possessed of imparting magnetism to ferruginous bodies. So powerful was the magnetism produced by the discharge of a small electrical battery, through a wire one-twentieth of an inch in diameter, that it rendered bars of steel two inches long highly magnetic. In Oersted's early experiments, he proposed to ascertain "whether electricity the most latent had any action on the magnet." He soon ascertained that an electrical current passing from the positive to the negative pole of a Voltaic battery would cause a magnetic needle, placed near it, to deviate from its natural position. He was thus led to believe that the electric action was not enclosed within the conducting wire, but that it must possess an extensive sphere of action around it. He was also led to conclude that the above influence acted by revolution; for without such a supposition it was impossible to conceive how the wire, when its position was changed with reference to the needle, should act in opposite directions. Oersted's experiments were no sooner published than they were repeated and varied by philosophers in every part of Europe; and a multitude of new facts were soon brought to light, through the labours of Davy, Faraday, Ampère, Biot, and Arago.*

The study of the heavenly bodies, and as such the dawn of astronomical science, must evidently be as old as the creation; though, in all probability, the astronomer of that period confined his observations to the more obvious motions of the sun and moon, the rising and setting of the principal stars, and the apparent motions of the planets. The progress of the sun being thus followed, the regular transitions from day to night would at once be understood. The level and extensive plains of Chaldea peculiarly fitted that favoured portion of the globe to the study of astronomy; and the clear nights which the inhabitants were wont to pass in the open air, united to a pure and serene sky and an unbroken horizon, all conspired to engage that people to contemplate the motions of the stars, and to lead them to conjecture on the laws by which they were governed. From Chaldea, astronomy passed into Egypt, and was soon afterwards carried into Phoenicia, where the people began to apply the observations which had been made to the uses of navigation, and thus rendered themselves both the masters of the sea and of commerce. Their guide, in steering their ships, when far from land, was one of the stars in the constellation called the Little Bear, which, unlike other stars, appeared always to retain the same situation. Other nations, less skilful in astronomy, observed only the Great Bear in their voyages, -a guide too imperfect to enable them to lose sight of land with safety. But the most ancient observations of which we are in possession, that are sufficiently accurate to be employed in astronomical calculations, were made at Babylon, about seven centuries prior to the Christian era: they relate to three eclipses of the moon; and Ptolemy, who has transmitted them to us, employed them for determining the period of the moon's mean motion, and therefore had probably none more ancient on which he could depend. To Anaximander, one of the disciples of Thales, is ascribed the invention of the terrestrial globe, and of a gnomon which he erected at Sparta, by means of which he observed the equinoxes and solstices, and

A very remarkable fact, for which we are indebted to the latter philosopher, may here be briefly noticed. He found that a needle, suspended over a revolving plate of copper, was drawn aside from the meridian in the direction of the motion, and that, when this became very rapid, it would even follow the tide of circumvolution. It was at first supposed that this very extraordinary effect was produced by the action of a current of atmospheric air; but, when the plate was enclosed in a glass box, the needle still continued to revolve. It may be proper to add that a variety of other bodies were afterwards substituted for the copper plate, with a similar result.

The connection which subsists between the electrical and magnetic influences influenced by heat was, we believe, first pointed out by Dr. Seebeck, of Vienna, in 1823. He found that, by combining metals of different kinds, deflections of the magnetic needle might be produced by a change of temperature alone, without the assistance of any Voltaic arrangement. A very valuable list of thermo-electrics has been furnished by professor Cumming, and M. Ampère ascribes the diurnal variation of the needle to the corresponding motion of the globe, which produces a continued variation in its temperature.

determined the obliquity of the ecliptic, more exactly than had ever been done before. The Greeks, assisted by the instructions they had received from Thales and Anaximander, ventured to make considerable voyages, and planted several colonies in remote countries; yet the latter philosopher and his children were proscribed by the Athenians; and their lives would have been sacrificed but for Pericles, through whose influence the sentence was commuted for banishment. The charge against him was the discovery of truth; for it was thought impious to suppose "that the works of the gods could be subject to im

mutable laws."

