen pounds of lead ; but on breathing out the air, he could sustain only eleven pounds. To show the practical purposes the principle here illustrated may serve, we will relate the story of Hiero 7 s crown. Hiero, king of Syracuse, had delivered a certain weight of gold to a workman, to be made into a crown ; the latter brought back a crown of the proper weight, which was afterwards suspected to be alloyed with silver. The king applied to the celebrated math- ematician, Archimedes, to know how he might detect the cheat, the difficulty being to measure the bulk of the crown, without melting it into a regular figure : silver being, weight for weight, of greater bulk than gold, any alloy of the former, in place of an equal weight of the latter, would mechanically increase the bulk of the crown. Archimedes was. at first embar- rassed with this problem ; but one day, on going into a bath, which happened to be quite filled with water, he was struck with the simple fact that a quantity of water, of the same bulk as his body, must flow over before he 108 HYDROSTATICS. could immerse himself. It immediately occurred to him that, by immersing a weight of pure gold, equal to that which the crown ought to have contained, in a vessel full of water, and observing how much water was left when the gold was taken out, and by after- wards doing the same thing with the crown itself, he could ascertain whether the latter exceeded the former in bulk. The moment he was struck with this thought, his exultation was so great that he leaped out of the bath, and, without stopping to put on his clothes, ran home, crying out, " Eureka ! " " I have found it ! " an expression which has become proverbial. HYDRAULICS. -^^ ^ifeti-* WATER, as we have already remarked, may be made a useful agent of power, merely by allowing it to act with the force of its own gravity, as in turning a mill ; and in this manner it is extensively employed in all civilized countries possessing streams which are suf- 30H. 10 110 HYDRAULICS. ficiently rapid in their descent. But water may be rendered otherwise useful as an agent of force in the arts. Although subtile in substance, and -eluding the grasp of those who attempt to handle it, water can, without alteration of temperature, be made to act, as a mechanical power, as conveniently and usefully as if it were a solid substance, like iron, stone, or wood. The lever, the screw, the inclined plane, or any of the ordinary mechanical powers, are not more remarkable as instruments of force than water, a single gallon of which may be made to perform what cannot be ac- complished, except at enormous cost and labor, by the strongest metal. To render water serviceable as an instrument of force, it must be confined, and an attempt then be made to compress it into less than its natural bulk. In making this attempt, the impressed force is freely com- municated through the mass, and, in the endeavor to avoid compression, the liquid will repel whatever mova- ble object is presented to it. The force with which water may be squirted from a boy's syringe gives but a feeble idea of the power of liquids, when subjected, in a state of confinement, to the impression of exter- nal force. We have already spoken of the tendency of water to seek every where a common level, on the principle of which aqueducts are constructed. Springs in the ground are natural hydraulic operations, and are ac- counted for on principles connected with the laws of fluids. One class of springs is caused by capillary at- traction, or natural attractive forces, by which liquids rise in small tubes, porous substances, or between flat Ill bodies, closely laid together. This species of power is a remarkable variety of the mutual attraction of matter, and is as unaccountable as the attraction of gravitation, or the attraction exercised by the load- stone. Springs from capillary attraction are believed to be less common, and of less importance, than springs which originate from the obvious cause of water find- ing its level. The water which falls in the form of rain sinks into the ground in high situations, and finds an outlet at a lower level, though perhaps at a con- siderable distance. The friction, or resistance, which fluids suffer when passing through pipes, is much greater than might be expected. It depends chiefly upon the particles being constantly driven from their direct course by the irregu- larities in the surface of the pipe. An inch tube, of 200 feet in length, placed horizontally, is found to dis- charge only a fourth part of the water which escapes by a simple aperture. Air, likewise, in passing through tubes, is retarded, as was discovered by a person who construct* 4 a great bellows at a waterfall, to blow a furnace two miles off. This resistance is so great, that when it was first proposed to lay gas-pipes in England, some engineers were of opinion that the gas could not be forced through them. All liquids flow faster through an orifice, or pipe, the higher their temperature is kept, as this diminishes that cohesion of parts which exists, to a certain degree, in all of them, and affects so much their internal movements. The flux of water through orifices, under uniform circumstances, is so regular, that, before the invention of clocks and watches, it was employed as a means of measuring time. These 1 12 HYDRAULICS. water-clocks were called clepsydra, and were often used by ancient orators, to show them when the time allotted to them for speaking had expired. The com- mon hour-glass of running sand is another modification of the same principle. The progress of water, in an open conduit, such as the channel of a river, or an aqueduct, is influenced by friction in the same manner. But for this, and the effect of bending, a river, like the Rhone, drawing its waters from an elevation of a thousand feet above the level of the ocean, would pour them out with the ve- locity of water issuing from the bottom of a reservoir a thousand feet deep ; that is to say, at the rate of 170 miles an hour. The ordinary flow of rivers is about 3 miles an hour, and their channels slope three or four inches a mile. Three feet fall, a mile, makes a mountain torrent. The friction of water, moving in water, is such, that a small stream directed through a pool, and rapid enough to rise over the opposite bank, will soon empty the pool. Large fenny tracts have been drained in this manner. The friction between air and water is also singularly strong, as is proved, on a great scale, by the magnitude of the ocean waves which are caused by it. A little oil, thrown upon the surface of the water, spreads as a thin film all over it, and defends it from further contact and friction of air. If this is done at the windward side of a pond, where the waves begin, the whole surface will soon become as smooth as glass ; and even out at sea, where the commencement of the waves cannot be reached, oil thrown upon them smooths their surface, and prevents their curling over and breaking. HTDRAITCJCS. 113 A stone thrown into a smooth pond causes a suc- cession of circular waves to spread from the spot where it falls. They become of less elevation as they expand, and each new one is less raised than the preceding, so that gradually the liquid mirror is again as perfect as before. Several stones falling at the same time in different places cause crossing circles, which, however, do not check the progress of each other a phenomenon seen hi beautiful miniature at each leap of the little insects which cover the surfaces of ponds in the calm days of summer. Such waves are caused in this manner : When the stone falls into the water, because the liquid is incompressible, a part of it is displaced laterally, and becomes an elevation, or circular wave, around the stone ; this wave then falls downward and outward, in obedience to the laws of fluidity, and the circle is seen to spread. In the mean- time, where the stone descended, a hollow is left for a moment in the water, but, owing to the surrounding pressure, it is soon filled up by a sudden rush from below. The rising water does not stop, however, at the exact level of that around, but, like a pendulum, sweep- ing past the centre of its arc, it rises as far above the level as the depression was deep. This central eleva- tion now acts as the stone did originally, and causes a second wave, which pursues the first, and, when the centre subsides, like the pendulum still, it sinks again as much below the level as it had mounted above ; hence it must again rise, again to fall, and this for many times, sending forth a new wave at each alterna- tion. Owing to the friction among the particles of H 10* 114 HYDRAULICS. water, each new wave is less raised than the preceding, and at last the appearance dies away. The common cause of waves is the friction of the wind upon the surface of the water. Little ridges, or elevations, first appear, which, by the continuance of the force, gradually increase till they become rolling billows. The heaving of the Bay of Biscay, or, still more remarkably, that of the open ocean between the southern capes of America and Africa, exhibits one extreme, and the stillness of the tropical seas, which are sheltered by encircling land, exhibits the other. In sailing round the Cape of Good Hope, waves are met with so vast, that a few ridges and depressions occupy the extent of a mile. But these are not so dangerous to a ship as a shorter sea, with more perpendicular waves. The slope in the former is so gentle,lhat the rising and falling are scarcely felt, while the latter, causing an abrupt and violent pitching of the vessel, are often destructive. The unfortunate steam-ship President doubtless perished from this cause. She encountered a tremendous gale on the day after leaving New York, and, during the height of its fury, she must have been at a point where the Gulf Stream approaches the shoal called George's Bank, and causes an almost perpendicular surge of the most dangerous character. The ship was last seen struggling ahead directly against the sea, and pitching violently. Her enormous length must have greatly added to the danger, and probably caused her soon to rack to pieces. The velocity of waves is in proportion to their mag- nitude. The largest waves move from thirty to forty flYDRAOTJCS. 115 miles an hour. It is a vulgar belief that the water itself advances with the speed of the wave, whereas it is only the/ora that advances, while the substance, with the exception of a little spray above it, remains rising and falling in the same place, with the regularity of a pendulum. A wave of water, in this respect, is exactly imitated by the' wave running along a stretched rope when one end is shaken, or by the mimic waves of our theatres, which are generally undulations of cloth shaken up and down. But when a wave reaches a shoal, or beach, the water becomes really progressive, because then, as it cannot sink directly downwards, it falls over and forwards, seeking its level. So terrific is the spectacle of a storm at sea, that it is generally viewed through a medium which biases the judgment ; and, lofty as waves really are, imagina- tion pictures them loftier still. Now, no wave rises more than ten feet above the ordinary sea level, which, with the ten feet that it afterwards descends below it, give twenty feet, from the hollow, or trough, of the sea, as the sailors call it, to the adjoining summit The spray of the sea, driven along by the violence of the wind, is, of course, much higher than the crest of the wave, and a wave coming against an obstacle may dash to a great elevation above it. At the Eddystone lighthouse, in the English Channel, in heavy storms, the waves dash over the top of the lantern. On a superficial view of the doctrine of resistance, in the case of bodies moving through a fluid, many persons would conclude that, if a body moving through the water at a given rate, meets a given resistance, it should encounter double that resistance when moving 116 HYDRAULICS. at double the rate ; but this is a fallacy ; the resistance is four times greater with a double rate. The reason is very clear. A boat which moves one mile an hour displaces a certain quantity of water, and with a cer- tain velocity ; if it moves twice as fast, it of course displaces twice as many particles in the same time, and requires to be moved with twice the force on that account. But it also displaces every particle with a double velocity, and requires another doubling of the power on this account; the power therefore, being doubled on two accounts, becomes a power of four. In the same manner, with a triple speed, three times as many particles are moved, and each particle with a triple velocity; therefore, a force of nine must be applied to overcome the resistance. For a speed of four, sixteen is wanted, and so on. Thus, even if the resistance against the bow of the vessel only were considered, one hundred horses would drag a canal- boat only ten times faster than one horse. But there is another important element in the calculation namely, the lessening of the usual water-pressure on the stern of the vessel as she moves forward, on account of which, the force required to produce an increased velocity is still greater than what is shown by the above calculation. There is not a more important truth in physics than this ; it explains so many phenomena of nature, and becomes a guide in so many matters of art. It ex- plains in what manner so great an expenditure of fuel is required to obtain high velocities in steamboats. It shows the folly of crowding sail upon a ship with a strong breeze, the trifling advantage in point of speed HYDRAULICS. 117 by no means compensating for the wear of the sails and the risk of accidents. No seamen practise this so much as the Americans, who are ready to incur any degree of expense, and run any risk, in the hope of gaining a little time. We remember an instance where a Boston merchant said to one of his shipmasters about to sail, " Wear out what you please, but make a quick passage." This ship returned from Europe, having worn out an entire new set of sails in one voyage. The above law explains, also, why a ship glides through the water one or two miles an hour with very little wind, although, with a powerful breeze, she would sail only eight or ten. Less than the 100th part of that force of wind which drives her ten miles an hour will drive her one mile ; and less than the 400th part will drive her half a mile. Thus, also, during a calm, a few men pulling in a boat can tow a large ship. If a ship be anchored in a stream where the current is four miles an hour, the strain on her cable is not one fourth part so great as if the current were eight miles. The rapid increase of resistance, in proportion to the increase of velocity, shows that we soon reach the maximum of speed in ships. Fifteen miles an hour is the utmost that a ship can sail. No fish swims faster than twenty miles an hour. The flight of birds, also, has a limited celerity ; but as the thin air opposes much less resistance than water, flying is, of course, more rapid than sailing or swimming. The crow, when flying homeward against the storm, cannot face the wind in the open sky, but skims along the surface of the earth in deep valleys, and wherever the swift- ness of the wind is retarded by terrestrial obstacles. 