[footnote] *Boussingault, 'Sur la Profondeus a laquelle se trouve la Couche de Temperature invariable, entre les Tropiques', in the 'Annales de Chimie et de Physique', t. liii., 1833, p. 225-247.
This mean temperature of the air at a fixed point, or at a group of contiguous points on the surface, is to a certain degree the fundamental element of the climate and agricultural relations of a district; but the mean temperature of the whole surface is very different from that of the globe itself. The questions so often agitated, whether the mean temperature has experienced any considerable differences in the course of centuries, whether the climate of a country has deteriorated, and whether the winters have not become milder and the summers cooler, can only be answered by means of the thermometer; this instrument has, however, scarcely been invented more than two centuries and a half, and its scientific application hardly dates back 120 years. The nature and novelty of the means interpose, therefore, very narrow limits to our investigation regarding the temperature p 176 of the air. It is quite otherwise, however, with the solution of the great problem of the internal heat of the whole Earth. As we may judge of uniformity of temperature from the unaltered time of vibration of a pendulum, so we may also learn, from the unaltered rotatory velocity of the Earth, the amount of stability in the mean temperature of our globe. This insight into the relations between the 'length of the day' and the 'heat of the Earth' is the result of one of the most brilliant applications of the knowledge we had long possessed of the planet. The rotatory velocity of the Earth depends on its volume; and since, by the gradual cooling of the mass by radiation, the axis of rotation would become shorter, the rotatory velocity would necessarily increase, and the length of the day diminish, with a decrease of the temperature. From the comparison of the secular inequalities in the motions of the Moon with the eclipses observed in ancient times, it follows that, since the time of Hipparchus, that is, for full 2000 years, the length of the day has certainly not diminished by the hundredth part of a second. The decrease of the mean heat of the globe during a period of 2000 years has not, therefore, taking the extremest limits, diminished as much as 1/306th of a degree of Fahrenheit.*
[footnote] *Laplace, 'Exp. du Syst. du Monde', p. 229 and 263; 'Mecanique Celeste', t. v., p. 18 and 72. It should be remarked that the fraction 1/306th of a degree of Fahrenheit of the mercurial thermometer, given in the text as the limit of the stability of the Earth's temperature since the days of Hipparchus, rests on the assumption that the dilation of the substances of which the Earth is composed is equal to that of glass, that is to say, 1/18,000th for 1 degree. Regarding this hypothesis, see Arago in the 'Annuaire' for 1834, p. 177-190.
This invariability of form presupposes also a great invariability in the distribution of relations of density in the interior of the globe. The translatory movements, which occasion the eruptions of our present volcanoes and of ferruginous lava, and the filling up of previously empty fissures and cavities with dense masses of stone, are consequently only to be regarded as slight superficial phenomena affecting merely one portion of the Earth's crust, which, from their smallness when compared to the Earth's radius, become wholly insignificant.
I have described the internal heat of our planet, both with reference to its cause and distribution, almost solely from the results of Fourier's admirable investigations. Poisson doubts the fact of the uninterrupted increase of the Earth's heat p 177 from the surface to the center, and is of opinion that all heat has penetrated from without inward, and that the temperature of the globe depends upon the very high or very low temperature of the regions of space through which the solar temperature of the regions of space, through which the solar system has moved. This hypothesis, imagined by one of the most acute mathematicians of our time, has not satisfied physicists or geologists, or scarcely indeed any one besides its author. But, whatever may be the cause of the internal heat of our planet, and of its limited or unlimited increase in deep strata, it leads us, in this general sketch of nature, through the intimate connection of all primitive phenomena of matter, and through the common bond by which molecular forces are united, into the mysterious domain of magnetism. Changes of temperature call forth magnetic and electric currents. Terrestrial magnetism, whose main character, expressed in the three-fold manifestation of its forces, is incessant periodic variability, is ascribed either to the heated mass of the Earth itself,* or to those galvanic currents which we consider as electricity in motion, that is, electricity moving in a closed circuit.**
[footnote] *William Gilbert, of Colchester, whom Galileo pronounced "great to a degree that might be envied," said "magnus magnes ipse est globus terrestris." He ridicules the magnetic mountains of Frascatori, the great contemporary of Columbus, as being magnetic poles: "rejicienda est vulgaris opinio de montibus magneticis, aut rupe aliqua magnetica, aut polo phantastico a polo mundi distante." He assumes the declination of the magnetic needle at any give point on the surface of the Earth to be invariable (variatio uniuscujusque loci constans est), and refers the curvatures of the isogonic lines to the configuration of continents and the relative positions of sea basins, which possess a weaker magnetic force than the solid masses rising above the ocean. (Gilbert, 'de Magnete', ed. 1633, p. 42, 98, 152 and 155.)