Pythagoras, another disciple of Thales, taught many important astronomical truths. To him is attributed the discovery of the true system of the world, which, after the lapse of many centuries, was revived by Copernicus, and which is now settled on the basis of so many truths that it can never be overthrown. It was thought, even in his school, that the planets were inhabited like the earth; and that the stars, which are disseminated through infinite space, are suns, and the centres of other planetary systems. He is also said to have considered the comets as permanent bodies, moving round the sun; and not as perishing meteors formed in the atmosphere, as they were thought to be in after times. The Arabian school of astronomy commenced with Almamoun, the son of the caliph Haroun al Raschid. This celebrated warrior and philosopher, having conquered the emperor Michael III., made it a condition of peace that a copy or the works of each of the best Greek authors should be delivered to him, and among them were the works of Ptolemy, of which he procured an Arabic translation. This occurred in the ninth century; and, about 400 years later, the work of Ptolemy was translated into the Latin language. The fifteenth century was rendered memorable by the revival of the Pythagorean system. This was effected by Nicholas Copernicus, a native of Thorn in Prussia. The Ptolemaic system, which supposes the earth to be fixed in the centre of the universe, and the sun and moon, with Mercury, Venus, and the other planets, revolving round it in concentric circles, he perceived to be inconsistent with the celestial phenomena, and encumbered with many absurdities, which did not affect the hypothesis which considered the sun to be in the centre, and the earth a planet revolving about it annually, with the rest, and daily turning upon its own axis.

The only opposition of any consequence which the theory of Copernicus ever met with from science and argument proceeded from Tycho Brahe, a celebrated Danish astronomer who attempted to set up against it a theory of his own. His system is not very different from the Ptolemaic, but is generally called by his name. He supposed the earth to be immovable in the centre of the universe, and the sun to revolve about it every twenty-four hours; the planets, he thought, went round the sun in their periodical times, Mercury being nearest to that luminary, then Venus, Mars, Jupiter, and Saturn; and of course to revolve also about the earth. But some of Brahe's disciples supposed the earth to have a diurnal motion round its axis, and the sun, with all the planets, to move round the earth in one year.

Kepler was one of the pupils of Tycho Brahe, and a man of a truly original genius. Hipparchus, Ptolemy, Tycho Brahe, and even Copernicus himself, were indebted for a great part of their knowledge to the Egyptians, Chaldeans, and Indians. Pursuing paths already pointed out, they did little more than separate fancy from fact, with more or less success; but Kepler, by his own talent and industry, made discoveries of which no traces are to be found in the annals of antiquity. Galileo was contemporary with Kepler; and whilst the latter was tracing the orbits of the planets, and settling the laws of their motions, he was investigating the doctrine of motion in general, which had been neglected for 2000 years; and, from the result of their united labours, Newton and Constantine Huyghens were afterwards enabled to establish the most complete theories of all the planetary motions.

The discoveries of Newton with reference to the laws of gravitation have already been

adverted to; and we may add that from the time of our illustrious countryman, who carried the theoretical part of the science to its present state of perfection, astronomy has never been without an illustrious phalanx of supporters, whose particular discoveries are fully illustrated in the various articles relating to the science of astronomy.

Practical astronomy, to which we must now more particularly direct our attention, appears to have originated in Chaldea; but it found still more active supporters in the Arabian caliphs, and the philosophers who surrounded their courts. The caliph Almanza employed many

able mechanics in the construction of instruments for his own astronomical observations; and he was so well provided with apparatus as to be enabled to measure, with tolerable accuracy, an arc of the meridian. Ulug Beg, grandson of the celebrated Tamerlane, was a great proficient in practical astronomy: he is said to have had very large instruments for making his observations, particularly a quadrant as high as the church of Sancta Sophia at Constantinople. But the practical mechanics of the east is, like their political institutions, of a very enduring character; and we still find their astronomical apparatus both accurate and colossal. The celebrated observatory at Benares still exists an enduring monument of the learning and mathematical science of the east; and we furnish our readers with a characteristic sketch of its appearance when visited by Sir Robert Barker.*

The earliest observatory of any importance, established in Europe, was erected by the

