Imágenes de páginas
PDF
EPUB

the year 868 Foke Vilgerdarson, the third discoverer of this island, a noted pirate, sailed from Rogaland, in Norway, in search of Iceland, or Gardarsholm, as it was then called, and took with him as pilots three ravens. To consecrate these to their important purpose, he instituted a grand sacrificial ceremony at Smoersund, when his ship was at anchor ready to sail; for, says Are Frode, the seamen in the northern regions were as yet unacquainted with the use of the leading stone. By the term leading stone the writer designated the natural magnet, which, in English, is still called loadstone or leadstone. It may, however, be presumed, from this form of expression, that in Frode's time the compass properly was not yet known, but that the natural magnet was suspended by a thread. According to the testimony of Hansteen, mention is made of the ift: stone or solar-stone, in the Sturlunga Saga. Gilbert, in his celebrated work “De Magnete,” relates that, according to the report of Flavius Blondus, the Amalfitanes (Amalfis? in Naples, first, about the year 1300, constructed and applied the mariner's compass, and this according to the direction of John Gioja, one of their fellow-citizens. He presumes, however, that more probabl the knowledge of this compass had been brought from China to Italy, by Paul Venetus, about the year 1260. Gioja, of Amalfi, was, nevertheless, at least the first who placed the magnetic needle on a point, and divided the compass, according to the points of the horizon, into eight divisions. That the mariner's compass, however, was known at an earlier period in the south of Europe, although in a rude form, is evident from a passage of a satirical poem, which was published by Guyot de Provins, in 1203, and of which the original manuscript is still preserved in the royal (imperial) library at Paris. It is mentioned in this poem that the seaman easily |. the northern direction by the assistance of an ugly, black stone, called mariniere, and this even under a cloudy sky; that for this purpose it was only necessary to rub a needle with the stone, and then, attaching the former to a straw, allow it to swim on water, when it would point to the north. Cardinal Vitri, who lived about the ear 1200, also makes mention of the magnetic needle in his history of Jerusacm, and remarks, moreover, that it is of inestimable value to mariners. That the mariner's compass was known to northern nations is evident from the history of Norway, by Torfaeus, in which it is stated therein that Yarl Stula was rewarded with a compass for a poem written on the death of the Swedish count Byrgeres. The directive force of the magnet is also distinctly alluded to in a letter to Peter Peregrinus de Marcourt, which was written towards the end of the thirteenth century. This letter was directed to “Sigerius de Foucancourt, a soldier in the service of magnetism,” and contains a description of the magnet, of the means to find its poles, and of its peculiar attractive property in regard to iron, and finally proves that the extremity of the magnet which turns towards the north is attracted by the one that turns towards the south. One of the oldest treatises on magnetism is contained in a Latin manuscript of Peter Alsiger, which is found in the University library at Leyden, and was written in 1269. This manuscript, which seems to have been composed for the instruction of a friend, is divided into two parts, of which the first contains ten, and the second three, chapters. In the second chapter of the second part the mariner's compass is distinctly and perfectly described; and what is still more interesting, the author does not only mention the variation of the magnetic needle from the true north pole, but also gives an account of the accurate observations which he had made in regard to the amount of this deviation. “Observe well,” says he, “that the ends of the magnet, and those of the needle rubbed with it, do not accurately turn toward the poles, but that the end which points toward the south inclines somewhat to the west, and the one pointing to the north in an equal proportion to the east.” The magnitude of this deviation amounts, according to numerous observations, to five degrees. The variation of the magnetic needle from the meridian, or its declination, as it is called, was known before the time of Columbus, to whom its discovery has been generally ascribed. His son, Ferdinand, in the biography of his father, written in Italian, and published at Venice, in 1571, relates that Columbus, on the 14th of September, (on the 13th according to Irving, 1492, when he was at the distance of 200 leagues from the isle of Ferro, first observed the deviation of the magnetic needle, “a phenomenon, which,” as the recorder says, “had never been observed before.” Columbus found that at the dusk of evening the needle, instead of pointing towards the north star, deviated about half a point, viz., from five to six degrees towards the northwest, and on the following morning still more. Astonished at this discovery, he observed the needle for three days, and found that the deviation increased the further he advanced to the west. At first he did not call the attention of the crew to this phenomenon, well knowing how easily they might be excited to revolt. The sailors, however, soon became aware of the fact, and, on account of it, fell into the greatest consternation. H appeared to them that the very laws of nature were changing as they advanced on their adventurous career, and that they were entering into a new world governed by entirely unknown influences. They saw the compass losing its truthful character, and asked with alarm what would become of them without this guide on the trackless inhospitable ocean? Columbus had to tax all his ingenuity to appease their terror. He stated to them that the needle does not direct itself strictly towards the polar star, but towards another invisible point in the sky, and that the variation of the magnetic needle was not due to a change in the compass, but to the motion and the diurnal revolution of this celestial point around its pole. The confidence which the sailors had in the astronomical knowledge of Columbus gave weight to this explanation, and their excisement was consequently calmed. Although, as we have seen before, the deviation was known two hundred years previous to the voyage of Columbus, it is, however, evident from the facts just related that he made another discovery of not less importance, namely, that of the difference of the declination in differ. ent places of the carth.

