obtained at four of these stations made by MM. de Humboldt and Dove, in the year 1830, shewed that the magnetic disturbances were simultaneous, and were for the most parallel in their progress. Important steps in the prosecution of this subject were soon after made by M. Gauss, the great mathematician of Göttingen. He contrived instruments and modes of observation far more perfect than any before employed, and organized a system of comparative observations throughout Europe. In 1835, stations for this purpose were established at Altona, Augsburg, Berlin, Breda, Breslau, Copenhagen, Dublin, Freiberg, Göttingen, Greenwich, Hanover, Leipsic, Marburg, Milan, Munich, Petersburg, Stockholm, and Upsala. At these places, six times in the year, observations were taken simultaneously, at intervals of five minutes for 24 hours. The Results of the Magnetic Association (Resultaten des Magnetischen Vereins) were published by MM. Gauss and Weber, beginning in 1836. British physicists did not at first take any leading part in these plans. But in 1836, Baron Humboldt, who by his long labours and important discoveries in this subject might be considered as peculiarly entitled to urge its claims, addressed a letter to the Duke of Sussex, then President of the Royal Society, asking for the co-operation of this country in so large and hopeful a scheme for the promotion of science. The Royal Society willingly entertained this appeal; and the progress of the cause was still further promoted when it was zealously taken up by the British Association for the Advancement of Science, assembled at Newcastle in 1838. The Association there expressed its strong interest in the German system of magnetic observations; and at the instigation of this body, and of the Royal Society, four complete magnetical observatories were established by the British government, at Toronto, St. Helena, the Cape of Good Hope, and Van Diemen's Land. The munificence of the Directors of the East India Company founded and furnished an equal number at Simla (in the Himalayah), Madras, Bombay, and Sincapore. Sir Thomas Brisbane added another at his own expense at Kelso, in Scotland. Besides this, the government sent out a naval expedition to make discoveries (magnetic among others,) in the Antarctic regions, under the command of Sir James Ross. Other states lent their assistance also, and founded or reorganized their magnetic observatories. Besides those already mentioned, one was established by the French government at Algiers; one by the Belgian, at Brussels; two by Austria, at Prague and Milan; one by Prussia, at Breslau; one by Bavaria, at Munich; one by Spain, at Cadiz; there are two in the United States, at Philadelphia and Cambridge; one at Cairo, founded by the Pasha of Egypt; and in India, one at Trevandrum, established by the Rajah of Travancore; and one by the King of Oude, at Lucknow. At all these distant stations the same plan was followed out, by observations strictly simultaneous, made according to the same methods, with the same instrumental means. Such a scheme, combining world-wide extent with the singleness of action of an individual mind, is hitherto without parallel. At first, the British stations were established for three years only; but it was thought advisable to extend this period three years longer, to end in 1845. And when the termination of that period arrived, a discussion was held among the magneticians themselves, whether it was better to continue the observations still, or to examine and compare the vast mass of observations already collected, so as to see to what results and improvements of methods they pointed. This question was argued at the meeting of the British Association at Cambridge in that year; and the conference ended in the magneticians requesting to have the observations continued, at some of the observatories for an indefinite period, at others, till the year 1848. In the mean time the Antarctic expedition had brought back a rich store of observations, fitted to disclose the magnetic condition of those regions which it had explored. These were discussed, and their results exhibited, in the Philosophical Transactions for 1843, by Col. Sabine, who had himself, at various periods, made magnetic observations in the Arctic regions, and in several remote parts of the globe, and had always been a zealous labourer in this fruitful field. The general mass of the observations was placed under the management of Professor Lloyd, of Dublin, who has enriched the science of magnetism with several valuable instruments and methods, and who, along with Col. Sabine, made a magnetic survey of the British Isles in 1835 and 1836. I do not dwell upon magnetic surveys of various countries made by many excellent observers; as MM. Quetelet, Forbes, Fox, Bache and others. The facts observed at each station were, the intensity of the magnetic force; the declination of the needle from the meridian, sometimes called the variation; and its inclination to the horizon, or the dip;-or at least, some elements equivalent to these. The values of these elements at any given time, if known, can be expressed by charts of the earth's surface, on