Sir Robert says, when speaking of the edifice, "We entered this building, and went up a staircase to the top of a part of it, near the river Ganges, that led to a large terrace, where, to my surprise and satisfaction, I saw a number of instruments yet remaining in the best preservation, stupendously large, immovable from the spot, and built of stone, some of them being upwards of twenty feet in height; and, though they were said to have been erected many hundred years before, the gradations and divisions of the several arcs appeared as well cut, and as accurately divided, as if they had been the performance of a modern artist. The execution in the construction of these instruments exhibited a mathematical exactness in the fixing, bearing, and fitting of the several parts, in the necessary and sufficient supports to the very large stones that compose them, and in joining and fastening them into each other by means of lead and iron cramps. The situation of the two large quadrants, whose radius is nine feet two inches, by being at right angles with a gnomon at twenty-five degrees elevation, are thrown into such an oblique situation as to render them the most difficult, not only to construct of such a magnitude, but to secure in the position for so long a period, and affords a striking instance of the ability of the architect in their construction; for, by the shadow of the gnomon thrown on the quadrants, they do not appear to have altered in the least from their original position; and so true is the line of the gnomon that, by applying the eye to a small iron ring of an inch diameter at one end, the sight is carried through three others of the same dimensions, at the extremity of the other end, distant thirty-eight feet eight inches, without obstruction; such is the firmness and art with which this instrument has been erected."

landgrave of Hesse-Cassel, in 1561. It occupied the whole upper portion of his palace, and it was well furnished with astronomical instruments. Tycho Brahe, who was engaged about the same period in astronomical observations, very materially improved on the landgrave's apparatus, and made a quadrant capable of showing single minutes. He afterwards erected an observatory in the island of Huenna; it was of a square form, and provided with two lofty round towers. The instruments consisted of quadrants, sextants, circles, astrolabes, globes, clocks, and sun-dials. These instruments were so divided as to show single minutes; and in some the arc was so divided as to be read off into ten seconds. Most of the divisions were diagonal; but he had one quadrant so divided as to form forty-seven concentric circles. The whole expense attendant on the erection of his observatory is said to have amounted to 200,000 crowns.

The erec

In 1671 the royal observatory at Paris was finished, and the use of it assigned to M. Cassini, after it had been furnished with instruments at a very great expense. tion of the Greenwich observatory, five years afterwards, may be said to form an era in practical astronomy. Mr. Flamstead was appointed the first astronomer royal, and the office has since been held by several distinguished mathematicians. In the beginning of the last century the practice of astronomy made but little progress, in consequence of the imperfect apparatus employed; but since that period the names of Hooke, Graham, Bird, Dolland, Ramsden, Troughton, and Herschel, as practical artisans, give abundant warrantry for the assertion that the natives of this country exceed those of all others in the construction of astronomical instruments.

The art of navigation is closely connected with astronomy, and as such will next engage our attention. Its origin, like that of most other arts, is involved in the fabulous history of early times; but, according to the accounts of the most ancient historians, it was in the Mediterranean and Arabian gulfs that the first attempts were made in the art, as in those places commerce assumed an active character in the intercourse between the Egyptians and Phonicians. The former, who lived in a fertile land and a genial climate, which rendered them independent of the productions of other countries, devoted themselves to the cultivation of the sciences rather than that of commerce, and, imbued with gross superstition, applied only a portion of their knowledge of astronomy to navigation. But the Phoenicians, whose country was unequalled in its beauty, less superstitious, and of a more active and commercial disposition, boldly extended their voyages, and established their colonies nearly in every country that was known. They improved the construction of their vessels; they forwarded navigation as much as the period admitted; and they became the teachers of this art to all nations, particularly to the Gaditanos (inhabitants of Cadiz) and the Greeks. The Carthaginians and Romans succeeded them; but, although each in their turn improved their ships, the knowledge, geography, and navigation, limited to the mere performance of short voyages, would have remained in its infancy, had not the progress of mathematics, and particularly that of astronomy, in modern ages, roused it from its lethargic condition. Navigation at that period consisted only in the knowledge of coasts, and in making short voyages from one place to another, without losing sight of the land; and when, by any unforeseen accident this became invisible, the motions of circum-polar stars, and the flight of birds, naturally directed towards the shore, served as guides to the bewildered mariner. The configuration of coasts, and their mountains and principal headlands, with the use of the lead, formed another species of knowledge, for which the mariner was indebted to his experience in navigation.*