118 HYDRAULICS. The great albatross can stem upon the wing the cur- rent of a gale, keeping company with a driving ship when the wind is passing at the rate of a hundred miles an hour ; but perhaps this is the limit to which winged speed can reach. If a flat surface experience a certain resistance, a projecting surface, like that of a ball or wedge, is resisted in a less degree. The explanation is, that a flat surface throws the particles of fluid almost directly outwards from its centre to its circumference ; but the convex or wedge-like surface, while displacing them just as far, does it more slowly, and therefore with less expenditure of force in proportion as its point is in advance of its shoulder, or broadest part. The shape of the hinder part of a solid moving through a fluid is of importance, for corresponding reasons. Fishes are wedge-like both before and behind : so are birds, and they stretch out their necks while flying, so as to be- come like sharp points dividing the air. In the form of the under part of boats and ships, men have imitated the shape of fishes. There are boats used by the Chinese called snake-boats, which are only a foot or two in width, but a hundred feet long, and when rowed, as they often are, by a hundred oars, their swiftness is excessive. Oars for boats are made flat, and often a little concave, that the mutual action between them and water may be as great as possible. The webbed feet of water-fowl are oars ; in advancing, they col- lapse, like a shutting umbrella, but open outwards in the thrust backward, so as to offer a broad concave surface to the water. The expanded wings of birds are, in like manner, a little concave towards the air HYDRAULICS. 119 which they strike. The sails of ships, when they are receiving a fair wind, are left slack, so as to swell and become hollow. We conclude this topic by the following striking example of the power of water, given by Mr. Olm- stead : " A waterfall like that of Niagara, where an immense body of water rolls first in rapids down a long inclined plane, and then descends perpendicularly from a great height, affords one of the greatest ex- hibitions of mechanical power ever seen. The Falls of Niagara contain power enough to turn all the mills and machines in the world. They waste a greater amount of power every minute than was expended in building the pyramids of Egypt ; for, in that short space of time, millions of pounds of water go over the falls, and each pound, by the velocity it gains in first falling down the rapids, and then perpendicularly, acquires resistless energy. Water falling one hundred feet would strike on every square foot with a force of more than six thousand pounds." PNEUMATICS; OR, THE MECHANICAL PROPERTIES OF AIR. THE earth which we inhabit is entirely enveloped, or surrounded, by a thin, transparent, and invisible fluid, called air. This air, together with the various gases, steams, vapors, and exhalations that are constantly thrown into it, and which form clouds, is called by the general name of the atmosphere. Consequently, atmospheric air is of a very mixed nature ; but when pure, it is found, by chemical examination, to consist of two permanently elastic gases, or airs, called nitrogen FNEUMATICS. 121 and oxygen, as we shall hereafter show in our chapter on Chemistry. Air, though invisible, is a material substance, and partakes of all the properties which belong in com- mon to other matter ; for it occupies space, attracts and is attracted, and, consequently, has weight. It likewise partakes of the nature of fluids, for it adapts itself to the form of the vessel which contains it ; and it presses equally in all directions ; consequently, it must be considered as a material fluid. But, inasmuch as it is highly elastic, a property which is common to all gases, steams, and vapors, while the more visible and tangible fluids, such as water, oil, spirits, &c., possess this character in a very slight degree, if at all, so they require a separate examination. The various airs, or gases, are called permanently elastic, because, under all changes which can be wrought upon them, they maintain their characters of fluidity and elasticity, and will not admit of being con- gealed, or rendered solid. With steams and vapors, the case is very different; for they arise from inelastic fluids by the application of heat, and they are highly elastic so long as they retain their form of vapor ; but on being cooled, they return again to their original state of inelastic fluid, and therefore differ very ma- terially from air, and cannot be said to be permanently elastic. Water affords a very good instance, for this is inelastic ; but its steam is elastic in the highest degree ; whenever this steam becomes cooled, it reverts back into its original state of water, and of course resumes all its former characters. Since air has weight, and every thing upon the earth XIII. 11 122 PNEUMATICS. is surrounded by it, it follows that all things must be subject to the pressure which will be exerted, not only upon them, but upon itself; and since air is elastic, or capable of yielding to pressure, so, of course, the lower part of the atmosphere will be more dense, or in a state of greater compression, than that which is above. Suppose, for example, that the whole haight of the atmosphere is divided into 100 equal parts, and that each of these may weigh an ounce, or may be equiv- alent to the production of that pressure ; then the earth, and all things upon its surface, will be pressed with the whole 100 ounces ; the lowest stratum of air will be pressed by the 99 ounces above it, the next by 98, and so on till we arrive at the 99th stratum from the bottom, which will, of course, be subject to no more than one ounce of pressure. Springs of metal, or wood, expand or contract, until they arrive at a state of equilibrium with the force that is acting upon them. The air acts in the same way ; for, being of an elastic nature, it will, of course, yield to any force that may be impressed upon it, until its spring becomes a balance to that force. It is on this account alone that we are insensible of the air's pressure ; for, notwithstanding the body of a man of ordinary stature is calculated to sustain no less a pressure of air than 32,400 pounds, yet the spring of the air contained within the body exactly balances, or counteracts, the pressure from without, and makes him insensible of the existence of any pressure at all. The spring and pres- sure of air will thus balance each other in all cases, except when the communication is cut off, and the nat- ural equilibrium is destroyed by some disturbing cause. PNEUMATICS. 123 The air-pump is the instrument that is generally used for the destruction of this equilibrium ; for, by means of this machine, the air may be taken from the interior of vessels which are put upon its plate, and then the effects of the external and undisturbed air immediately begin to show themselves. Thus, for example, if a small glass receiver, which is open both above and below, be placed upon the plate of an air- pump, and the palm of the hand be put upon it, so as to cover it completely, without leaving any orifice for the admission of the external air, as soon as the pump is set in motion, the hand will be forcibly held down to the receiver, and cannot be released without difficulty ; for the air within the glass being rarefied or diminished in quantity, that without will preponderate by its weight, which keeps the hand down, while the spring of that air which is contained in the hand will cause its lower side to swell, and enter the glass to a considerable depth. This shows the necessity of having all glasses, to be used upon the air-pump, with hemispherical or rounded tops, that they may present a dome, or arched form, to the pressure of the external air ; and all such glasses are called by the general name of receivers. If an open-topped receiver be covered with a piece of flat glass, the pressure from without will break it. If a small portion of the shell of an egg be broken away at the small end, and it is then placed under a receiver, and the air is exhausted, the bubble of air that is always contained in the large end will expand, and force out the contents of the egg. A withered apple, placed under a receiver, will expand, and appear fresh, provided its skin be not broken. That air is 124 PNEUMATICS. contained in water appears plain from the following experiment : Place a tumbler of clear water, in which not a single bubble of air is visible, under a receiver, and then exhaust it ; the water will instantly appear full of bubbles, which become large, and rise to the top ; but as soon as the air is returned into the receiver, they are all instantly compressed, and disappear. The ascent of water in a common pump is caused by atmospheric pressure ; for the water in the pump being raised by the action of the upper pump-box, a vacuum is created below, which is immediately filled by the pressure of the air from without, which forces the water in the bottom of the well upward, to supply that vacuum. But, as equal weights must, of course, exactly balance each other, and as the weight of the atmosphere is limited, it is evident that only a column of water of a certain height can be raised by that weight. Accordingly, it has been found that water cannot be raised in a pump, by the mere pressure of the external air, higher than 32 or 33 feet ; whence the inference is plain, that a column of water of this height is exactly equal in weight to that of the atmos- phere on the same surface. The diameter of the column of water, in this case, is of no consequence ; be- cause, whatever it may be, an equal-sized column of air always acts against it. This balance of power between a perpendicular column of water and atmospheric pressure was first observed by Galileo, in erecting a pump for the grand duke of Tuscany ; but he appears not to have been aware of its cause. This was first investigated by Torricelli, who made use of quicksilver, a fluid 14 PNEUMATICS. 125 times heavier than water, by which he was enabled to produce a pressure equal to that of water with one fourteenth part of its height, and accordingly, his ex- periments were very neat and accurate. He filled glass tubes of different sizes, having one end closed with quicksilver ; and then, by covering the open end, he inverted them into basins filled with the same metal. Thus he found that the diameters of the tubes had no effect on the experiments, but that all those which were less than 28 inches in height, remained full of quicksilver, when inverted, and that in all those which were taller, the quicksilver descended until it became stationary at between 28 and 31 inches above the surface of that which was in the basin. An empty space was thus left at the upper end of the tube, which has since been found to be the most perfect vacuum producible by art. This is known by the name of the Torricellian vacuum. A tube filled with quicksilver, and thus disposed, is called a barometer. In this instrument, the column, being maintained by the pressure of the air, must of course be a balance to that pressure ; and if the amount of pressure changes, as it is found to do, then the height of the mercurial column will change also. It is on this account that the quicksilver in the barom- eter moves up and down through a space of three inches, because the density of the air is never so great as to cause it to sustain the quicksilver at more than 31 niches from the surface of that in the basin below, nor does it ever diminish so as to allow the column to descend lower than 28 inches. The falling of the mercury in the barometer always indicates that a 11* 126 PNEUMATICS. storm is approaching ; for this fall takes place in con- sequence of the rarefaction of the air, which presently causes the surrounding air to rush in to restore an equilibrium. The barometer thus becomes a most in- valuable instrument to the mariner ; for on many oc- casions, when the weather is perfectly serene, and the sky exhibits not the smallest token of approaching bad weather, the mercury is seen to sink with uncommon rapidity. The prudent seaman immediately takes in sail, and makes every preparation against the coming danger. Scarcely has the ship been put into the con- dition which the sailors emphatically call " snug," when a squall, or perhaps a hurricane, bursts from the sky, and tears away the sails, although furled and secured to the yards, disabling spars and masts, and, but for the timely preparation made against it, would have ren- dered the ship a complete wreck. Another useful purpose to which the barometer is made subservient, is to measure the height of moun- tains ; for as the mercurial column is always an exact indication of the pressure produced by the mass of air above its level, the mercury must fall when the in- strument is carried from any lower to any higher situa- tion, and the degree of falling must always tell exactly how much air has been left below. When the barome- ter, on the surface of the earth, stands at 30 inches, and the temperature is 32 Fahrenheit, it has been ascer- tained, by trial, that taking such a barometer to the per- pendicular height of 87 feet lowers the quicksilver just one tenth of an inch. But as the atmosphere decreases in density and weight as we ascend, something more than 87 feet must be ascended, to lower the mercury PNEUMATICS. 127 another tenth, and so on. By nice calculations of this sort, the system of measurement has been brought to such perfection, that the height of any accessible mountain may be ascertained with the utmost ac- curacy. That water is at all times contained in air is evident from the cloud of vapor which we constantly observe to be precipitated whenever a very clear receiver is ex- hausted upon the air-pump, and which is neither more nor less than a shower of rain in miniature. The damp on our walls and windows, which precedes wet weather, arises from the same cause ; for then the air is overcharged with water, and begins to return a part of it : the pressure of water in the atmosphere Is detected by the instruments called hygrometers, which measure the moisture of the air. They are of various forms, and are constructed of different materials ; but, unfortunately, most of them lose their action in course of time. One of the simplest of these instruments may be formed of a considerable length of well-twisted flaxen string, suspended from the ceiling of a room about 4 inches from the wall, and stretched tight by a leaden ball, above which is fixed a circle of pasteboard with divisions upon the edge of it, and a fixed mark on the wall for observing their motion. In wet weather the string twists tighter, and of course turns the circle round, and in dry weather it uncoils. There is a toy called the weather-house, constructed on this principle : in this, by the twisting and untwisting of the string, a woman comes out at the door in fine weather, and a man when it is wet. The most com- mon hygrometer, which somewhat resembles a watch 128 PNEUMATICS. in shape, is made of the beard of a peculiar species of wild oat, which possesses the singular property of coiling up in dry weather, and unfolding when wet. A scale-beam, with any substance capable of absorbing moisture, such as a sponge, at one end, counterpoised by a metal weight at the other, becomes an hygrometer since the sponge will absorb moisture from the air, and become dry again, by which it is made heavier or lighter than the counterpoising weight. Air incorporates not only with water, but with a great variety of other volatile materials, by which many of its characters become much changed ; and since heat assists in these combinations, so all warm or hot fluids will evaporate more readily than such as are cold. Put a few drops of ether into a large drinking- glass, and cover it with a plate for a few minutes, the ether will evaporate into the air, and will ren- der it so inflammable that it will take fire on the approach of a taper. Notwithstanding the attraction that thus appears to exist between the air and various fluids, yet the very pressure of the atmosphere pre- vents their rising in vapor, or evaporating, upon a slight increase of temperature. Thus ether is the rarest and most volatile of all the visible fluids ; and when a cup containing a little of this is placed under the receiver of an air-pump, a very trifling action of the pump will make it boil. Water in the open air will not boil unless heated to 212 degrees, but when the atmospheric pres- sure is removed, it boils at a much lower temperature ; and a glass of strong ale, when heated in the slightest degree under an exhausted receiver, will put on the ap- pearance of boiling. From these facts it follows that, PNEUMATICS. 