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great sensation, and in order to verify it and to discover from observations the law of the variation of the magnetic needle, Halley obtained, through the influence of King William, the command of a small vessel of the royal navy, in which he made two voyages in the years 1698 and 1699. He soon returned from the first voyage on account of his crew having fallen sick after passing the equator, and also on account of the mutiny of his lieutenant. In 1699 he sailed again and cruised in the Atlantic and Pacific oceans in various directions. From these voyages he gathered a sufficient number of observations to enable him to prepare his celebrated prospective chart of the variations of the magnetic needle.

On this chart he connected with continued lines all the places on the earth where similar and equal deviation of the needle had been observed, and thus produced a projection of what is called the lines of equal variation, or isogonic lines. These lines afford a ready means of presenting at once to the eye the totality of the phenomenon. They are also sometimes called Halley's lines, although, as may be inferred from a passage in Kircher, he was not the first who constructed such charts. Kircher, in fact, states, at page 443 of his Nautica Magnetica, that a Father Chr. Burrus had thought he had discovered a process by which longitude at sea might be determined, and had on account of it claimed a reward of 50,000 ducats from the King of Spain. His statement is as follows: On his voyage to India he observed, under the widely dif ferent meridians, the deviation of the magnetic needle, and collected also observations made by others. These observations, the number of which was not inconsiderable, he projected on a map, and then connected the places of equal variation by lines, which he called chalyboclitic lines. He asserted confidently that, by means of these lines, he could accurately determine the geographical longitude of a place by merely observing its magnetic variation. The insufficiency of this method was, however, recognized at the time. Gilbert made a similar proposal for determining longitude; but, instead of applying the variation, he thought to use the inclination or dip of the magnetic needle to obtain the object sought.

Enler, the great geometrician, also occupied himself with the theory of the magnetism of the earth, and endeavored to show that the hypothesis of Halley respecting four magnetic poles was unnecessary, and to prove from mathematical deduction that the assumption of the existence of two poles was sufficient; he determined the position of them for the year 1757. The north pole was beyond latitude 76° north, and longitude 96° west from Teneriffe; the south pole at Latitude 58° south, and longitude 158° west.

In recent times a large number of the most accurate and valuable observations on the declination and inclination of the magnetic needle, and on the force of terrestrial magnetism in different parts of the earth, and especially in the neighborhood of the equator, have been made by Alexander von Humboldt during his travels. It was principally from these observations that the French physicist, Biot, endeavored to give an improved theory of the magnetism of the earth. He assumes in this theory that the magnetic poles are not situated on the earth's surface, but in its centre, and in close proximity to each other, and by means of a somewhat complicated mathematical process he succeeds in bringing the results of observations into apparent harmony with his theory.

But one of the most zealous promoters of our knowledge of the magnetism of the earth is Professor Christopher Hansteen, of Christiana, who, in 1817, published his work entitled "Investigations relative to the Magnetism of the Earth." An incident in the beginning of the year 1807 gave the first impulse to these investigations. Examining a physical globe constructed for the Cosmographical Society of Upsala, Hansteen found, at its south pole, an elliptic figure, designated by the name of "magnetic polar region," and it was further inscribed on the globe that this magnetic polar region had been delineated by

Wilke from observations made by Captains Cork and Fourneaux. One focus of the ellipsis was designated as the stronger, the other as the weaker region. Hansteen was induced to compare these statements with the observations, and the comparison being satisfactory, he was led to investigate more thoroughly the theory of Halley, which, until then, he had looked upon as a wild speculation. The result of these investigations was that he became a convert to the theory of the existence, of four movable magnetic poles.

In 1811 the Royal Danish Society of Sciences had offered the annual prize for the best answer to the question, "Whether it is necessary, in order to explain the magnetic phenomena of the earth, to admit the existence of several magnetic axes, or whether one is sufficient?" At the beginning of the following year Hansteen presented the greatest part of his work, as far as it was completed, and the society crowned his labors with its principal prize.

The most important part of Hansteen's work is that in which he treats of the number, the position, and the motion of the magnetic poles. From all the observations collected by him on the variations of the magnetic needle, he concludes that there are four points on the earth through which the lines of equal deviation pass, viz., a, stronger and a weaker one in the vicinity of each geometric pole. Both the stronger poles, as well as the two weaker ones, are situated opposite to each other, as if they were extreme points of the same axes. All four have a regular rotation, the two northern ones from west to east, and the southern ones from east to west.