* Until the properties of the magnet were discovered, followed by the invention of the compass, the progress of navigation could not be otherwise than slow; and therefore, from this important period, which was about the end of the thirteenth century, the real advancement of this art towards perfection can only be dated. The valuable invention of the compass is equally involved in mystery, and its real discoverer is unknown

"But little avail would it be to the mariner," says Salazar, "to know the course which his vessel may make with respect to the shore; or the means of measuring the distance sho might sail, unless he possessed some method of comparing his situation with reference to the various parts of the earth's surface, when alone and forsaken in the vast watery desert, where the eye can discover only sea and sky. In this condition, astronomy is ready to assist him, and to afford him the means by which his latitude and longitude may be ascertained, and with these data he finds out the position he occupies on the globe. But even this is insufficient to enable him to shape his course in his lonely situation, surrounded on all sides by the element alone in which his vessel moves, nor can he yet direct her to the port he is seeking, or avoid the dangers he may meet in his way. The mariner, unless he knows the relative situation of the place to which he would go, must still be at a loss what to do; and, to ascertain this, he must be informed of the contour and direction of coasts, their respective positions, the motions of the tides and currents, the gulfs and depths of the sea, as well as the innumerable rocks with which it is scattered. Such are the important objects which constitute hydrography, all of which are essential to his safety and convenience.

The exact epoch is not known, with any probable degree of certainty, when seamen first made use of maritime charts, the ingenious invention and valuable guides with which geography has furnished navigation, and which, being gradually improved, have now arrived at that state of perfection which enables the navigator to traverse the ocean with the same confidence that a traveller would perform his journey across a country.

The celebrated prince Henry of Portugal, seeing the importance of advancing this branch of navigation, in the year 1417, founded an academy for pilots and mathematicians at Sagres, and established as the president of it Maestro Jayme, an experienced pilot of Majorca, one who was well acquainted with such matters. This person instructed young Portuguese officers in the use of the astrolabe, and the method of finding the latitude by means of the sun's altitude, having previously constructed tables of declination.

Hutton, in his Mathematical Dictionary, says that Ptolemy was the first to whom the idea occurred of varying the proportion of the degrees of latitude, and that Gerard Mercator, in 1556, published charts on the principle of those which now bear his name; but that Edward Wright was the first who conceived the true principles of constructing these charts in 1599, and that Mr. Blundeville published an account of Wright's charts in 1594. Such was the state of navigation at the end of the fifteenth century, when Columbus, well versed in the astronomical and mathematical knowledge of his day, and endowed with an extraordinary degree of intrepidity, boldly ventured across the ocean, traversed unknown seas, and, by discovering new countries, produced a total change in politics and science. In his first celebrated voyage, he discovered the variation of the needle, a phenomenon which, to his companions, appeared so wonderful, that, fearful of its losing altogether the virtue of pointing to the north, they imagined they must become victims to the ambition of this great man. But the safety of the navigator was yet imperfect, and a method was still wanted by which he might find his longitude at sea, and which, although it might not arrive at that degree of precision by which the course of a vessel could be found, should at least afford an approximation Lafiteau, in his History of the Portuguese Discoveries in the New World, says that Vasco di Gama brought it to Lisbon from the coast of Africa, on his return from Melinda, where the Arabs then used it, and he believed the Portuguese to have been then ignorant of it. Some attribute it to Flavio Gioja, of Amalphi, about the year 1302; while others again are of opinion that the invention is due to the Chinese, and that one of their emperors, a celebrated astronomer, was acquainted with it 1120 years before the Christian era; nor have others again been wanting who have supported the opinion that it was known in the time of Solomon. The ancient Greeks and Romans are supposed by some to have used it, but the silence of Pliny on this subject renders this doubtful.