[footnote] ** Gauss, 'Allgemcine Theorie des Erdmagnetismus', in the 'Resultate aux den Beob. des Magnet. Vereins', 1838, s. 41, p. 56.
The mysterious course of the magnetic needle is equally affected by time and space, by the sun's course, and by changes of place on the Earth's surface. Between the tropics, the hour of the day may be known by the direction of the needle as well as by the oscillations of the barometer. It is affected instantly, but only transiently, by the distant northern light as it shoots from the pole, flashing in beams of colored light across the heavens. When the uniform horary motion of the needle is disturbed by a magnetic storm, the perturbation manifests itself 'simultaneously', in the strictest sense of the word, over hundreds and thousands of miles of sea and land, or propagates itself by degrees, in short intervals of time, in p 178 every direction over the Earth's surface.*
[footnote] *There are also perturbations which are of a local character, and do not extend themselves far, and are probably less deep-seated. Some years ago I described a rare instance of this kind, in which an extraordinary disturbance was felt in the mines at Freiberg, but was not perceptible at Berlin. ('Lettre de M. de Humboldt a Son Altesse Royale le Duc de Sussex sur les moyens propres a perfectionner la Connaissance du Magnetisme Terrestre', in Becquerel's 'Traite Experimental de l'Electricite' t. vii., p. 442.) Magnetic storms which were simultaneously felt from Sicily to Upsala, did not extend from Upsala to Alten. (Gauss and Weber, 'Resultate des Magnet. Vereins', 1839, 128; Lloyd, in the 'Comptes Rendus de l'Acad. des Sciences', t. xii., 1843, Sem. ii., p. 725 and 827.) Among the numerous examples that have been recently observed, of perturbations occurring simultaneously and extending over wide portions of the Earth's surface, and which are collected in Sabine's important work ('Observ. on Days of unusual Magnetic Disturbance', 1843), one of the most remarkable is that of the 25th of September, 1841, which was observed at Toronto in Canada, at the Cape of Good Hope, at Prague, and partially in Van Diemen's Land. The English Sunday, on which it is deemed sinful, after midnight on Saturday, to register an observation, and to follow out the great phenomena of creation in their perfect development, interrupted the observations in Van Diemen's Land, where in consequence of the difference of the longitude, the magnetic storm fell on the Sunday. ('Observ.', p. xiv., 78, 85, and 87.)
In the former case, the simultaneous manifestation of the storm may serve, within certain limitations, like Jupiter's satellites, fire-signals, and well-observed falls of shooting stars, for the geographical determination of degrees of longitude. We here recognize with astonishment that the perturbations of two small magnetic needles, even if suspended at great depths below the surface, can measure the distances apart at which they are placed, teaching us, for instance, how far Kasan is situated east of Gottingen or of the banks of the Seine. There are also districts in the earth where the mariner, who has been enveloped for many days in mist, without seeing either the sun or stars, and deprived of all means of determining the time, may know with certainty, from the variations in the inclination of the magnetic needle, whether he is at the north or the south of the port he is desirous of entering.*