129 on the top of a mountain, water will boil with a less degree of heat than in a lower region; and this has been verified by actual experiment From the highly elastic nature of air, there is no limit to its condensation, which may be continued as long as there is strength in machinery to force it It has been carried to great extent ; but, from all the ex- periments that have been tried, it does not appear that condensation produces any effect on the fluidity, trans- parency, or other characters, of air. Various machines have been invented for this purpose. The air-gun is the best example of the surprising force which air is capable of exerting when condensed to a considerable degree ; for by means of this instrument, bullets may be propelled with a force very nearly equal to that of gunpowder. It is a curious fact, that, although the air-pump is a modern invention, yet the air-gun, which is so nearly allied to it in the construction of its valves and condensing syringe, existed long antecedent to it ; it was invented as early as the year 1408. The ail- gun of the present day, however, is very different from that of former times, which discharged but one bullet after a long and tedious process of condensation, while it now discharges five or six without any visible dimi- nution of force, and will even act upon a dozen, though with less effect The alternate rarefaction and condensation of the atmosphere is the cause of most of the changes of the weather; for thus are produced not only wind and storms, but dew, fog, rain, hail, and snow. The air, being saturated with moisture, lets fall a part of it on any reduction of the temperature : the atmosphere, *30 PNEUMATICS. which has been heated by the sun during the day, and has received much moisture, lets it fall again during the night, and thus causes the night fogs of certain seasons, which float near the surface of the earth until again acted upon by the beams of the next morning's sun. Fog, when condensed by the combination of the minute particles, forms rain ; and rain, when frozen, becomes snow or hail. The general principles of aerostation, or navigating the air in balloons, are so little different from hydro- statics, that the reader may be supposed already to understand them, from what has been said. It is a fact universally known that, when a body is immersed in any fluid, if its weight be less than an equal bulk of that fluid, it will rise to the surface ; but if heavier, it will sink ; and if equal, it will remain stationary. For this reason smoke ascends into the atmosphere, and heated air into that which is colder. The ascent of the latter is shown in a very easy and satisfactory manner, by bringing a red-hot iron under one of the scales of a balance ; the balance is instantly made to ascend ; for as soon as the iron is brought under the scale, the hot air, being lighter than that which is colder, moves upward, strikes the scale, and elevates it. Upon this simple principle depends the whole theory of aeros- tation ; for it is the same thing whether we render the air lighter by introducing a quantity of heat into it, or enclosing a quantity of gas, specifically lighter than the common atmosphere, in a certain space ; both will ascend, and for the same reason. The power of hot air, in raising weights, may be shown in the following manner. Roll a sheet of paper into a conical form, PNEUMATICS. 131 and fasten it, by its apex, under the scale of a balance ; apply the flame of a candle underneath, and the scale will rise, and will not be brought into an equilibrium with the other, except by a much greater weight than would be imagined by a person who had never seen the experiment. The first balloon was made by a man ignorant of what he was about to discover. Seeing the clouds float high in the atmosphere, he thought that, if he could make a cloud and enclose it in a bag, it might rise, and carry him with it Then, erroneously supposing cloud and smoke to be the same, he made a fire of green wood, and placed a great bag over it, with the mouth open. He soon had the joy of finding himself in the possession of a bag-full of smoke, which pres- ently rose to the ceiling of the room ; but he under- stood not that the cause of its rising was the hot, rarefied air within, which, being lighter than the sur- rounding air, was buoyed up, while the visible part of the smoke, which chiefly engaged his attention, was really heavier than the air, and impeded the ascent of the bag. The hot air or fire balloon was afterwards better understood, and was used by aeronauts, until the more commodious and less dangerous modification, called the inflammable air balloon, or balloon of hydro- gen gas, was substituted. The first aeronautic expedi- tions astonished the world, and endless speculations were indulged as to the important uses to which the new discovery might be applied ; but more mature re- flection, and recent trials, have shown that the balloon is interesting chiefly as a philosophical toy, and as having furnished the means of making some observa- 132 PNEUMATICS. tions in elevated regions of the atmosphere. An aero- naut may rise or descend in the air, by throwing out ballast or letting off gas ; but he has no power of pro- ducing a lateral motion. The diving-bell is a large, heavy, open-mouthed vessel, which is let down in the water, bottom upwards, with men inside. The enclosed air keeps out the water at first ; but as the bell descends, the pressure of the water increasing according to its depth, the air is compressed within the bell, and, at 34 feet depth, it is reduced to half its bulk. The bell is then half full of water, and a person within breathes twice as much air, at an inspiration, as he does at the surface. When men are required to remain long under water, a supply of fresh air is conveyed down by means of a forcing- pump, and the heated and contaminated air, which has served for respiration, and which rises to the top of the bell, is allowed to escape through an opening. Men can work at a distance from the bell, and breathe the air from it, through tubes of communication. These operations are so little hazardous, or uncomfortable, that the wages of submarine labor are very little higher than any other. OPTICS, Luminous Insects. THIS science treats of the phenomena of light and vision. Of the precise character of light there are various theories, but none which admit of actual dern. xin. 12 134 OPTICS castration, or proof. By some, it has been described as consisting of very minute particles, which are thrown off from what are called luminous bodjes, in all directions, and with immense velocity ; while others consider it as the effect of an undulation, cr vibration, produced by luminous bodies in the thin and elastic medium which is interposed between them and the seat of our vision ; this vibration producing an effect upon our organs, which we recognize as light, analogous to the impression of sound upon the ear, caused by the atmosphere. This theory is called the undulatory theory of light ; and the former one, in which light is supposed to consist of material particles, is called the theory of emission. Whatever may be the cause, or absolute nature, of light, we know it is a remarkable property of luminous bodies ; that it enables us to see the luminous objects themselves, as well as others ; and that its absence produces darkness. All visible bodies may be divided into two classes self-luminous and non-luminous. Under the first head are comprised all those bodies which possess in them- selves the property of exciting the sensation of light, or vision ; such as the heavenly luminaries, terrestrial flames of all kinds, phosphorescent bodies, and those substances which shine by being heated, or by friction. Under the second class, we recognize such bodies as have not, of themselves, the power of throwing off par- ticles or undulations of light, but which possess the power of reflecting the light which is cast upon them by self-luminous bodies. A non-luminous body may thus, by reflection, receive light from another non- luminous body, and communicate it to a third, and so OPTICS. 135 on ; all reflected light, however, is inferior, in point of brilliancy, to that which comes direct from a self-lu minous body. The transmission of light was formerly supposed to be instantaneous ; but recent observations have shown that, like sound, it requires a certain time to pass from one place to another, though the veloci- ty of its motion is truly astonishing, as has been man- ifested in various ways. Astronomers have proved, by observing the eclipses of Jupiter's satellites when that planet is nearest, and when it is farthest, from the earth, that light moves from the sun to the earth, a distance of 95 millions of miles, in seven and a half minutes, or about 200,000 miles during a single vibra- tion of a pendulum ! So prodigiously great is this velocity, that, as far as any of the common affairs of life may be concerned, light may be said to be instanta- neous in its universal action. Light proceeds in a straight direction from the luminous body which produces it The direct shining of the sun, or any other luminous body, is in the form of rays, or thin, ethereal lines, each acting independ- ently of the other. No such separation of parts, how- ever, is observable in common circumstances, in con- sequence of the diffusive properties of our atmosphere. Seeing is simply the reception of the direct or reflect- ed ray from an object, by our eye. Until the rays of the sun reach the spot on which we are placed, we are neither conscious of light, nor of the presence of the sun as an object. In the same manner, a candle, being lighted, and exposed in the open country in a dark night, all who are able to see it are within the influence of its rays ; but beyond a given distance, these rays are too weak to produce vision; and all who are in this remote situation cannot see the small- est appearance of the light. Yet the number of rays which proceed even from a common candle is so vast as to be beyond the power of imagination to conceive ; for if such a light is visible within a sphere of 4 miles, it follows that, if the whole of that space were sur- rounded with eyes, each eye would receive the im- pression of a ray of light. In proportion as light ad- vances from its seat of production, it diminishes in intensity. The ratio of diminution is agreeable to that which governs physical forces ; that is, the intensity of the light will diminish as the square of the distance increases, or at the rate of 1, 4, 16, &c. But, in pro- portion as we lose in intensity, we gain in volume ; the light is the weaker the farther it is from the candle, but it fills a wider space. In discussing the properties of light, it is important to consider the medium through which it passes, as air, water, glass, &c. Any parcel of rays passing from a point, is called a pencil of rays ; the point at which converging rays meet, is called a focus. Rays may be parallel, convergent., or divergent, which terms will not require an explanation. The point towards which they tend, but which they are prevented from reaching by some obstacle, is called the imaginary focus. REFRACTION is the bending of rays of light from the course they first pursued. If the rays, after passing through a medium, enter another of different density, perpendicular to its surface, they are not refracted, but proceed through this medium, in their original direc- tion. For instance, if the rays of the sun were to OPTICS. 137 strike upon the surface of a river at right angles, or perpendicularly to its surface, they would go straight to the bottom, and the line which they pursued in the air would be continued in the water. But if they enter obliquely to the surface of a medium either denser or more rare than what they moved in before, they are made to change their direction in passing through that medium ; in other words, they are refracted. The mode of refraction depends on the comparative density or rarity of the respective media. If the medium which the rays enter be denser, they move through it in a direction nearer to the perpendicular drawn to its surface. On the contrary, when light passes out of a denser into a rarer medium, it moves in a direction farther from the perpendicular. This refraction is greater or less; that is, the rays are more or less bent, or turned aside, from their course, as the second medium, through which they pass, is more or less dense than the firsL To prove this in a satisfactory way, take an upright empty vessel into a darkened room, which admits but a single beam of light obliquely through a hole in the window-shutter. Let the empty vessel stand on the floor a few feet in advance of the window which admits the light, and let it be so ar- ranged that, as the beam of light descends towards the floor, it just passes over the top of the side of the ves- sel next the window, and strikes the bottom on the side farthest from the window. Let the spot where it falls be marked. Now, on filling the vessel with water, the ray, instead of striking the original spot, will fall considerably nearer the side towards the window. And 12* 138 OPTICS. if we udd a quantity of salt to the vessel of water, so as to form a dense solution, the point where the rays strike the bottom will move still nearer to the window. In like manner, if we draw off the salt water, and supply its place with alcohol, the beam of light will be still more highly refracted ; and oil will refract yet more highly than alcohol. The following simple experiment is well known: Take an empty basin, and place it on a table ; then lay a silver dollar at the bottom of the basin, and let the spectator withdraw so far that the brim of the basin hides the dollar. Now, fill the basin with water, and the dollar, though lying unmoved, will come com- pletely into sight. The explanation of this phenomenon is, that the ray of light producing vision in the eye is bent, as it emerges from the water, and has all the effect of conveying our sight round a corner. The refractive power of water is also observable when we thrust a straight stick into it; we see that the stick seems to be bent, and fails in reaching the point which we desired it should. On this account, the aim, by a person not directly over a fish, must be made at a point apparently below it, otherwise the weapon will miss, by striking too high. With regard to the refrac- tive power of transparent substances, or media, the general rule, with certain limitations, is in proportion to the densities of the bodies ; it increases, for instance, from the most perfect vacuum which can be formed, through air, fresh water, salt water, glass, and so on. But those substances which contain the most inflam- mable matter have the highest refractive power. It was from the great refractive powers of the diamond OPTICS. 