We find more accurate notions of the declination of the magnetic needle, but these are as late as the middle of the seventeenth century. In the year 1541 the deviation of the needle from the meridian at Paris was found to be from seven to eight degrees to the east; in 1550 from eight to nine degrees; and, in 1580, cleven degrees and a half to the east. Norman, who first observed the deviations in London, found it to be 114 degrees in 1596, and Gellibrand, at the same place, in 1634, four degrees towards the east.

We have seen by what precedes that the magnetic needle does not point in all parts of the earth precisely to the geometric pole of the globe, and also that the amount of the deviation is not the same in all places. But it is important further to remark that a magnetic bar, free to move in every direction, will not remain stationary if placed in a horizontal position; on the contrary, in the northern hemisphere the north end of the bar will turn down towards the earth, and in the southern hemisphere the south end will assume a similar position. The bar will only remain horizontal in the region of the equator. The discovery of this important property, which is called the dip or inclination of the magnetic needle, has been generally ascribed to Itobert Norman, (whose name has just been mentioned in connexion with the variation,) an Englishman, an experienced sailor, and, as William Gilbert calls him, an artist of genius. It is said the discovery was made by Norman in the year 1576, but, according to authentic documents, it was known as early as 1544 to George Hartmann, vicar of the church of St. Sebaldus, in Nuremberg. Hartmann was in correspondence with Albert, Duke of Prussia, one of those enlightened minds who recognized the importance of the sciences even at their early dawn. Their correspondence, commencing in 1541, was principally on scientific subjects, but the letter, which is of the most interest to us at present, is dated March 4, 1544, and contains accurate descriptions of three magnetic discoveries which Hartmann had shown the year before, at Nuremberg, to Ferdinand, King of Bohemia, a brother of Charles V. This letter is found in the secret archives at Berlin, and was published by Mosor. Hartmann states in this that he had discovered that the extremity of the needle, which is intended to point to the north, must be rubbed with the end of the stone which points to the south, and that a needle so rubbed which has previously been accurately balanced so as to rest horizontally, will, after the magnetization, incline or dip at one end below the horizon. Further, that a large bar of iron placed vertically becomes so strongly magnetic as to repel with its lower end the northern point of a compass needle. This fact is best shown by using a large bar and a small needle. The fact that rusted iron bars, which have remained for a long time in a vertical position, exhibit always more or less magnetism, was first observed in 1590 by Julius Caesar, a surgeon, at Rimini, who observed that an iron rod, which had been placed for the support of the wall of the tower of the church of the Augustines, had become magnetic. Gassendi observed the same, in 1630, in an iron cross which had been thrown down by lightning from the church tower at Aix. He found that the rusted extremities of this cross had the qualities of the loadstone. When, about the year 1722, the iron cross which had adorned for several centuries the spire of the church tower at Delft was taken down for repair, the celebrated Loewenhoeck, on the suggestion of a stranger, as he says, obtained a piece of the iron from one of the laborers, but no influence was exhibited by it on the compass needle. Some time afterwards, lowever, the same laborer brought him a rusted piece from the foot of the vertical bar, which exhibited more power of attraction than the two natural magnets which Loewenhoeck possessed. Whilst magnetism made but slow progress by incidental observations, it received suddenly a powerful impulse from the investigations of Dr. William Gilbert, of Colchester, England. This distinguished individual, who was physician to Queen Elizabeth, published in 1600 his “Dissertation on the Physiology of the Magnet,” a work which not only contained everything known of magnetism and electricity up to that period, arranged in a truly scientific manner, but also a numerous and ingenious series of investigations on the subject by himself. He was the first who advanced the proposition that the earth itself acts, in all its parts, as a great magnet, in opposition to the opinions of those who, either with Olaus Magnus, supposed that there existed great magnetic mountains of such power that ships, in the construction of which iron had not been entirely omitted, would be attracted and held fast, or with those who placed the power