In order to elucidate the nature of the magnetism of the earth in each of its relations, Hansteen also undertook to make numerous observations, and even made a journey to Siberia, in order to carry on his investigations within the region of greatest intensity of the magnetic phenomenon. This journey, besides directly enriching our knowledge of the magnetism of the earth with valuable results, had other consequences of great importance; it called the attention of the Russian government to this subject, and thus prepared the way for the labors of Alexander von Humboldt, at whose request the Emperor of Russia, with great liberality, ordered a number of magnetic observatories to be erected in his empire. Humboldt, immediately after his return from his travels in America, (1799, 1804,) had erected, in a garden at Berlin, an observatory, exclusively devoted to magnetism, and in which observations were made, often from four to six consecutive days, every half hour without interruption. The proposal of Humboldt, to erect similar observatories in other places of Germany, was not responded to partly on account of the political disturbances which were then visiting that country, partly because its celebrated citizen was intrusted with a mission from his government to France, and was thus hindered, for the time, in the pursuit of his favorite object. Arago commenced in 1818, at Paris, an exceedingly valuable series of magnetic observations, and by comparing them with such as were made simultaneously at Kasan, he confirmed the assertion of his friend Humboldt in regard to the importance and necessity of corresponding observations.

Humboldt returned to Germany in 1827, and established in the autumn of 1828 a continuous and regular series of observations. In consequence of his solicitation, the Imperial Academy of St. Petersburg and the curator of the University at Kasan, erected an observatory at St. Petersburg and Kasan, and under the protection of the chief of the mining corps, Count Canain, magnetic stations were established from the south of Russia through the whole of northern Asia. The Russian Academy sent George Fuss to Pekin, where he erected a magnetic observatory in the garden of the Greek convent, in which Kowanko made a continued series of observations corresponding with those of all the other stations. Admiral Greig also erected a magnetic observatory at Nicolajeff, in the Crimea; and, at the instance of Humboldt, a subterranean magnetic station was established under the supervision of Professor Reich, in the

mines at Freiberg, in Saxony, whilst Arago, at his own expense, had a declination compass placed in the interior of Mexico, at the height of 6,000 feet above the level of the sea. On the suggestion of Admiral Labord, the secretary of the navy of France directed the establishment of a magnetic observatory in 1836, at Reikiavik, in Iceland, and Humboldt sent instruments for an observatory to Havana.

In 1832 a new epoch commenced in the history of magnetic investigations; in that year Frederic Gauss, the renowned author of the general theory of the magnetism of the earth, as Humboldt calls him, erected in the observatory of Gottingen a set of instruments, constructed upon an entirely new principle. In 1834 this apparatus was transferred to a new observatory, expressly prepared for the purpose, and placed in charge of William Weber. After this, from Gottingen, as from a centre, was diffused over Germany, Sweden, and Italy, a spirit of magnetic observation with the improved methods and the instruments of Gauss. In 1836 four annual terms, each of twenty-four hours, were agreed upon by all the observers, during which a continued series of observations were to be simultaneously made, although the hours of these terms did not exactly correspond with those which Humboldt had proposed, yet they were unanimously adopted.

England had thus far taken no part in the general movement, although the celebrated English physicist, Sir David Brewster, made application to the government for the establishment of magnetic stations at different points of the British possessions, but it was here again, through the influence of Humboldt, that the desired result was obtained. He addressed a letter in April, 1836, to the Duke of Sussex, then president of the Royal Society in London, strongly recommending the establishment of permanent magnetic stations in Canada, at St. Helena, the Cape of Good Hope, on the Isle of France, Ceylon, and New Holland. In consequence of this letter, a committee of the Royal Society was appointed in order to examine and report upon the subject. It was proposed by this committee, in a letter to the government, not only to establish permanent magnetic observations, but also to equip ships for an expedition to the Antarctic ocean for the purpose of magnetic observations in that region.

(TO BE CONTINUED IN THE NEXT REPORT.)

ACCOUNT

OF SOME

RECENT RESEARCHES RELATIVE TO THE NEBULE.

BY PROFESSOR GAUTIER

Translated for the Smithsonian Institution from the Archives des Sciences Physiques et Natu relles, Geneva, 1862.

THERE is no part of the vast field of the astronomy of observation which is not at present the object of persevering explorations. I propose on this occasion to give a cursory view of those which relate to a widely extended and highly curious class of celestial objects, which was first made a subject of special study by the distinguished astronomers Herschel and Messier, and since by Lord Ross, by Fathers De Vico and Secchi, and by MM. Lamont, Lassell, and Bond; a subject which presents peculiar difficulties, and respecting which there remains much to be cleared up. I allude to the nebulæ, those small whitish patches, of feeble light, which the telescope reveals to us in great numbers in the heavens, and which powerful instruments enable us, for the most part, to recognize as assemblages of stars, situated at enormous distances from the earth.