139 and water, that Newton, with admirable sagacity, pre- dicted that they contained inflammable principles. The refraction of rays of light is observable in the case of common window-glass. The two sides of a pane not being perfectly parallel to each other,' bodies seen through it appear as if distorted ; and as the obli- quities in the glass are very various, the distortions are equally grotesque and numerous. Some windows are purposely ground on the surface, to produce universal and minute refraction ; and thus so great a confusion is in- troduced among the rays, that objects are not distin- guishable through the glass. When the obliquities on the surface of one side of a piece of glass stand distinct from each other, so as to admit of refraction in a clear and distinguishable manner, then each obliquity affords a separate view of an object on the opposite side, and thus an object seems to be multiplied as many times as there are obliquities. The refraction of light is also observable, on a great scale, in relation to our atmos- phere. The rays of the sun, on reaching the confines of the atmospheric fluid which envelops the earth, enter a medium of greater density than that through which they have previously passed, and consequently are refracted, or bent. One obvious effect of this is, that we never see the sun in the actual position which he occupies. He always appears more or less raised in relation to our eyes, as was the case with the dollar in the above- described experiment of the basin of water. This is peculiarly the fact in the morning, when his earliest rays meet our eyes. Entering a denser medium, these rays bend round to meet our vision, apd we actually see the body of the sun a few minutes before he has 140 OPTICS. risen above the horizon ; like the dollar in the basin, we see him round a corner. In proportion as the sun approaches the zenith, the refraction diminishes ; and as he recedes toward setting, it increases. So con- siderable is it, in the hazy atmosphere of evening, that we retain a sight of the sun's disk after it has sunk. The same phenomena occur in relation to the other heavenly luminaries. From these explanations, it will appear that the di- rectness of our vision is at all times liable to be dis- turbed by atmospheric conditions. So long as the atmosphere between our person and the object we are looking at is of the same density, we may be said to see in a straight line to the object. But if, by any cause, a portion of that atmosphere is rendered less or more dense, the line of vision is bent, or refracted, from its course. A thorough comprehension of this truth in science has banished a mass of superstition. It has been found that, by means of powerful refraction, ob- jects at great distances, and round the back of a hill, or considerably beneath the horizon, are brought into sight. In some countries this phenomenon is called mirage. The following is one of the most interesting and best-authenticated cases of the kind. In a voyage performed by Captain Scoresby, in 1822, he was able to recognize his father's ship, when below the horizon, from the inverted image of it which appeared in the air. " It was," says he, " so well defined, that I could distinguish, by a telescope, every sail, the general rig of the ship, and its particular character, insomuch that I confidently pronounced it to be my father's ship, the Fame, which it afterwards proved to be, though, OPTICS. 141 on comparing notes with my father, I found that oar relative position, at the time, gave our distance from one another very nearly thirty miles, being about seventeen miles beyond the horizon, and some leagues beyond the limit of direct vision ! " Dr. Vince,an English philosopher, was once looking through a telescope at a ship which was so far off, that 142 OPTICS. he could only see the upper part of the masts. The hull was entirely hidden by the bending of the water ; but, between himself and the ship, he saw two perfect images of it in the air. These were of the same form and color as the real ship ; but one of them was turned completely upside down. In the sandy plains of Egypt, the mirage is seen to great advantage. These plains are often interrupted by small eminences, upon which the inhabitants have built their villages in order to escape the inundations of the Nile. In the morning and evening, objects are seen in their natural form and position ; but when the surface of the sandy ground is heated by the sun, the land seems terminated, at a particular distance, by a general inundation ; the villages which are be- yond it appear like so many islands in a great lake ; and an inverted image of a village appears between the hills. The Swedish sailors long searched for a supposed magic island, which, from time to time, could be descried between the Island of Aland and the coast of Upland. It proved to be a rock, the image of which was pre- sented in the air by mirage. At one time, the English saw with terror the coast of Calais and Boulogne, in France, rising up on the opposite side of the Channel, and apparently approaching their island. But the most celebrated example of mirage is exhibited in the Straits of Messina. The inhabitants of the Calabrian shore behold images of palaces, embattled ramparts, houses, and ships, and all the varied objects of towns and landscapes, in the air being refracted images from the Sicilian coast. This wonderful phenomenon is OPTICS. 143 regarded by the common people as the work of fairies, and is known by the name of the fata morgana. COLOR BY REFRACTION. One of the most remark- able phenomena attending refraction, is, that the rays of light, which seem to us to be white, may be sep- arated into rays of various colors. It will be obvious that light has the effect of representing colors when no color substantially exists, by noticing the glancing and varied hues on irregular surfaces of glass, ice, or other crystallized substances. The proper method of analyzing the rays of light, and discovering into what colors they may be resolved, is by the use of ajjrwm, or three-sided rod of glass. The experiment may be made in the following manner : Into a darkened room admit a beam of sunlight through a hole in the shut- ter ; let this fall upon the prism, and, instead of passing in a direct line through it, and forming a circular white spot upon the wall opposite, the rays will be refracted upwards, and form an oblong image upon the wall, divided into seven colors red, orange, yellow, green, blue, indigo, and violet. This lengthened image of the sun is called the solar or prismatic spectrum. No lines are seen across the divisions between the different colors, and it is extremely difficult for the sharpest eye to point out their boundaries. This experiment shows that common white light is compounded of seven dif- ferent colors, and that they all differ in their powers of refraction ; that is, the glass, or whatever medium through which they pass, attracts no two of them with the same degree of force. As they differ in refraction, so also they differ in their powers of reflection ; and hence arise all the various colors of bodies. Those bodies 144 OPTICS. which reflect only the red rays, for instance, and ab- sorb all the others, appear red ; and so of the other colors. Those which reflect all the rays appear white, and those which absorb all the rays, or nearly so, ap- pear black. Hence it is that black clothes are warmer than any other color, as they absorb more light, and light is never unaccompanied by heat. On the other hand, white is the coolest dress that can be worn. The rainbow is formed by a combined process of reflection and refraction. It is never seen, except when rain is falling between the spectator and the sky opposite the sun. If we look into a globe of glass, or water, held above the head, and opposite to the sun, we shall see a prismatic spectrum reflected from the farther side of the globe. In this spectrum, the violet rays will be innermost, and the spectrum vertical. If we hold the globe on a level with the eye, so as to see the sun's light reflected in a horizontal plane, we shall see a horizontal spectrum with the violet rays inner- most ; and a corresponding variation will be observed in other positions. Now, since, in a shower of rain, there will be drops in all positions relative to the eye, the eye will receive spectra inclined at all angles to the horizon ; so that, when combined, they will form the large, curved spectrum called the rainbow. In a very strong sunlight, a secondary bow is seen outside of the primary one : the colors are fainter, because the bow consists of rays that have suffered two reflections in- stead of one. Red rainbows, distorted rainbows, and inverted rainbows on the grass, have been observed. The latter are formed by the drops of rain suspended on the spiders' webs in the fields. OPTICS. 145 Light is diffused around us by the refractive power of the atmosphere, and therefore objects are quite visible, though the rays of the sun do not strike directly upon them. The atmosphere being thus a vehicle of light, it may be supposed that, if we were to ascend to a great height above the level of the earth, or beyond the atmosphere, we should be almost in darkness, although we were, in reality, nearer the sun. There is reason to believe that such would be the case ; for travellers, who have ascended to the sum- mit of Mont Blanc, or about 15,000 feet above the level of the sea, mention that, at that height, the sky appears to be of an exceedingly dark blue color, or almost black, and the light so faint that the stars are visible. We may understand, from this, that the rays of the sun travel through immense regions of darkness before .they reach our atmosphere, and are difiused into that universal, soft light which we observe around us. REFLECTION adds to the brilliancy of the great mass of light transmitted from the sun. If all the objects on the surface of our planet were black, which is a negation of all color, the sun's light would be absorbed, and we should, even while the sun shone, possess much less light than we now enjoy. But, in consequence of the varied coloring in which our earth is dressed, the sun's rays are more or less reflected, and sent back into the general mass of light If the object on which the rays fall be clear, and polished on its surface, it will possess the power of representing the image of any object within the reach of its rays. Thus the surface of a smooth lake will represent the image of the sky above, of the neighboring hills, or of any j xra. 13 146 OPTICS. object floating on its surface. But the phenomena of reflection are too familiar to the reader, to require any very minute description. A lens is a thin piece of glass, or any other trans- parent medium, having one or both sides either con- vex or concave. The convex surface magnifies ob- jects, and the concave diminishes them ; for, accord- ing to the laws of refraction already explained, the rays of light, falling upon a convex lens, are refracted towards its centre, or drawn to a focus ; and as the eye judges of the position of an object from the direction in which the rays last proceed, the converging rays will appear to come from a wider extent of space than is real. In a concave lens, the rays, being refracted in a different direction, diverge, instead of converging, and strike the eye as if coming from a narrower space than the reality ; for this reason, the apparent. size of the object is diminished. Concave mirrors magnify, and convex mirrors reduce objects, on the same prin- ciple. The human eye contains a natural convex lens, through which all the rays of light which cause our vision pass, and are brought to a focus on the retina, a delicate membrane, lining the back part of the eye ; this is connected with the optic nerve, which commu- nicates with the brain the organ, or centre, of all sensation. ACOUSTICS. THE term ACOUSTICS is derived from a Greek word which signifies, to hear, and is applied to that branch of natural philosophy which treats of the nature of sound, and the laws of its production and propagation. Atmospheric vibration is allowed to be the cause of sound. For instance, a bell is struck by its clapper ; the body of the bell consequently vibrates, as we may assure ourselves by applying one of our nails lightly to the edge : in its agitation, it beats, or makes im- pulses upon the air, which, yielding under the stroke, or pressure, is compressed, or condensed, to a certain distance around. The compressed air instantly ex- pands, and, in doing so, repeats the pressure on the air next in contact with it, and thus each one of the ori- ginal strokes of the vibrating metal sends out a series of shells of compressed air, somewhat like the waves dispersed over a lake from the dropping of a stone into its placid bosom, and, like them, always lessening in bulk and force. These shells are from 2 inches to 30 feet in thickness. The air they agitate finally reaches the ear, where it gives a similar impulse to a very fine nervous membrane, in the ear, called the drum, which communicates with the auditory nerve, and this conveys to the brain the sensation of sound. With regard to the velocity with which the impulse 148 .ACOUSTICS. of sound advances, it appears, from the most accurate experiments, on the discharge of pieces of ordnrnc^, and marking the interval between the flash and the report, at a distance carefully measured, that, when the atmosphere is at the temperature indicated by 62 of Fahrenheit, sound travels at the rate of 1125 feet per second, which is nearly equal to the velocity of a can- non-ball, the moment it issues from the piece. The ball is very speedily retarded by the resistance of the air, but the sound advances with undiminished velocity, though unequal intensity. It will travel a mile in little more than four seconds and a half, or twelve miles and three fourths a minute. On this depends an easy method of determining, in many cases, our distance from objects, and which may often prove useful, particularly in thunder-storms. We have only to observe, in seconds, the interval between the flash and the report, and allow four seconds and a half to every mile, or 1125 feet to every second. It is remarkable, also, that all kinds of sounds, strong or weak, acute or grave, advance with the same velocity ; and this arises from the circumstance, that all the oscil- latory movements in the air, however minute or ex- tended, are performed each in the very same interval of time. For every degree of Fahrenheit above 62, tne velocity of sound is increased one foot and about a seventh ; and for every degree below 62, it is les- sened in the same measure ; so that, when the tem- perature is at the freezing point, the rate is only 1090 feet per second. That water is a vehicle of sound, as well as the air, is proved by various circumstances, particularly by the ACOUSTICS. 