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.