BOOK XII.
MAGNETISM.
Recent Progress of Terrestrial Magnetism.
IN Chapter II., I have [noticed] the history of Terrestrial 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.[18] 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 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 [614] the south, till we arrive at the 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 of 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.
[18] These maps are published in Mr. Keith Johnstone’s Physical Atlas.
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 labor approaching to that employed upon reductions of astronomical observations. And the results have been curious and unexpected.
The declination was first examined.[19] This magnetical element is, as we have already seen ([p. 232]), 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. 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.
[19] Phil. Trans. 1852 and 1856.
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,[20] 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 [615] 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,[21] 282, 330 groups of spots. The minimum years, 1833, 1843, had only 33 and 34 such groups. This curious fact[22] 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.
[20] Phil. Trans. 1856, p. 382.
[21] In 1837 there were 333.
[22] The observations up to 1844 were published in Poggendorf’s Annalen.
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 endeavored 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 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,[23] 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.
[23] See K. Johnstone’s Physical Atlas.
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;[24] and he calculated the Declination, Inclination, and Intensity at above 100 places, and found [616] the agreement, as he says, far beyond his hopes. They show, he says, that the Theory comes near to the Truth.
[24] Atlas des Erdmagnetismus nach den Elementen der Theorie Entworfen. See Preface.
Correction of Ship’s Compasses.
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. The 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. Probably 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 meantime come into view; the effect of the iron in the ship upon the Compass. And this has gone on increasing as guns, cables, stays, knees, have been made of iron; then steam-engines with funnels, wheels, and screws, have been added; and finally the whole ship has been made of iron. How can the compass be trusted in such cases?
I have already [said] in the history that Mr. Barlow proposed to correct the error of the compass by placing near to the compass an iron plate, which from its proximity to the compass might counterbalance magnetically the whole effect of the ship’s iron upon the compass. This correction was not effectual, because the magnetic forces of the plate and of the ship do not change their direction and value according to the same law, with the change of position. I have further stated that Mr. Airy devised other means of correcting the error. I may add a few words on the subject; for the subject has been further examined by Mr. Airy[25] and by others.
[25] Phil. Trans. 1856.
It appears, by mathematical reasoning, that the magnetic effect of the iron in a ship may be regarded as producing two kinds of deviation which are added together;—a “polar-magnet deviation,” which changes from positive to negative as the direction of the ship’s keel, in a horizontal revolution, passes from semicircle to semicircle; and a “quadrantal deviation,” which changes from positive to negative as the keel turns from quadrant to quadrant. The latter deviation may be remedied completely by a mass of unmagnetized iron placed on a level [617] with the compass, either in the athwartship line or in the fore-and-aft line, according to circumstances. “The polar-magnet-deviation” may be corrected at any given place by a magnet or magnets, but the magnets thus applied at one place will not always correct the deviation in another magnetic latitude. For it appears that this deviation arises partly from a magnetism inherent in the materials of the ship, not changing with the change of magnetic position, and partly from the effect of terrestrial magnetism upon the ship’s iron. But the errors arising from both sources may be remedied by adjusting, at a new locality, the positions of the corrective magnets.
The inherent magnetism of the ship, of which I have spoken, may be much affected by the position in which the ship was built; and may change from time to time; for instance, by the effect of the battering of the waves, and other causes. Hence it is called by Mr. Airy “sub-permanent magnetism.”
Another method of correcting the errors of a ship’s compass has been proposed, and is used to some extent; namely, by swinging the ship round (in harbor) to all points of azimuth, and thus constructing a Table of Compass Errors for that particular ship. But to this method it is objected that the Table loses its value in a new magnetic latitude much more than the correction by magnets does; besides the inconveniences of steering a ship by a Table.