149 fact, that a bell rung under the water can be heard above ; and if the head of the auditor be also under water, it will be still more distinctly heard. The sound which the sonorous body produces, however, is graver than that which it gives forth in the air. That the atmosphere is necessary for the transmission of sound is evident from the fact, that a bell rung in the exhausted receiver of an air-pump can scarcely be heard. Smooth bodies form favorable channels of sound ; as, for example, the surface of ice, snow, water, or the hard ground. Savages, it is well known, are in the habit of putting their ear to the ground in order to discover the approach of enemies, or beasts of prey. Tubes convey sounds with great accuracy, and to great distances ; and this property has been applied to various useful objects. The most valuable of these purposes is that of examining the chests of persons supposed to possess pulmonary affections. This is done by means of the stethoscope, an instrument which resembles a small trumpet. The wide end of the in- strument is applied to the body, and the other is held to the ear by the physician, who then has a very clear perception of the sounds caused by the action of the lungs, and can judge whether they be healthy or the reverse. A person of skill can exactly describe the condition of the lungs, from the nature of the sounds that thus reach his ear. In a public exhibition in London, there has long been shown an apparatus, consisting of a four-footed stand, and several trumpet-mouthed tubes, from any one of which a spectator will receive a ready answer to a question. The answer is said to come from the " in- 13* 150 ACOUSTICS. visible girl ; " and the true explanation of the puzzle is, that a secret tube, in the legs of the apparatus, commu- nicates the sounds to a girl in a neighboring apartment. Sound requires a certain length of time to travel from one place to another. It is on account of this principle that, in long ranks of soldiers, where two bands of music are placed at a considerable interval from each other, it is impos- sible for the two bands to keep time. They may, indeed, play together, but each soldier will hear the nearest sounds quickest, and thus they will seem to be out of time. It is often noticed, too, that if, from an eminence, we look upon a long column, which is marching to a band of music in front, the various ranks do not step exactly together. Those in the rear are, in each step, a little later than those before them. This produces a sort of undulation in the whole column, which is difficult to describe, but which all who have noticed it will understand. Each rank steps, not when the sound is made, but when, in its progress down the column, at the rate of 1125 feet per second, it reaches their ears. Those who are near the music hear it as soon as it is produced, while the others must wait till sufficient time shall have elapsed for it to have passed through the air to them. Should a commander stand at a distance of a fifth of a mile from his army, and command them to fire, they might all obey at the moment when the word of command reached them ; but the officer will hear the report of the guns from those at the side nearest him first, then those a little farther off, and so on to the most remote. Thus, though all might obey with equal ACOUSTICS. 151 alacrity, the sounds will not, and cannot, appear simul- taneous, for the report of the distant guns must be delayed long enough for the command to pass from the officer to the men, and then for the sound to return. All attempts, therefore, to make the firing appear exactly simultaneous from a long line, must be in vain. An echo, or duplication, of sound, is one of the most interesting phenomena in acoustics. The cause of it is precisely analogous to the reaction of a wave of water. When a wave of water strikes the precipitous bank of a river, it is thrown back in a diagonal direc- tion to the side whence it came, and then again strikes on the bank. In the same manner, the pulses, or waves, of sound are reflected, or thrown back, from flat surfaces which interrupt them, and, thus returning, pro- duce what we call an echo. It is evident that the smoother the surface which reflects the sound, the more perfect will be the reverberation. An irregular surface, by throwing back the wave of sound at irreg- ular intervals, will so confound and distract it, that no distinct or audible echo will be reflected. On the con- trary, a regular concave surface will be concentrated into a focus capable of producing a very powerful effect. The velocity with which an echo returns to the spot where the sound originates, depends, of course, upon the distance of the reflecting surface ; and since sound travels at the rate of 1 125 feet in a second, a rock situated at half that distance will return an echo in exactly one second. The number of syllables which we pronounce in a second will, in such a time, be repeated distinctly, while the end of a long sentence would blend with the commencement of the echo. 152 ACOUSTICS. An echo may be double, treble, or even quadruple according to the nature and number of the projecting surfaces from and to which the sound is allowed to play. Distinctly-marked echoes of this combined and planned order may sometimes be heard in the vaults of cathedrals in which case, the waves of sound are driven from side to side of a deeply-groined arch, and reverberate in protracted peals. One of the most inter- esting echoes of this kind in nature is that which occurs on the banks of the Rhine, at Luxley. If the weather be favorable, the report of a musket fired on one side is repeated from crag to crag, on opposite sides of the river alternately. There are some remarkable echoes in churches, arising from peculiarities in the construction. In erecting the baptistry of the church of Pisa, the archi- tect disposed the concavity of the cupola in such a manner, that any noise from below is followed with a very loud and long double echo. Two persons whis- pering, and standing opposite to each other, with their faces near the wall, can converse together without being overheard by the company between. This arises from the elliptical form of the cupola, each per- son being placed in the focus of the ellipsis. In the ca- thedral church of Gloucester, England, there is, or was, a whispering gallery about the eastern extremity of the choir, which extends from one end of the church to the other. If two persons placed at distant points speak to one another in the lowest voice, it is distinctly heard. A similar effect is produced in the vestibule of the ob- servatory of Paris, and in the cupola of St. Paul's, London. A tourist has mentioned that in Italy, on the ACOUSTICS. 153 way to Naples, and two days' journey from Rome, he saw in an inn 3 square vault, where a whisper could easily be heard at an opposite corner, but not at all on the side corner that was near to you. This property was common to each corner of the room. He saw another, on the way from Paris to Lyons, in the porch of a common inn, which had a' round vault. When any person held his mouth to the side of the wall, sev- eral persons could hear his whisper on the opposite side. The whispering gallery in St. Paul's, London, is a great curiosity. It is 140 yards, in circumference, and is just below the dome, which is 430 feet in circumfer- ence. A stone seat runs round the gallery along the front of the wall. On the side directly opposite the door by which visitors enter, several yards of the seat are covered with matting, on which the visitor being seated, the man who shows the gallery whispers, with- the mouth near the wall, at the distance of 140 feet from the visitor, who hears his words in a loud voice, seemingly at his ear. The mere shutting of the door produces a sound like a peal of thunder rolling among the mountains. The effect is not so perfect if the visitor sits down half way between the door and the matted seat, and much less if he stands near the man who speaks, but on the other side of the door. It is of great importance that buildings designed for large auditories should be constructed in such a man- ner that the voice of the speaker will neither echo from the walls, nor be lost to the hearers. The best-known form of apartment, for the proper distribution of sound, is that in which the length is from a third to a half more than the breadth ; the height somewhat greater 154 ACOUSTICS. than the breadth, and having a roof bevelled off all round the sides. This species of ceiling, technically called a coved or coach roof, from its being lower at the sides than at the centre, is, in all cases, best suited for conveying sounds clearly to the ears of auditors. MUSICAL SOUNDS. Thei'e is a peculiar character in sounds, depending on "the nature of the sounding body. A blow with a hammer, or the report of a pistol, pro- duces only a noise. But if a body be of such a thin- ness and tightness as to produce a succession of im- pulses of a sufficient degree of quickness, a tone is the result namely, a sound composed of a great number of noises, all so close upon each other that they bring but one result to the ear. Wires and strings of metal and catgut, slips of metal, fine membranes, and columns of the air itself, enclosed in tubes, are the most familiar means of producing sounds of this kind. Such sounds are said to be musical. The study of musical sounds, as a branch of natural philosophy, is calculated perhaps to give as much pleas- ure to the man of science as music itself can convey to those who are gifted with what are called good ears. The natural character of these sounds, and their rela- tions to each other, are very remarkable ; while the relation of the whole to the human mind must be regarded as one of the most interesting proofs of cre- ative design which the entire circle of nature presents. The principal sounds in music may be said to be only seven in number. There are other five, which may be produced by the voice with some difficulty ; but the voice in an untutored condition gives forth only seven. The notes are of different degrees of shrill- ACOUSTICS. 155 one rising above another, in succession. A per- son who knows nothing of music beyond having heard another sing or play, and having seen the key-board of a piano-forte, will be ready to say that there are more notes than seven ; but there are only seven that are, strictly speaking, various. The voice, or an instru- ment, may run up into other notes ; but all of these are repetitions of the first seven, and identical respectively with them, in all regards except shrillness. In ordi- nary piano-fortes, there are at least six repetitions of the seven notes, so that the uppermost keys are more piping than the voice of a child, while the lowest rum- ble like a drum. The seven notes are named Do, Re, Mi, Fa, Sol, La, Si, or by the first seven letters of the alphabet, in a peculiar arrangement, namely, C, D, E, F, G, A, B. There are many curious facts connected with the harmonious notes. The cries of a city that is, the scarcely articulate, but often very musical sounds uttered by persons selling things in the streets gen- erally rise on thirds or fifths, sometimes on octaves ; and this, although few of these poor people have ever been taught music. The cry of oysters by women in Edinburgh is always on an octave. Teachers of elo- cution are always aware that human beings, in general, make such transitions of voice naturally, under the in- fluence of certain feelings. For example, a person indifferently surprised at hearing a friend say, " I was the person who did so and so," will say, " Was it you ? " rising only a third at the last word. If greatly sur- prised, the rise will be a fifth. There may even be so great a degree of astonishment, that the word " you " 156 ACOUSTICS. may begin on one note and terminate on its octave. The answer, " Yes, it was I ! " will show corresponding declension^ or falls of voice. We thus see how truly music is a species of natural language. Unquestion- ably, every shade of human feeling can be represented by successions of its sounds, apart altogether from its words. With respect to the sounds produced by wind in- struments, the effect is caused by the vibration of a column of air confined at one end, and either open or shut at the other. The length of the sounding column determines the nature of the vibrations ; but along with the fundamental tone, there are interior and sub- ordinate vibrations. The whole column divides itself into regular portions, equal to the half, the third, and so on, of the longitudinal extent, in the same man- ner as is the case in stringed instruments. We may observe something similar to these vibrations in the contraction and expansion of a long and very elastic string, to one extremity of which a ball is attached. A spiral spring also shows, and perhaps more clearly, the repeated stretching and recoil. If suddenly struck at one end, it will exhibit not only a vibration through- out its whole extent, but likewise partial ones, which wind, like a snake, along the chain of elastic rings. If the air be struck with great force, the subordinate vi- brations sometimes predominate, and yield the clearest and loudest tones. This may be observed in the dying sounds of a bell, which rise one or two octaves, and expire in the acutest note. Upon the degree of force with which the instrument is blown, depends the per- formance of the bugle-horn, whose compass is very ACOUSTICS. 157 small, consisting only of the simplest notes. In other wind instruments, the nature of several notes produced depends upon the length and size of the tube, or the positions of the holes in its sides. In the organ there is a pipe for each note, and wind is admitted from the bellows to the pipes by the action of keys similar to those of a piano-forte. The organ may be played, also, by a barrel, made to turn slowly under the keys, and to lift them, in passing, by means of pins projecting, at certain determinable intervals, from the surface of the barrel. In wind instruments which are furnished with reeds, the tone depends on the stiffness, weight, length, &c., of the vibrating plate, or tongue, of the reed, as well as on the dimensions of the tube, or space, with which it is connected. xm. 14 ELECTRICITY. ELECTRICITY, from the Greek word electron, amber, properly signifies the science which treats of the phe- nomena of attraction and repulsion produced by the friction of amber, in which substance these phenomena were first observed. As similar appearances, however, were afterwards observed in sealing-wax, glass, and a vast number of other bodies, the term has been ex- tended so as to embrace the operation of this principle wherever it is found. The property exhibited by amber in attracting light bodies was known more than 600 years before Christ ; and Thales of Miletus, in en- deavoring to account for it, ascribed to this substance the functions of an animated being. Singularly enough, although this property was known to both ancients and moderns, no experiments seem to have been made upon the subject before the 17th century, when Dr. Gilbert discovered that the electrical attraction resided, not only in amber, but in the diamond and many other stones, glass, sulphur, sealing-wax, resin, alum, &c. After this, experiments and researches were made by many eminent men, among whom were Sir Isaac Newton and Dr. Franklin ; and the electric phenomena, connected as they are now known to be by certain well-ascertained laws, form together the most complete and important addition to the physical sciences which has been made since the time of Newton. ELECTRICITY. 