of attraction in the sky, as, for instance, the astrologer, Lucas Gauricus, who supposed that a great magnet existed under the tail of Ursa Major, a constellation in the northern hemisphere to which all compass needles pointed. Gilbert logically refuted these and similar fanciful hypotheses, and substituted his own rational theory in their stead—a theory which, in its general principles, has been retained to the present time. He also attempted to explain, but with less success, the declination of the needle by ascribing magnetism merely to the solid parts of the earth, and not to the water, so that the needle would incline towards the continent, because a greater amount of magnetic power existed there. It could, moreover, not escape the sagacity of a man like Gilbert, that the magnetic terminology, as he found it, was liable to great inconsistencies. Even in our days we are still accumstomed to call the end of the needle which points to the north its north pole, and the one pointing to the south its south pole. This form of expression is, nevertheless, incorrect, for if we admit that the earth is a great magnet, and that in the vicinity of the geographical north pole a magnetic north pole is situated, this north pole could only attract the south pole of another magnet, and consequently the end of the magnetic needle which turns towards it should be called the south pole. In like manner the end of the needle which points to the south should be called its north pole. Gilbert objected to the use of this inconsistency, and introduced in its stead the correct appellation. He did not succeed, however, in abolishing the old terms, although physicists agreed with him, and even some of the more recent writers on this subject have adopted his forms of expression. This is the case with the French authors on magnetism, and some of the English physicists have endeavored to avoid the difficulty by using the term north end for the extremity of the needle which points to the north, and the south end for that which is directed to the south. The fact was still unknown, even to Gilbert, that the deviation of the magnetic needle changes with time, and, upon the whole, there is but little trust. worthy testimony to show to whom the discovery of the secular variation of the magnetic needle is to be attributed. Although observations made at Paris and London exhibit in different years a difference in the variations, the idea could not be seized upon at once that the needle changed its position from one year to another; on the contrary, it appears that the differences observed were considered as errors of the observations. Gellibrand, however, who observed the variation in 1634, in London, finding it different from that observed by Gunter in 1622, and that by Burrows in 1580, concluded that the deviation was variable, and therefore the discovery is generally ascribed to him. Although the French had observed as early as 1541, 1550, 1580, and 1603, in Paris, four different variations, and although Gunter, in London, had also found a deviation different from that of Burrows, the honor of the discovery cannot be ascribed to any of them, since the one who makes a discovery is he who first clearly perceives the essential particulars of the phenomena and gives an intelligible account of them; for this reason, and, indeed, with justice, the discovery of Uranus is ascribed to Herschel, although Flamstead had observed it nearly a hundred years before, but had mistaken it for a fixed star. The fact of the yearly variation of the magnetic needle was adopted and defended by Gassendi, in France, and was soon generally admitted, although it was thought at the time that the motion was regular, or that the north end of the needle moved every year an equal amount towards the west. It was, however, soon discovered that its progress was far from being regular, but it was still thought that the motion was so slow that the needle might be considered stationary at least for a few days. But this also proved to be incorrect when Father Guy Tuchart, in 1682, observed the deviation in the city of Louvo, in Siam, in presence of the King; he found it on four, and again on three successive days to assume different directions, either increasing or decreasing. The celebrated mechanist, Graham, in London, repeated these observations with better instruments in 1722, and discovered that the needle changes its position not only from day to day, but even from hour to hour; that, indeed, it does the same continually, and is, in fact, in a state of perpetual motion. Assessar Swedenborg, in his treatise on magnetism, expressed a . as to the correctness of these propositions, and asserted that they were based upon errors of observation. This induced the celebrated professor Celsius, at Upsala, to repeat the observations of Graham. As early as in 1740 he communicated a few results to the public, which showed the correctness of Graham's discovery. Celsius was also the first who, in company with Hiarter, observed the remarkable and violent disturbances of the magnetic needle which accompany the appearance of the aurora borealis, and it was he who also first established the fact of the simultaneous motion of the needle at different places on the earth. He had induced Graham, in London, to make observations simultaneously with him, in order to ascertain whether the disturbances of the needle depends on local changes, or on those affecting large portions of the earth. After the death of Celsius, Olav Hiarter continued his observations and published the records of

