[footnote] *I have described, in Lametherie's 'Journal de Physique', 1804, t. lix., p. 449, the application (alluded to in the text) of the magnetic inclination to the determination of latitude along a coast running north and south, and which, like that of Chili and Peru, is for a part of the year enveloped in mist ('garua'). In the locality I have just mentioned, this application is of the greater importance, because, in consequence of the strong current running northward as far as to Cape Parena, navigators incur a great loss of time if they approach the coast to the north of the haven they are seeking. In the South Sea, from Callao de Lima harbor to Truxillo, which differ from each other in latitude by 3 degrees 57' I have observed a variation of the magnetic inclination amounting to 9 degrees (centesimal division); and from Callao to Guayaquil, which differ in latitude by 9 degrees 50', a variation of 23.5 degrees. (See my 'Relat. Hist.', t. iii., p. 622.) At Guarmey (10 degrees 4' south lat.), Huaura (11 degrees 3' south lat.), and Chancay (11 degrees 4' south lat.), Huaura (11 degrees 3' south lat.), and Chancay (11 degrees 32' south lat.), the inclinations are 6.80 degrees, 9 degrees, and 10.35 degrees of the centesimal division. The determination of position by means of the magnetic inclination has this remarkable feature connected with it, that where the ship's course cuts the isoclinalline almost perpendicularly, it is the only one that is independent of all determination of time, and consequently, of observations of the sun or stars. It is only lately that I discovered, for the first time, that as early as at the close of the sixteenth century, and consequently hardly twenty years after Robert Norman had invented the inclinatorium, William Gilbert, in his great work, 'De Magnete', proposed to determine the latitude by the inclination of the magnetic needle. Gilbert ('Physiologia Nova de Magnete', lib. v., cap. 8, p. 200) commends the method as applicable "aëre caliginoso." Edward Wright, in the introduction which he added to his master's great work, describes this proposal as "worth much gold." As he fell into the same error with Gilbert, of presuming that the isoclinal lines coincided with the geographical parallel circles, and that the magnetic and geographical equators were identical, he did not perceive that the proposed method had only a local and very limited application.
p 179 When the needle, by its sudden disturbance in its horary course, indicates the presence of a magnetic storm, we are still unfortunately ignorant whether the seat of the disturbing cause is to be sought in the Earth itself or in the upper regions of the atmosphere. If we regard the Earth as a true magnet, we are obliged, according to the views entertained by Friedrich Gauss (the acute propounder of a generaltheory of terrestrial magnetism), to ascribe to every portion of the globe measuring one eighth of a cubic meter (or 3 7/10ths of a French cubic foot) in volume, an average amount of magnetism equal to that contained in a magnetic rod of 1 lb. weight.*
[footnote[ *Gauss and Weber, 'Resultate des Magnet. Vereins', 1838, 31, s. 146.
If iron and nickel, and probably, also, cobalt (but not chrome, as has long been believed),* are the only substances which become permanently magnetic, and retain polarity from a certain coerceive force, the phenomena of Arago's magnetism of rotation and of Faraday's induced currents show, on the other hand, that all telluric substances may possibly be made transitorily magnetic.
According to Faraday ('London and Edinburgh Philosophical Magazine', 1836, vol. viii., p. 178), pure cobalt is totally devoid of magnetic power. I know, however, that other celebrated chemists (Heinrich Rose and Wohler) do not admit this as absolutely certain. If out of two carefully-purified masses of cobalt totally free from nickel, one appears altogether non-magnetic (in a state of equilibrium), I think it probable that the other owes its magnetic property to a want of purity; and this opinion coincides with Faraday's view.
According to the experiments of the p 180 first-mentioned of these great physicists, water, ice, glass, and carbon affect the vibrations of the needle entirely in the same manner as mercury in the rotation experiments.*
[footnote] *Arago, in the 'Annales de Chimie', t. xxxii., p. 214; Brewster, 'Treaties on Magnetism', 1837, p. 111; Baumgartner, in the 'Zeitschrift fur Phys. und Mathem.', bd. ii., s. 419.
Almost all substances show themselves to be, in a certain degree, magnetic when they are conductors, that is to say, when a current of electricity is passing through them.
Although the knowledge of the attracting power of native iron magnets or loadstones appears to be of very ancient date among the nations of the West, there is strong historical evidence in proof of the striking fact that the knowledge of the directive power of a magnetic needle and of its relation to terrestrial magnetism was peculiar to the Chinese, a people living in the extremest eastern portions of Asia. More than a thousand years before our era, in the obscure age of Codrus, and about the time of the return of the Heraclidae to the Peloponnesus, the Chinese had already magnetic carriages, on which the movable arm of the figure of a man continually pointed to the south, as a guide by which to find the way across the boundless grass plains of Tartary; nay, even in the third century of our era, therefore at least 700 years before the use of the mariner's compass in European seas, Chinese vessels navigated the Indian Ocean* under the direction of magnetic needles pointing to the south.
[footnote] *Humboldt, 'Examen Critique de l'Hist. de la Geographie', t. iii., p. 36.