159 The simplest and most usual mode of producing electricity is by friction. If we rub a piece of amber with dry fur or woollen cloth, and then hold the amber over any light substance, as small pieces of paper, or the down of a feather, the light body will be attracted by the amber. The same effect will be produced by rubbing the crystal of a watch upon the sleeve of the coat, and still more powerfully by rubbing a glass tube with a piece of dry silk. In this latter case, when the tube is rubbed in the dark, sparks of brilliant light, ac- companied with a crackling sound, will be emitted as long as the friction is continued. In like manner, if a dry black silk ribbon, about two feet long, be laid upon a white one of the same length, and be drawn over woollen cloth, or silk velvet, or even between the finger and thumb, they will be found to adhere strongly to each other. In a dark room, the separation of the ribbons will be accompanied with a flash of light, and either of the ribbons, when separated from the other, will attract light substances. Now, in these three simple experiments, the amber, the glass, and the silk ribbons, have obviously received new properties, which they did not possess before they were rubbed namely, the property of attracting light bodies, and the property of emitting light in the dark. These properties are called electrical. The amber, the glass, and the ribbons, are said to be excited by friction. The power of drawing to themselves light bodies is called electrical attraction, to distinguish it from the attraction of cohesion, of gravity, and of magnetism. The light emitted in the dark is named the electric spark, and the body which is capable of 160 ELECTRICITY. acquiring these properties is called an electric. By rubbing a great number of other bodies with woollen cloth, fur, silk, &c., they are found to exhibit the same properties as amber and glass ; while another class of bodies exhibits no such properties. Hence bodies are divided into two great classes namely, electrics and non-electrics. The following is a list of electrics ar- ranged in the order of their perfection : glass, the precious stones, amber, sulphur, shell-lac, and all resin- ous bodies ; bituminous substances ; silk, wax, cotton, dry paper ; dry animal substances, as feathers, wool, hair, parchment, and leather ; dry sugar, ice of dis- tilled water, oils, metallic oxides, ashes, dry vegetable substances, and hard stones. The electrical machine consists of a cylinder, or cir- cular plate of glass, mounted in a frame, so that it can be turned rapidly round on its axis by means of a handle. On one side of it is placed a small cushion covered with silk, against which the glass is rubbed during its rotatory motion ; and on the other side, a brass or metal tube, resting upon a stand of glass, for the purpose of collecting the electricity generated during the excitation of the cylinder or plate. When this is turned briskly round, the motion will be accompanied by a crackling noise ; and if in the dark, streams of bluish light will be perceived directed toward the sharp points with which the metal tube is furnished for the purpose of drawing off the electricity from the glass. This tube may thus be highly charged with elec- tricity, and when removed from the machine, will retain its electrical properties, and will, by simple con- tact, communicate a portion of its electricity to another ELECTUICITT. 161 isolated conducting substance, or be discharged by touching one not isolated. If, with a moderate charge, it be touched with the finger, a sensation like the pricking of a needle is felt, accompanied with a feint spark apparently penetrating the finger. Bodies are either conductors or non-conductors of electricity. The best conductors are metals and water ; the best non-conductors are glass, wax, gum, resin, &c. The quantity of electricity which can be communicated to a perfect conductor is very great, but it appears to have its limit If, from different sources of electricity, we charge a metallic ball, and so continue to charge, we shall presently find that the ball will discharge itself through the air into the nearest conducting body, when a spark, describing apparently a zigzag course, will be observed. This spark travels with immense velocity, and is accompanied by a very audible sound. If received by the body of a man or animal, it produces through a part or whole of the system an instant muscular contraction, which may be rendered ciently strong to cause death, but in more moderation has been used to advantage in some diseases. Electricity had long been an object of study with men of science, yet little was done towards elucidating the theory of it, when a discovery was unexpectedly made which raised the science to an extraordinary degree of estimation. This discovery consisted in the art of accumulating electricity by means of the Leydem, jar. In the year 1745, Von Heist, a German, made an experiment of which he gives the following curious account : " When a nail, or piece of thick brass wire, is put into a small apothecary's phial, and electrified, K 14* 162 ELECTRICITY. remarkable effects follow ; but the phial must be very dry or warm. I commonly rub it over beforehand with a finger, on which I put some pounded chalk. If a little mercury, or a few drops of spirit of wine, be put into it, the experiment succeeds the better. As soon as this phial and nail are removed from the electrifying glass, or the prime conductor to which it has been ex- posed is taken away, it throws out a pencil of flame so long, that, with this burning machine in my hand, I have taken above sixty steps in walking about my room. When it is electrified strongly, I can take it into another room, and fire spirit of wine with it. If, while it is electrifying, I put my finger, or a piece of gold which I hold in my hand, to the nail, I receive a shock which stuns my arms and shoulders. A tin tube, or a man, placed upon electrics, is electrified much stronger by this means than in the common way. When I present this phial and nail to a tin tube fifteen feet long, nothing but experience can make a person believe how strongly it is electrified." In the above experiment, as the phial was of small dimensions, and as the circumstances essential to its greatest effects were not combined, because they were unknown, the power of the electricity accumulated was inconsiderable : but soon afterwards the art of giving a strong shock was discovered in Holland, because the vessels employed happened to be larger. As it was known that the air, or the particles floating in it, ab- stracted the power of electrified bodies, so that even insulation was no remedy against their being in a short time deprived of it, the idea occurred to Muschenbroek and some of his friends, that, if the electrified body ELECTRICITY. 163 were entirely surrounded by a non-conductor, the dis- sipation of the electricity would in a great measure be prevented. To ascertain this, a quantity of water was put into a bottle, and electrified till it was thought to be fully charged ; but here the original design of the ex- periment was lost sight of by an unexpected result, which absorbed all their attention. One of the per- formers, happening to hold his vessel in one hand, while he endeavored to disengage it from the conductor with the other, suddenly received a shock which stunned and terrified him in a high degree. In this manner was discovered what still continues to be called the electric shock. At this day, it excites a smile to ob- serve the terms in which the shock is spoken of by several of those who first submitted to its effects. Muschenbroek, who tried the experiment with a thin glass bowl, told his friend Reaumur, that he felt him- self struck in his arms, shoulders, and breast ; that he lost his breath for a time, and did not feel himself well again for two days. He adds, that he would not take a second shock for the whole kingdom of France. In terms almost equally heightened by terror, speaks Allemand. Though he made the experiment with only a common beer-glass, he declares that he lost his breath for some moments, and felt such an intense pain in his nght arm, that he was alarmed for the consequences. Other philosophers, however, were found, who had the resolution to take shocks of great intensity ; and one of the most hardy wished that he might die by the electric shock, that his death might furnish an article for the Memoirs of the Parisian Academy ! After the art of giving a shock by means of a phial 164 ELECTRICITY. or jar had been discovered, the art of combining sev- eral jars, so as to unite their powers in one discharge, soon followed, and this improvement constituted what is now called a 'battery. It was made by Dr. Franklin, and resulted from his reflections on the phenomenon of the Leyden jar. It had been found that, by coating the outside of the jar with a conducting substance, which communicated by a wire with the person who dis- charged it, the strength of the shock was exceedingly increased ; and that, unless some conducting substance was in contact with the outside of the jar, no charge could be given. Franklin, in accounting for this cir- cumstance, suggested that a charged phial or jar con- tained no more electricity than before ; that as much was lost on one side as was gained by the other ; and that, to discharge it, nothing more was necessary than to make a communication between the two sides : the elec- tricity being by this means enabled to regain its equi- librium, that equilibrium was instantly restored, and no signs of electricity remained. He also demonstrated by experiments that the electricity did not reside in the coating, as had been supposed, but in the glass ; for after a phial was charged, he removed the coating, and found that, by applying a new one, the shock might be received. From these facts, Franklin proposed to distinguish the two opposite electrical states by the terms plus and minus, or positive and negative. Thus, when a body or surface has more than its usual portion of electri- city, it is said to be positively electrified ; when it has less than its usual share, it is said to be negatively electrified. These two conditions exercise a constant ELECTRICITY. 165 effort to balance each other ; and when a communica- tion is made by a conducting substance between two bodies or surfaces, in these conditions, a discharge ensues, and equilibrium is restored. Another theory was broached by M. Du Fay, according to which there are two electric fluids, the one called vitreous and the other resinous, which attract each other after separa- tion ; but the theory of Franklin has prevailed, and the phraseology introduced by him is the only one used in treating of the science. The preceding view of the subject induced Frank- lin to suppose that, if the insides of several Ley- den jars were connected by means of a conducting substance, and their outsides connected with each other in like manner, they would receive and impart a charge like a single jar ; that the shock would be in- creased in proportion to their number, and thus a bat- tery of any force would be obtained. Accordingly they were soon constructed of sufficient power to kill small animals. Franklin did not stop here, but pursued his researches with a success that astonished the world. In the year 1749, he suggested an explana- tion of the phenomena of thunder-gusts, and of the aurora borealis, on electrical principles. He pointed out many particulars in which lightning and electricity agree ; and, in adverting to the power of pointed rods in drawing off lightning, he supposed that, when fixed in the air at the time when the atmosphere was charged with lightning, they might, without noise or danger, draw from it the matter of the thunderbolt into the body of the earth. The manner in which he proposed to bring his speculations to trial was, to erect on the top 166 ELECTRICITY. of a tower, or other elevated place, a sentry-box, from which might rise a pointed iron rod, insulated by being fixed in a cake of resin. Electrified clouds passing over this would, he conceived, impart to it a portion of their electricity, which would be rendered evident by the sparks it yielded on being touched by any con- ductor. Philadelphia contained at that time no build- ing which Franklin deemed proper for his purpose ; he therefore laid aside the thoughts of realizing his conjecture at that time. But while he thus postponed the completion of his views, they were actually car- ried into effect in France, and caused incredible sur- prise and admiration. Franklin had communicated to his friend Collinson, in England, regular accounts of his experiments and theories ; and the latter had communicated them to the public. These publications were widely circulated, and translated into different languages. In France, the principles of Franklin, and several of the experiments by which they were supported, soon became familiar to some of the chief men of science, and his proposed scheme for drawing lightning from the clouds was ac- tually accomplishe.d in the year 1752. A month after this, but before any intelligence of it had reached America, Franklin himself had demonstrated the truth of his theory by an invention of his own. He prepared a silken kite, with a pointed iron at the top, communicating with the string, to which, at the lower end, was attached a key. Having raised the kite into the air while a thunder-storm was approaching, he held it by a band of silk attached to the key. As the cloud passed over it, he beheld the fibres of the string ELECTRICITY. 167 suddenly bristle up, and, holding his knuckle to the key, he received a strong electric spark. As soon as the string became wet, the supply of electricity was co- pious. He afterwards prepared an insulated iron rod, to draw the lightning into his house, and by means of real lightning he performed all the experiments usually executed by common electrical machines. In this dis- covery the French had the precedence in point of time, but they had only followed in the path which Franklin explicitly pointed out. Upon the public announcement of this discovery, the experiment was repeated in various parts of Europe, and the consequences were such as occasioned alarm and terror. Many persons who incautiously attempted to bring down the ethereal fire suffered much by violent shocks, while they incurred the most imminent danger. In one instance, a fatal catastrophe ensued. On the 6th of August, 1753, Professor Richman, of St. Petersburg, was making experiments on lightning drawn into his own room. He had provided himself with an instrument for measuring the quantity of electricity communicated to his apparatus ; and as he stood with his head inclined to it, his attendant observed a globe of blue fire, as large as his fist, jump from the instrument, which was about a foot distant, toward his head. Rich- man was instantly killed, and the attendant was much hurt The latter could give no particular account of the way in which he was affected ; for at the time the professor was struck, he stated that there arose a sort of steam or vapor which entirely benumbed him, and made him sink down to the ground, so that he did not even hear the clap of thunder, which was very loud. The 168 ELECTRICITY. globe of fire was attended with an explosion like that of a pistol ; the electrical instrument was broken to pieces, and the fragments were thrown about the room. A red spot appeared on the forehead of the dead body, and a blue spot on the foot, from which the shoe had been torn ; whence it was inferred that the lightning had entered at the head, and passed off at the foot. On the back of the attendant's coat appeared long, narrow streaks, as if red-hot wires had burnt off the nap. Common electrical experiments, however, are per- fectly safe, provided an ordinary degree of caution is observed. A great multitude of devices may be re- sorted to for showing the singular properties of elec- tricity. The artificial aurora borealis may be ex- hibited in the following manner. Take a glass phial, m shape and size like a Florence flask, with a stop- cock fitted to it. Place it under the receiver of an air- pump, exhaust the air, and close the phial. Rub the glass in the usual manner for exciting electrics, and it will immediately appear luminous within, and a flashing light will be observed, forming a striking min- iature resemblance of the northern lights. The phial may be made luminous by holding it in the hand, with one end presented to the prime conductor of an electrical machine : the strong, flashing light which then appears will remain for some time after it is with- drawn ; and even after several hours have elapsed, on grasping the phial with the hand, strong flashes of light will reappear. If a bundle of hair or feathers be hung upon the prime conductor, the moment they are electrified, by ELECTHICITY. 