William Gilbert, a physician of Colchester, first showed in 1600 that the earth as a whole has the properties of a magnet, and consequently that the directive action exerted by it upon a compass needle represents only a special case of the mutual action of two magnets. In 1845, Faraday established the fact that susceptibility to magnetic force is not, as was generally believed, confined to iron, nickel, and a few other substances, but is a property of all substances. According to Balfour Stewart, auroræ and earth currents may be regarded as secondary currents resulting from changes in the earth’s magnetism. Magnetic phenomena are included under the general term terrestrial magnetic elements, and consist of magnetic declination, inclination, and intensity. These are for convenience determined separately; the first by an instrument called a _Declinometer_, and the second by an _Inclinometer_ or _Dipping Needle_. The Declinometer is also made to serve the additional purpose of measuring the _intensity_ of the earth’s magnetic force, which it effects on a principle similar to that by which the force of gravity is determined by the oscillations of a pendulum of known length on any given portion of the earth’s surface. The declinometer needle is made to oscillate, and the number of oscillations in a given time counted; due allowance being made for the strength of the needle, it is obvious that the force which restores the needle to rest can be estimated. To ascertain the angle of _declination_, the zero line of the compass card is made to coincide with the geographical north and south line; and the angle which the direction of the needle makes with this line is then read off on a graduated circle over which the needle turns. The magnetic _inclination_ or _dip of the needle_ is estimated by observing the inclination to a horizontal plane of a needle turning on the vertical plane which passes through the magnetic north and south points. [Illustration: 62.] Fig. 62 shows a simple form of magnetic needle suspended on a fine steel point, which is supported by a brass stand; the addition of a graduated circle would constitute such an arrangement a Declinometer. [Illustration: 63.] [Illustration: 64.] Fig. 63 gives the appearance of the dipping needle, or Inclinometer, and Fig. 64 an arrangement by which both kinds of terrestrial as well as local attraction may be shown. These components of the earth’s magnetism undergo not only an annual but a daily and even hourly variation, apparently connected in some occult manner with the frequency of the sun’s spots. The needle sometimes suffers such exceptional perturbations as to suggest the idea of a magnetic storm. These disturbances are usually accompanied (in polar regions) by luminous phenomena called auroræ. Continuous automatic records of them, therefore, is of great value, as facilitating inductive research which may lead to valuable practical results.