of the conductor, are the causes, I conceive, which lead him to the conclusion that Induction takes place in curved lines of such contiguous particles. With reference to this, I may remark that, as I have said, the distribution of electricity on a conductor in the presence of an electrized body is so complex a mathematical problem that I do not conceive any merely popular way of regarding the result can entitle us to say, that the distribution which we find cannot be explained by the Coulombian theory, and must force us upon the assumption of an action in curved lines-which is, indeed, itself a theory, and so vague a one that it requires to be made much more precise before we can say what consequences it does or does not lead to. Professor W. Thomson has arrived at a mathematical proof that the effect of induction on the view of Coulomb and of Faraday must, under certain conditions, be necessarily and universally the same. With regard to the influence of different Dielectrics upon Induction, the inquiry appears to be of the highest. importance; and may certainly necessitate some addition to the theory. 1 Researches, 1165, &c. BOOK XII. MAGNETISM. Recent Progress of Terrestrial Magnetism. IN Chapter II., I have noticed the history of Ter restrial Magnetism; Hansteen's map published in 1819; the discovery of magnetic storms' about 1825; the chain of associated magnetic observations, suggested by M. de Humboldt, and promoted by the British Association and the Royal Society; the demand for the continuation of these till 1848; the magnetic observations made in several voyages; the magnetic surveys of various countries. And I have spoken also of Gauss's theory of Terrestrial Magnetism, and his directions and requirements concerning the observations to be made. I may add a few words with regard to the more recent progress of the subject. The magnetic observations made over large portions of the Earth's surface by various persons, and on the Ocean by British officers, have been transmitted to Woolwich, where they have been employed by General Sabine in constructing magnetic maps of the Earth for the year 1840. Following the course of inquiry described in the part of the history referred to, these maps exhibit the declination, inclination, and intensity of the magnetic force at every point of the earth's surface. The curves which mark equal amounts of each of these three elements (the lines of equal declination, inclination, and force: the isogonal, the isoclinal, and the isodynamic lines,) are, in their general form, complex and irregular; and it has been made a matter of question (the facts being agreed upon) whether it be more proper to say that they indicate four poles, as Halley and as Hansteen said, or only two poles, as Gauss asserts. The matter appears 1 These maps are published in Mr. Keith Johnstone's Physical Atlas. to become more clear if we draw magnetic meridians; that is, lines obtained by following the directions, or pointings, of the magnetic needle to the north or to the south, till we arrive at their points of convergence of all their directions; for there are only two such poles, one in the Arctic and one in the Antarctic region. But in consequence of the irregularity of the magnetic constitution of the earth, if we follow the inclination or the magnetic force round the earth on any parallel of latitude, we find that it has two maxima and two minima, as if there were four magnetic poles. The isodynamic map is a new presentation of the facts of this subject; the first having been constructed by Colonel Sabine in 1837. I have stated also that the magnetic elements at each place are to be observed in such a manner as to bring into view both their periodical, their secular, and their irregular or occasional changes. The observations made at Toronto in Canada, and at Hobart Town in Van Diemen's Land, two stations at equal distances from the two poles of the earth, and also at St. Helena, a station within the tropics, have been discussed by General Sabine with great care, and with an amount of labour approaching to that employed upon reductions of astronomical observations. And the results have been curious and unexpected. This mag The declination was first examined.2 netical element is, as we have already seen (p. 53), liable both to a diurnal and to an annual inequality; and also to irregular perturbations which have been termed magnetic storms. Now it was found that all these inequalities went on increasing gradually and steadily from 1843 to 1848, so as to become, at the end of that time, above twice as large as they were at the beginning of it. A new periodical change in all these elements appeared to be clearly established by this examination. M. Lamont, of Munich, had already remarked indications of a decennial period in the diurnal variation of the declination of the needle. Phil. Trans. 1852 and 1856. The duration of the period from minimum to maximum being about five years, and therefore the whole period about ten years. The same conclusion was found to follow still more decidedly from the observations of the dip and intensity. This period of ten years had no familiar meaning in astronomy; and if none such had been found for it, its occurrence as a magnetic period must have been regarded, as General Sabine says,3 in the light of a fragmentary fact. But it happened about this time that the scientific world was made aware of the existence of a like period in a phenomenon which no one would have guessed to be connected with terrestrial magnetism, namely, the spots in the Sun. M. Schwabe, of Dessau, had observed the Sun's disk with immense perseverance for 24 years:-often examining it more than 300 days in the year; and had found that the spots had, as to their quantity and frequency, a periodical character. The years of maximum are 1828, 1838, 1848, in which there were respectively 225, 282, 330 groups of spots. The minimum years, 1833, 1843, had only 33 and 34 such groups. This curious fact was first made public by M. de Humboldt, in the third volume of his Kosmos (1850). The coincidence of the periods and epochs of these two classes of facts was pointed out by General Sabine in a Memoir presented to the Royal Society in March, 1852. Of course it was natural to suppose, even before this discovery, that the diurnal and annual inequalities of the magnetic element at each place depend upon the action of the sun, in some way or other. Dr. Faraday had endeavoured to point out how the effect of the solar heat upon the atmosphere would, according to the known relations of heat and magnetism, explain many of the phenomena. But this new feature of the phenomena, their quinquennial increase 3 Phil. Trans. 1856, p. 382. 4 In 1837 there were 3335 The observations up to 1844 were published in Poggendorfs Annalen. and decrease, makes us doubt whether such an explanation can really be the true one. Of the secular changes in the magnetic elements, not much more is known than was known some years ago. These changes go on, but their laws are imperfectly known, and their causes not even conjectured. M. Hansteen, in a recent memoir,6 says that the decrease of the inclination goes on progressively diminishing. With us this rate of decrease appears to be at present nearly uniform. We cannot help conjecturing that the sun, which has so plain a connexion with the diurnal, annual, and occasional movements of the needle, must also have some connexion with its secular movements. In 1840 the observations made at various places had to a great extent enabled Gauss, in connexion with W. Weber, to apply his Theory to the actual condition of the Earth;7 and he calculated the Declination, Inclination, and Intensity at above 100 places, and found the agreement, as he says, far beyond his hopes. They show, he says, that the Theory comes near to the Truth. Correction of Ships' Compasses. The The magnetic needle had become of importance when it was found that it always pointed to the North. Since that time the history of magnetism has had its events reflected in the history of navigation. The change of the declination arising from a change of place terrified the companions of Columbus. determination of the laws of this change was the object of the voyage of Halley; and has been pursued with the utmost energy in the Arctic and Antarctic regions by navigators up to the present time. bably the dependence of the magnetic declination upon place is now known well enough for the purposes of navigation. But a new source of difficulty has in the 6 See K. Johnstone's Physical Atlas. Pro 7 Atlas des Erdmagnetismus nach den Elementen der Theorie Entworfen. See Preface. |