169 working the machine, they begin to repel and fly from one another, and will not again collapse until the elec- tricity is taken off. A fanciful mode of showing this experiment consists in making the form of a human head, with hair on it ; and, upon placing this image up- on the electrified conductor, the hair immediately shoots up " like quills upon the fretful porcupine." If two per- sons one standing on an insulated stool, and com- municating with the prime conductor, while the other stands upon the floor hold in their hands plates of metal in such a manner that the flat sides shall be opposite to each other at the distance of about two inches, on strongly electrifying the insulated person, dense and frequent flashes will be observed between the plates, forming a kind of artificial lightning. Insulate two bodies, and charge one of them plus, and the other minus ; then suspend between them, by a silken string, an artificial spider, of which the body may be cork, and the legs the fibres of feathers ; the spider will move from the one to the other, till their charge is equalized. Place a cap or covering of metal upon the two extremities of a glass tube, four or five inches long, and enclose in the tube some sawdust or pith-balls ; then charge one of the plates plus, and the other minus ; when, as glass is a non-conductor, the equilibrium can be restored only by the sawdust or balls, which will accordingly jump up and down, till the charge of each plate is the same. To illuminate water, connect one end of a chain with the outside of a charged jar, and let the other end lie upon the table ; place the end of another piece of chain at the distance of about a quarter of an inch from the former ; then xui. 15 170 ELECTKICITV. set a decanter of water upon these separated ends, and, on making the discharge, the water will be illu- minated. Every one knows the use of lightning-rods, which, by means of their sharp points at the top, draw off gradually the electric matter in the clouds, and convey it to the earth, where it is dissipated. Personal security during a thunder-storm forms another important object of consideration. Franklin advises persons apprehen- sive of lightning to sit in the middle of the room, not near a metal chandelier, or any other conductor, and to lay their feet in a chair. A precaution of this kind is the easiest that can be observed, and insures a high degree of safety. It will be still safer to lay two or three beds or mattresses in the middle of the room, and, folding them double, to place the chairs upon them, A hammock, suspended by silken cords, would be an improvement even upon this apparatus. The floor should be dry, or the lightning, if it strikes the house, will fly all over the room. As the walls and floors of houses are usually dry, and therefore non-con- ductors, the lightning is prevented from spreading, and seizes with the more avidity the slightest articles of metal in its way. A person has been known to be struck dead by having his head near a bell-wire during a thunder-storm. Even the electricity conducted by the gilding of a picture-frame would be highly dan- gerous. Dr. Priestley observes, that the safest place is the cellar, and especially the middle of it ; for when a person is lower than the surface of the earth, the light- ning must strike the earth before it can reach him ; it is therefore most probable that it will become imme- ELECTRICITY. 171 diately diffused, and not enter the cellar, especially if it be dry. The best situation for a person who happens to be in the fields during a thunder-storm is within a short distance of a tree, but not immediately under it, as the lightning generally strikes first the highest and best conductors. The frequency with which barns in the country are struck by lightning has been the subject of much remark ; it seems not unlikely that, from their contents of fresh hay, grain, and other matters, they are constantly sending upward a column of vapor, which may serve as a conductor to bring down the electric fluid. It would appear, therefore, that they are unsafe places of shelter when there is lightning .n the atmosphere. GALVANISM. THIS science is a branch of natural philosophy which has originated within a few years, and derives its name from Galvani, a professor of anatomy at Bologna, in Italy. He had the good fortune to make some observa- tions on the electricity of the muscles of frogs, which ap- peared to him to depend on a new power in the animal body ; and although it is now generally admitted that he drew an erroneous inference from his observations, yet they led to a train of experiments which have associated his name with some of the most brilliant discoveries of modern science. To this supposed new power he gave the name of animal electricity, conceiving it to depend upon something inherent in the animal body itself; but we now regard these effects as produced by minute quantities of the electric fluid set at liberty by a certain agency of substances upon each other. The original discovery took place by accident. The wife of Galvani, being in ill health, employed, as a restorative, frog-soup, according to the custom in that country. A number of these animals, skinned for the purpose of cooking, chanced to lie near an electrical machine. While the machine was in action, an at- tendant happened to touch, with the point of a scalpel, the thigh of one of the frogs, when it was observed that the muscles of the limb were instantly thrown into GALVAX1SM. 173 strong convulsions. This experiment was performed in the absence of the professor, but it was noticed by his wife, who communicated it to her husband. He repeated the experiment, varied it in different ways, and perceived that the convulsions took place only when a spark was drawn from the prime conductor, while the nerve of the thigh was at the same time touched with a substance which was a conductor of electricity. When a frog was so placed as to form a part of an electric circuit, it was found that an ex- tremely minute quantity of electricity produced con- vulsions in the muscles : if the hind legs were dissected from the body, the connection being kept up by the crural nerves only, and the electric fluid was passed through it in this state, a still more minute quantity produced a visible effect, so that a frog prepared in this manner was capable of exhibiting very decisive marks of electricity, when none could be detected by an electrometer. After employing the electric fluid as disengaged from the common machine, he next tried the atmos- pherical electricity ; and it was in this experiment that he was first led to observe the effects of galvanism, properly so called. Having suspended a number of frogs by metallic hooks to an iron railing, he found that their limbs were frequently thrown into convul- sions when it did not appear that there was any elec- tricity in the atmosphere. Having duly considered this phenomenon, he discovered that it did not originate from an extraneous electricity, but that it depended upon the position of the animal with respect to certain metallic bodies. 15* 174 GALVANISM. The first stage or epoch in the history of galvanism must be considered that in which it was observed that ex- cited electricity produced muscular contractions in dead animals ; the second is that in which it was observed that different metallic bodies, by mere contact, produced the same kind of contractions ; the third and most remark- able one commences with Volta's admirable discovery of the means of accumulating the galvanic influence. This invention, which justly confers so much celebrity on its author, is, in galvanism, analogous to that of the Leyden phial in common electricity ; and became, like the phial, the precursor of the most brilliant discoveries. We can form, as yet, but a very imperfect judgment of the importance of the consequences to which it will lead. It is called the Voltaic pile, and is made by combining the effects of a number of plates of differ- ent metals, by which means a galvanic battery, capa- ble of giving a shock, is produced. As silver and zinc had been found when a single plate of each was em- ployed to have the greatest effect in producing mus- cular contractions, these metals were selected by Volta for his battery. The silver plates generally consisted of coins, and the plates of zinc were of the same size. The like size and number of pieces of cloth, paste- board, or leather, steeped in salt water, were also pro- vided. These substances were piled upon each other alternately, and the whole pile was supported by some non-conducting substance. The Voltaic pile is now but little used in its original shape, having been superseded by galvanic batteries of a more convenient form, particularly when a great accumulation of galvanism is required. The pile or GALVANISM. 175 battery is fojnd to unite the effects of as many pair of plates as it contains. A pile of 50 pair will give a pretty smart shock, similar to that from an e'wtrical machine when touched by the two hands f'^iultane- ously at the two extremities ; but little or no shock is perceived unless the hands are moistened. The efiects are also increased wheu a larger surface of the body is exposed to action. Thus, if the communication is made by touching with the tips of the fingers only, the effect is not perceived beyond the joints of the knuc- kles ; but if a spoon or other metallic substance be grasped in moistened hands, the shock is felt up to the shoulders. If the communication be made between any part of the face, particularly near the eyes, and another part of the body, a vivid flash of light, corre- sponding with the shock, is perceived. This phenome- non may be more faintly observed by putting a piece of silver between the upper lip and the gum, and lay- ing a piece of zinc at the same time on the tongue ; upon bringing the two metals into contact, a faint flash of light generally appears. It is singular that this light is equally vivid in broad day as in the dark, and whether the eyes be shut or open. Frogs have been found the most convenient subjects for galvanic operations. Galvani prepared these ani- mals by skinning their legs when recently dead, and having them attached to a small part of the spine, but separated from the rest of the body. Any other limb may be prepared in a similar manner, by depri- ving it of its integuments, and laying partly bare the nerve which belongs to it The strongest contractions are produced when the, galvanic electric'ty is made to 176 GALVANISM. pass through the nerve to the muscles. Frogs which have been galvanized quickly become putrid. Per- haps most persons who try galvanic experiments mere- ly for the purpose of amusement would choose to dis- pense with the operation of decapitating and skinning frogs. We may therefore observe that an ample proof of the power of galvanism over the dead muscle may be obtained by galvanizing any animal killed for the kitchen. It will be necessary only to point the wires from the battery, and to penetrate the skin with them, at the two parts between which a communication is intended to be made. Those animals only which possess distinct limbs and muscles can be convulsed by galvanism ; yet reptiles may be affected by it. Thus, if a leech or worm be laid upon a plate of zinc, and surrounded at a little dis- tance by half dollars, every time the animal touches one of the pieces of silver, it will be observed to shrink back. The medical uses of galvanism cannot yet be fully estimated. In some cases, it has proved bene- ficial ; in others, it has had no effect whatever ; and in others still, an unfavorable effect has been ascribed to it. The cases in which it is in general most proper to try it, are those in which common electricity has failed. In instances of numbness, palsy, and suffoca- tion, it has proved highly advantageous. The electricity of the torpedo, and the electrical eel, has a considerable resemblance to galvanism ; it gives a sensible shock, but has little power of any other sort, and might be well imitated by a vast number of mi- nute plates put in action by a fluid feeble in its power of oxidation. GALVANISM. 177 The history of science affords many examples of observations which have remained isolated and useless for ages, and which, though often denied and discredited, have, by the progress of discovery, grown into impor- tance, and become parts of a beautiful system con- tribut'ng, at the same time, essentially to the early ma- turity of some departments of knowledge. Hence those who have accurately detailed a single new phe- nomenon, which appeared to have no connection with any thing useful or any thing known, have, in fact, often been performing a work which should give celebrity to their names, by the direction which it has given to in- quiry, and the light it has afforded to subsequent re- searches. In galvanism, several instances of the kind have occurred, and some of them are so curious as to deserve mention. A long time prior to the establishment of galvanism as a science, it had been observed that, if two different metals were placed in contact under water, they were subject to a rapid oxidation, though the water had no perceptible action upon them when they were apart. It had also been observed that ancient inscriptions made of mixed metals were totally defaced, while those made of pure metals, equally old, were in excel- lent preservation. When metals have been soldered bv means of other metals, they were found to contract a tarnish about the parts where they were joined ; and the copper sheathing of ships, when fastened with iron nails, soon corrodes where the different metals are in contact It had been generally affirmed that porter drunk out of a pewter vessel had a taste different from that drunk out of glass or earthenware, &c. It is now 178 GALVANISM. evident that, in all these cases, the effects were pro- duced by galvanic action. Several persons may receive the galvanic shock to- gether, by joining hands, in the same manner as in receiving the shock from a Leyden jar. Their hands should be well moistened ; but, unlike electricity, the strength of the shock diminishes as it proceeds, in con- sequence of which, the last person feels it much less violently than the first. After receiving the shock, a slight numbness of the part exposed to it remains for some time. The shock may be also conveniently given by placing the hands or feet in salt water, and bringing wires from each end of the battery into the liquid. If any other part of the body is intended to be operated upon, a sponge moistened with salt water, and fastened to a metal plate connected with one end of the battery, may be applied to the part, and the hand or foot put into a vessel of the same liquid, con- nected by a wire with the other end of the battery. The decomposition of water by galvanism is easily effected. The simplest mode of performing this ex- periment is, to bring the wires coming from each end of the battery into a vessel of water. A profusion of bubbles of gas will appear to be given out from each wire as far as it is immersed in the liquid. The closer the wires are brought together, so as not to touch, the more rapidly decomposition goes on. The gas produced from the wire coming from the zinc end of the battery, if the wire be of gold or platina, will be oxygen ; but if the wire be of any metal more oxidable, no gas will appear, but the wire will be oxidated. The gas furnished by the wire from the copper end of the GALVAXISM. 