the whole. From the comparison of these observations with those of Graham it was found that in Europe the needle is furthest to the east in the morning from eight to nine o'clock, and furthest to the west in the afternoon from one to two o'clock, when it again proceeds eastwardly until eight or nine o'clock in the . evening, when it either remains stationary for a few hours, or makes a small movement of a few minutes back towards the west. During the night it generally moves somewhat towards the east, so that in the morning at eight o'clock it is found a little more to the eastward than in the evening. About the year 1756, John Canton, in London, made observations on the daily deviations, or of the variations, as they were called, from which the result was deduced that the regular daily motion about the time of the summer solstice is nearly twice as great as at the winter solstice. In the first instance, it amounts to about 4, in the latter to about 4 of a degree. Canton endeavored to explain the daily western and the subsequent eastern variation of the needle by referring it to the influence of solar heat on the magnetism of the earth. He supposes, since magnetism is weakened by heat, that, if in the forenoon the sun warms the eastern parts of the earth, the needle will be more attracted towards the western parts, and in a similar manner in the afternoon, when the sun has weakened the western side, the greater influence of the eastern will draw the needle more towards that direction. Before proceeding further in the exposition of this subject, we are obliged to take a step backwards and direct our attention to an individual who produced an epoch in the theory of the magnetism of the earth. We allude to Dr. Edmund Halley, of England, who in 1683 published his theory of terrestrial magnetism, which, in some particulars, still forms the basis of our present theories. He advanced the hypothesis that there were four magnetic poles, two in the vicinity of each geometrical pole of the earth, so that in different parts of the earth the needle always directs itself in such a manner that the influence of the nearest poles overcomes that of the more distant one. He further assumed that the pole which at that time was nearest to England was situated on the meridian of Cape Landsend, at the distance of seven degrees from the north geometrical pole, and that the other magnetic north pole was on the meridian of California, at the distance of 15 degrees from the north geometrical pole. He placed one of the two magnetic south poles 16 degrees from the geographical south pole, and 95 degrees west from London, and the other, the strongest of the four, at the distance of 20 degrees from the south pole, and 120 degrees west from London. In order also to explain the successive variations of deviations, he advanced the remarkable hypothesis that our earth is a hollow sphere within which is a solid globe; that the two revolve around the same centre of gravity in nearly, though not in exactly the same time; and furthermore, that the solid globe is separated from the exterior hollow shell by a liquid medium. He also supposed that the internal globe, as well as the external shell, have each two magnetic poles, and that the changing deviation of the needle was produced by the want of perfect simultaneousness in the rotation of the two spheres. According to this hypothesis the magnetic poles of the external shell, while they do not coincide with the geometric poles of the same shell, always retained the same position, and, therefore, if the needle was only affected by them, the variation would always remain the same at the same place; but the needle being also acted upon by the magnetic poles of the interior globe, and as these slowly change their position relative to those of the exterior shell on account of the difference of velocity in the revolution of the two spheres, a change in the direction of the needle on all points of the earth's surface must be constantly going on. Also after a complete rotation of the exterior within the interior sphere the

~~~~ +**** -----

« AnteriorContinuar »