In this rapid review I shall follow, in general, the order of dates, and I shall commence by saying a few words of a catalogue of the positions in the heavens of fifty-three nebulæ, the result of observations made at the observatory of Paris by M. Langier, principally in 1848 and 1849, and by him presented to the Academy of Sciences of Paris at its sitting of December 12, 1853. This catalogue, published in the Compte Rendu of that sitting, gives with the precision of seconds of a degree the right ascensions and mean declinations of the centre or most brilliant point of those nebula to January 1, 1850, as well as the differences between these positions and those resulting from the catalogues of Herschel and Messier. It is a first attempt at precise determinations of the position of a certain number of nebulæ, undertaken with a view of serving to decide, in the sequel, the question whether these bodies are really situated beyond the fixed stars which are visible to us.

RESEARCHES RELATIVE TO THE NEBULA OF ORION.

M. Liapounoff, director of the observatory of Kazan, in the beginning of 1856 presented to the Academy of Sciences of Petersburgh, through the medium of M. W. Struve, a memoir on the great nebula of Orion, being the result of observations made for four years with an equatorial telescope of the power of that of Dorpat and a meridian circle of Repsold. He has applied himself

* I know this memoir only from a very succinct mention of it at the end of the number of the Monthly Notices of the Astronomical Society of London for March 14, 1856, vol. xvi, p. 139. As I shall frequently have to cite this compilation, as well as that published at Altona by Dr. Peters under the title of Astronomische Nachrichten, I shall designate them respectively by their initial letters, M. N. and A. N.

to a very exact determination, by a process of triangulation, of the positions of all the stars which his instruments have enabled him to see in that nebula, and to a most careful delineation of all the parts of that remarkable celestial object, of which more than one chart had been already constructed, while assigning particular names to its several regions. M. Struve, in comparing the results of Liapounoff with those of Sir John Herschel, Lamont, and Bond, has expressed the opinion that this nebula must be subject to changes of form and relative brightness in its different parts.

M. Otto Struve has continued, at the observatory of Poulkova, the labors of M. Liapounoff, and has reported the first results of his researches in a communication, of the date of May 1, 1857, presented to the Astronomical Society by M. Airy, June 12 of the same year, and published in the seventeenth volume of the M. N., pp. 225-230.

In this, M. Struve begins by describing the variableness of the lustre of different small stars situated in the nebula of Orion-a variableness which he has verified as well by a comparison of his observations with those of other astronomers as by different observations of his own.* "The existence of so many variable stars," he continues, "in so limited a space of the central part of the most curious nebula of the heavens must naturally lead to the supposition that these phenomena are intimately connected with the mysterious nature of this body. * Admitting that the rapid changes of light observed in these small stars, whether in the region called Huygens or in that called Subnebulosa, are connected with the nature of the nebula, it might be presumed that changes would be equally observed in the appearance of the nebula and in the distribution of the nebulous matter. But observations of this kind are subject to so many illusions, that we can scarce be sufficiently reserved in the conclusions drawn from them. I cannot think that the course commonly pursued by astronomers in this species of researches-the comparison, namely, with one another of graphic representations made at different epochs by different observers-ever conducts to results which can be regarded as indubitable. The optic power of the telescope, the transparency of the atmosphere, varying with different stations, the peculiarities of the observer's eye, the measure of skill and of experience in graphic representations of the kind-all this, joined with the influence of the imagination of the observer, forms obstacles which it will always be difficult to overcome in proceeding after this manner. It might perhaps be possible, by following this method for centuries, to discover progressive changes, if any exist; but those can never be thus verified which take place in short intervals of time. Now, the rapid variations of light in the stars may well cause us to expect similar, and perhaps periodical, variations in the appearances of the nebulous matter. It is therefore to rapid changes of this sort that we should particularly direct our attention, and we shall be better able to verify their existence by comparative observations on the degree of light and the forms of some prominent portions of the nebula than by representing it in its entireness. It was in this way that I endeavored to proceed during last winter, and the impression produced upon me was a strong one that, at different points, considerable changes occurred within the short period of my observations. I do not venture, however, to regard them as positive facts until they shall have been corroborated, especially by observers stationed in more favorable climates and provided with optical instrumentalities sufficient for the purpose."†

* I have heretofore had occasion to speak of this work of M. O. Struve, in a Notice on the Stars of Variable Brightness, published in the numbers of the Bibliotheque Universelle (Archives, vol. xxxvi, pp. 5-89) for September and October, 1857. M. Otto Struve has recently succeeded his father in the direction of the great Russian observatory of Poulkova. + The memoir of M. O. Struve on this subject has been published, I believe, in vol. ii of a collection entitled, Melanges Mathematiques et Astronomiques.

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