179 battery, of whatever metal the wire may be, is of pure hjdrogen. Both the gases are produced by the decom- position of the water. Batteries containing 6000 or 8000 square inches of zinc and copper surface, furnish the means of per- forming a variety of experiments in which light and heat are abundantly extricated. Such a battery, in its highest state of energy, will make red-hot, and even fuse, a considerable length of fine steel wire, when it forms part of the circuit in making the connection be- tween the two ends of the battery. Attach to the end of each wire of the battery a small piece of charcoal : on completing the circuit, by bringing the two pieces of charcoal into contact, a light, the most vivid that the eye can behold, immediately appears. The charcoal should be prepared for the purpose by burning some very bard, close-grained wood in a closed vessel. The foils, or thin leaves, of gold, silver, tin, and other metals, may be consumed by the help of mercury. Let the conducting wire from one end of the battery terminate in the mercury, in a small iron dish ; to the other conducting wire attach the foil or wire to be deflagrated, and, upon touching the mercury with the latter, the effect will follow. The light afforded by the combustion of different metals is of different colors. Copper or brass leaf, commonly called Dutch gold^ burns with a green light ; silver with a pale blue light; gold with a yellow light ; and all with a slight crackling. The galvanic discharge fires gunpowder, hydrogen gas, oils, alcohol, &c. One of the most brilliant discoveries in modern chemistry was effected by the application of galvanism. 180 GALVANISM. This was the decomposition of the fixed alkalies, by Sir Humphry Davy. These alkalies namely, soda and potass were supposed to be simple bodies ; but Davy discovered them to be metallic oxides. A small piece of one of these oxides being laid upon a piece of plat- ina connected with one end of a powerful battery, and another piece of platina, connected with the other end of the battery, being brought into contact with it, a portion of black matter is soon formed, in which are found imbedded small metallic globules. These glob- ules are the base of the alkali, which has been de- prived of its oxygen by the action of the battery. Ex- periments made by Davy, and other chemists, also showed that many other substances before supposed to be simple as lime, barytes, strontites, magnesia, zir- con, &c. were capable of analysis; and though silex, alumina, and others, offered great resistance to the appli- cation of galvanism, in the majority of cases the anal- ysis was successful. In giving a theory of galvanism, we are struck with a primary question : How does galvanism differ from common electricity ? This query may refer both to the nature of the phenomena themselves, and to the means employed for their pro- duction. It is in some cases difficult to draw the exact line of distinction between the two principles, and many persons doubt whether any precise distinc- tion actually exists. For, as it is conceived that they both depend upon the same agent, having merely ex- perienced some modification in its nature, or mode of action, we must conclude that there may be some inter- mediate or indeterminate state which might be referred to the one or the other with almost equal propriety. GALVANISM. 181 The electricity produced by the galvanic battery is of the same nature as that given by the common electrical machine ; the only difference being that the mode of producing galvanism is continuous ; that is, when in any way discharged, it is immediately reproduced by the oxidation of the zinc ; and hence many galvanic phenomena have been successfully imitated by a series of sparks of ordinary electricity. 91O xm. 16 MAGNETISM. THE word MAGNETISM, in its original and particular acceptation, is employed to denote that invisible force with which certain ores of iron, called in Greek mag- nes, attract pieces of iron to themselves. This prop- erty is found naturally in all the ores of oxidulated iron ; but when the laws of its action are known, we may excite it artificially in metallic iron or steel by a particular process. Of the nature of the principle which produces the phenomena of magnetism, we are entirely ignorant. Of magnets there are two kinds the natural and the artificial. The natural magnet, or loadstone, is an ore of iron, hard enough to strike fire with steel : its color is dull, generally dark gray, brown, or nearly black. The power of magnetic attraction may be communi- cated to iron in any state ; and a bar of iron possessing it in any considerable degree is called an artificial magnet. Magnetism is an accidental property of iron, and the metal may either possess or be deprived of it without losing any of its essential characteristics as a metal. Magnetic attraction was, till lately, supposed to be exerted by ferruginous bodies alone on other ferru- ginous bodies, and hence the use of the magnet was resorted to as a sure means of detecting the presence MAGNETISM. 183 of iron ; but modern investigations have shown that nickel is also susceptible of it ; cobalt is likewise sup- posed to be magnetic. A magnet suspended by a thread, or placed in any situation that leaves it at liberty to move with freedom, turns one end toward the north. The two ends of a magnet are, therefore, called its poles; they are not reversible points, but the pole which is at any time ob- served to point toward the north will always point in the same direction, or nearly so. The attractive prop- erties of the magnet have been known from time im- memorial, and its polarity was known in Europe as early as the middle of the 12th century. Of course there is no ground for the current belief that the mari- ner's compass was invented by Gioja of Amalfi, in the 14th century. It is pretty satisfactorily ascertained that the Chinese had compasses long before the Chris- tian era ; but although the use of them on land was common, they do not appear to have been applied to the purposes of navigation till the 3d or 4th century, when they are distinctly mentioned in the Chinese his- tories. That the Europeans derived the compass from the East, is evident from the fact, that it was known on the coast of Syria before it appeared in Europe, whither it was undoubtedly carried by the crusaders. The first distinct mention of it in Europe is in a satire, written by Guyot de Provins, about the year 1190. The French, in consequence, lay claim to the discovery of the compass ; and this notion has been strengthened by the circumstance of the north pole being marked on the card with a fleur de lis ; but this figure is supposed to be only an ornamented cross. 184 MAGNETISM. The most simple method of exhibiting the power and distribution of magnetism, in a piece of natural loadstone, is to roll it in the filings of iron : on taking it out, it will be perceived that the filings have accu- mulated at the two ends of the loadstone, leaving the middle comparatively bare. If we examine these crests of filings attached to the poles of the loadstone, we shall observe that they radiate by adhering end to end to one another. This phenomenon is particu- larly deserving of attention, as it informs us that iron, placed in contact with a loadstone, becomes itself magnetic, in the same manner that an insulated body becomes electric when held in the presence of another body that is electrified. Magnetism may be communicated to a bar of steel in a more prompt and energetic manner by two loadstones than by one, by placing its two extremities in contact, at the same time, with the contrary poles of the load- stones. The same loadstone may thus successively render magnetic any number of bars, without losing any portion of its original virtue ; from which it is evident that it communicates actually nothing to the bars, but only develops, by its influence, some hidden principle. In the same manner, a stick of sealing-wax, when rubbed, loses nothing of its electricity by the decomposition which its influence effects at a distance in the natural electricities of other bodies. When we hold one of the poles of a loadstone at a distance from a magnetic needle, suspended horizontally by its centre, the two poles of the loadstone act at once upon the needle, but the action of the nearest pole is always the strongest. The needle then turns toward MAGXETISX. 100 Die loadstone the pole which is attracted, and keeps at a distance the one which is repelled. If, after it has taken a position of equilibrium, we turn it ever so little from its place, it will return to it by a series of oscilla- tions, in the same manner as a pendulum, pushed from the perpendicular line, will return to it by the influence of gravity. A motion absolutely similar to this is observed in magnetic needles, freely suspended, when they are pushed ever so little out of the magnetic me- ridian. From this circumstance, therefore, as well as from the constant direction which it gives them, it ap- pears that the terrestrial globe acts upon them exactly like a true magnet. Whether this faculty is owing to the mines of iron and magnetic substances which it contains, or whether it depends upon some other cause, we are yet ignorant The directive property of the magnet is one of the most important discoveries ever made by man. It gives to navigators an infallible guide to point their course across the trackless ocean, in the midst of the darkest nights, and when fogs or tempests have entirely obscured the heavens : a magnetic needle, balanced upon a pivot, points out to them the fixed direction in which they ought to keep, and this valuable indication conducts them as accurately as even the observation of the stars. Previous to this invention, so useful and simple, the seaman could not venture to a distance from the coast The discovery of the compass has given him the means of launching into the farthest depths of the ocean, and of seeking regions unknown to the most powerful and enlightened nations of antiquity. The magnetic needle does not, in general, point 16* 186 MAGNETISM. exactly north and south ; and this deviation, which is different in different parts of the globe, and is even different according to the hour of the day, was first observed by Columbus on his voyage of discovery to America, in 1492. The phenomenon caused great alarm at the time, for it was feared that the only guide which the mariner possessed, to conduct him across the trackless waste, was about to fail him. Succeeding observations, however, have shown that this irregularity is subject to certain laws, and that the earth has a magnetic pole, which does not exactly correspond to the rotatory pole. The aberration from the true north and south line is called the variation of the compass. It occurs at different hours of every day, and at differ- ent seasons of the year, but it is not exactly periodical. It is also very observable at the time of the appearance of the northern lights. The greatest variation from the north toward the west takes place about 2 o'clock in the afternoon, and the nearest approach of the needle to the pole is about 8 in the morning. The needle has an annual progress, between January and March, toward the west ; between March and May it returns toward the north ; in June it is stationary ; in July it varies again to the west ; in August, September, and October, it returns again toward the pole, and during the re- mainder of the year it varies westerly. Before vol- canic eruptions and earthquakes, the needle is often subject to extraordinary agitations. Before 1657, the variation was easterly ; during that year, the needle pointed due north ; and the variatkn to the west has constantly increased ever since. The magnetic needle is also subject to an influence MAGNETISM. 187 called the dip. When a bar of iron unmagnetized is balanced in an exact horizontal position, and the mag- netism is afterwards applied, the north pole of the magnet will dtp, or point, below the horizon. This movement varies in different latitudes ; in the southern hemisphere, it is the south pole of the magnet which dips ; at the equator there is hardly any dip. Magnetism is transmitted through all bodies ; and, apparently, through those which are the most solid with as much ease as through the most porous. In moving a magnet to and fro, under a slice of cork or a plate of gold, the effect upon bits of iron lying upon these substances appears to be the same ; and no difference is observed whether magnetical experiments are tried in ractto, or in the open air. But there are other causes which render magnetism one of the most mu- table of powers. It is weakened by an increase of temperature ; and a white heat almost entirely erad- icates it. Magnetic repulsion takes place only between poles of the same name. Thus a north pole always repels a north pole, and a south pole repels a south pole : yet it is observed that, when the north pole of a weak magnet is presented to the north pole of a powerful one, an attraction often appears. But when this occurs, it is found that the poles of the weaker magnet have in reality been reversed. The middle part of a magnet, exactly between the extremities of the poles, possesses no power either of attraction or repulsion ; but if the magnet be divided in the middle, each half will be- come a distinct magnet ; and those parts which were the north and south poles of the single original magnet 188 MAGNETISM. will still retain their character. The position in which a magnet is kept, and the manner in which it is loaded, nave an effect upon its power. If it be constantly kept with its pole to the north, and be loaded with a weight which is gradually increased, it acquires additional magnetism. But in proportion as its position deviates from the pole, and if at the same time it is kept with little or no weight upon it, the magnetical power is soon materially impaired. In the northern hemisphere, the north pole of a mag- net is considered the most powerful ; in the southern the south pole predominates. But, in order to render a magnet capable of raising the greatest weight possible, an artifice is adopted to render both poles active in lifting the same load. This is done, in what is called the horse-shoe magnet, by bending the bar into a horse- shoe till the two poles nearly touch. By combining many bars into one magnet, an enormous power may be obtained. Magnets of this sort are used by artisans to touch, or magnetize, compass-needles. A magnet employed in the communication of magnetism rather gains than loses in strength, but it cannot impart a greater degree of power than its own. Every kind of violent percussion, or whatever disturbs or deranges the disposition of the particles of a magnet, weakens its power. A strong magnet has been entirely deprived of its magnetism by several smart strokes of a hammer. The effect of the hammer is in some measure corre- spondent to what takes place in the tube-magnet. A glass tube filled with iron filings may be magnetized like a steel bar, and become a perfect magnet ; but when the situation of the filings among themselves is MAGNETISM. 189 altered by shaking the tube, the magnetism disap- pears. In some instances, magnetism may be obtained with- out the agency of a magnet Thus, if a bar of iron, three or four feet long, be held in a vertical position, or, what is more proper, in the direction of the dipping- needle, it will immediately show signs of magnetism, by attracting light pieces of iron. The lower end of the bar will be the north pole ; but if the bar be in- verted, the pole will change likewise. On the south side of the equator, the lower extremity is always the south pole. To succeed in this experiment, the iron should be soft. Bars of iron which have for a long time remained entirely or for the most part in a vertical position as fire-irons, bars of windows,