which are drawn the isodynamic, isogonal, and isoclinal curves. The second of these kinds of charts contain the 'Halleian lines' spoken of in a previous page. Moreover the magnetic elements at each place are to be observed in such a manner as to determine both their periodical variations, (the changes which occur in the period of a day, and of a year,) the secular changes, as the gradual increase or diminution of the declination at the same place for many years; and the irregular fluctuations which, as we have said, are simultaneous over a large part, or the whole, of the earth's surface. When these Facts have been ascertained over the whole extent of the earth's surface, we shall still have to inquire what is the Cause of the changes in the forces which these phenomena disclose. But as a basis for all speculation on that subject, we must know the law of the phenomena, and of the forces which immediately produce them. I have already said that Euler tried to account for the Halleian lines by means of two magnetic 'poles,' but that M. Hansteen conceived it necessary to assume four. But an entirely new light has been thrown upon this subject by the beautiful investigations of Gauss, in his Theory of Terrestrial Magnetism, published in 1839. He remarks that the term 'poles,' as used by his predecessors, involves an assumption arbitrary, and, as it is now found, false; namely, that certain definite points, two, four, or more, acting according to the laws of ordinary magnetical poles, will explain the phenomena. He starts from a more comprehensive assumption, that magnetism is distributed throughout the mass of the earth in an unknown manner. On this assumption he obtains a function V, by the differentials of which the elements of the magnetic force at any point will be expressed. This function V is well known in physical astronomy, and is obtained by summing all the elements of magnetic force in each particle, each multiplied by the reciprocal of its distance; or as we may express it, by taking the sum of each element and its proximity jointly. Hence it has been proposed 16 to term this function the integral proximity' of the attracting mass.17 By using the most refined mathematical artifices for deducing the values of V and its differentials in converging series, he is able to derive the coefficients of these series from the observed magnetic elements at certain places, and hence, to calculate them for all places. The comparison of the calculation with the observed results is, of course, the test of the truth of the theory. The degree of convergence of the series depends 16 Quart. Rev. No. 131, p. 283. 17 The function V is of constant occurrence in investigations respecting attractions. It is introduced by Laplace in his investigations respecting the attractions of spheroids, Méc. Cel. Livr. III. Art. 4. Mr. Green and Professor MacCullagh have proposed to term this function the Potential of the system; but this term (though suggested, I suppose, by analogy with the sub stantive Exponential,) does not of both the upon the unknown distribution of magnetism within the earth. If we could venture to assume,' says M. Gauss, 'that the members have a sensible influence only as far as the fourth order, complete observations from eight points would be sufficient, theoretically considered, for the determination of the coefficients.' And under certain limitations, making this assumption, as the best we can do at present, M. Gauss obtains from eight places, 24 coefficients (each place supplying three elements), and hence calculates the magnetic elements (intensity, variation and dip) at 91 places in all parts of the earth. He finds his calculations approach the observed values with a degree of exactness which appears to be quite convincing as to the general truth of his results; especially taking into account how entirely unlimited is his original hypothesis. It is one of the most curious results of this investigation that according to the most simple meaning which we can give to the term 'pole,' the earth has only two magnetic poles; that is, two points where the direction of the magnetic force is vertical. And thus the isogonal curves may be looked upon as deformations of the curves deduced by Euler from the supposition of two poles, the deformation arising from this, that the earth does not contain a single definite magnet, but irregularly diffused magnetical elements, which still have collectively a distant resemblance to a single magnet. And instead of Hansteen's Siberian pole, we have a Siberian region in which the needles converge; but if the apparent convergence be pursued it nowhere comes to a point; and the like is the case in the Antarctic region. When the 24 Gaussian elements at any time are known the magnetic condition of the globe is known, just as the mechanical condition of the solar system is known, when we know the elements of the orbits of the satellites and planets and the mass of each. And the comparison of this magnetic condition of the globe at distant periods of time cannot fail to supply materials for future researches and speculations with regard to the agencies by which the condi |