MAGNETISM. 



55 



CTease("''), the mean temperature of the body 

 of the earth is discovered not to have altered, 

 in the course of 2000 years, by the y|^th part 

 of a thermometrical degree. 



This invariableness of form farther implies 

 great invariability in the distribution of density 

 in the interior of the earth. The translatory 

 movements effected by the erfiptions of our 

 present volcanoes, the outbursts of ferruginous 

 lavas, and the filling up of empty chasms and 

 hollows with dense masses of rock, are there- 

 fore to be regarded as mere superficial phe- 

 nomena, as peculiarities of parts of the earth's 

 crust, which, in point of magnitude, when con- 

 trasted with the semidiameter of the earth, are 

 utterly insignificant. 



The internal heat of the planet, in its course 

 and distribution, I have described almost ex- 

 clusively from the results and beautiful experi- 

 ments of Fourier. Poisson, however, doubts 

 the uninterrupted increase of the terrestrial 

 heat from the surface to the centre. He be- 

 lieves that all the heat has penetrated from 

 without inwards, and that the temperature of 

 the interior of the earth depends on the very 

 high or very low temperature of the universal 

 space through which the solar system has mo- 

 ved. This hypothesis, devised by one of the 

 most profound mathematicians of the age, has 

 satisfied himself only ; it has met with little 

 countenance from other natural philosophers 

 and geologists. 



But whatever be the cause of the internal 

 temperature of our planet, and of its limited or 

 unlimited increase in the deeper strata, it still 

 leads in this Essay to present a general picture 

 of nature, through the intimate connection of 

 all the primary phenomena of matter, and 

 through the common bond which surrounds 

 the molecular forces, into the obscure domain 

 of Magnetism. Changes of temperature elicit 

 magnetical and electrical currents. Terres- 

 trial magnetism, whose principal character in 

 the threefold manifestation of its force is an 

 uninterrupted periodic changeableness, is EJscri- 

 bed either to the unequally heated mass of the 

 earth itself(^^'), or to those galvanic currents 

 which we consider as electricity in motion, as 

 electricity in a circuit returning into itself("=). 

 The mysterious march of the magnetic needle 

 is equally influenced by the course of the sun, 

 and change of place upon the earth's surface. 

 The hour of the day can be told between the 

 tropics by the motion of the needle, as well as 

 by the oscillations of the mercury in the barom- 

 eter. It is suddenly, though only passingly, 

 affected by the remote Aurora, by the glow of 

 heaven, which emanates in colours at one of 

 the poles. When the tranquil hourly motion 

 of the needle is disturbed by a magnetical 

 storm, the perturbation frequently proclaims 

 itself over hundreds and thousands of miles, in 

 the strictest sense of the word simultaneously, 

 or it is propagated gradually, in brief intervals 

 of time, in every direction over the surface of 

 the earth("'). In the first case the simultane- 

 ousness of the storm might serve, like the 

 ci.hpses of Jupiter's satellites, fire signals, and 

 well-observed shooting stars, within certain 

 limits, for the determination of geographical 

 longitudes. It is seen with amazement, that 



the tremblings of two small magnetic needles, 

 were they suspended deep in subterraneous ' 



space, measure the distance that intervenes 

 between them ; that they tell us how far Kasan 

 lies east from Gottingen, or from* the banks of 

 the river Seine. There are regions of the earth 

 where the seaman, enveloped for days in fog, 

 without sight of the sun or stars, without all 

 other means of ascertaining the time, can still 

 accurately determine the hour by the variation 

 of the dip of the needle, and know whether he 

 be to the north or south of the port towards 

 which he w^ould steer his courseC^*). 



If the sudden perturbation of the needle in 

 its hourly course makes known the occurrence 

 of a magnetic storm, the seat of the perturbing 

 cause — whether it be to seek in the crust of 

 the earth itself, or in the upper regions of the 

 air — remains, to our extreme regret, as yet un- 

 determined. If we regard the earth as an ac- 

 tual magnet, then are we compelled, according 

 to the decision of the deep-thinking founder of 

 a general theory of terrestrial magnetism, 

 Frederick Gauss, to admit that every eighth of 

 a cubic metre, or y^ths of a cubic foot of the 

 earth, possesses, on an average, at least as 

 much magnetism as a one-pound magnetic 

 bar(*"). If iron and nickel, and probably co- 

 balt also — not chrome, as was long sup- 

 posed("*), be the only substances which be- 

 come permanently magnetic, and retain polar- 

 ity by a certain coercive force, the phenomena 

 of Arago's rotative magnetism, and Faraday's 

 induced currents, assure us, on, the other hand, 

 that probably all terrestrial substances may 

 passingly comport themselves magnetically. 

 From the experiments of the first of the great 

 natural philosophers just mentioned, water, 

 ice(^^^), glass, and charcoal, affect the oscilla- 

 tions of the needle precisely as quicksilver does 

 in the rotatory experiments. Almost all sub- 

 stances show themselves in a certain degree 

 magnetic when they are conductors ; that is to 

 say, when they are traversed by a current of 

 electricity. 



How ancient the knowledge of the attractive 

 power of natural magnetic iron appears to have 

 been among the western nations (and this his- 

 torically well-authenticated fact is remarkable 

 enough), the knowledge of the polarity or di- 

 rective force of the magnetic needle, and its 

 connection with terrestrial magpetism, was, 

 nevertheless, confined to the extreme east of 

 Asia, to the Chinese. A thousand years and 

 more before the commencement of our era, in 

 the dark epoch of Codru and the return of the 

 Heraclidae to the Peloponnesus, the Chinese 

 had already magnetic cars, upon which the 

 moveable arm of a human figure pointed inva- 

 riably to the south, as a means of finding the 

 way through the boundless grassy plains of 

 Tartary ; in the third century, indeed, of the 

 Christian era, at least seven hundred years, 

 therefore, before the introduction of the ship's 

 compass upon European seas, Chinese craft 

 were sailing the Indian ocean under the gui- 

 dance of M.\GNETIC SOUTHERX INDICATI0N("'). 



I have shown in another work('i'), what ad- 

 vantages this method of determining topograph- 

 ical position, this early knowledge and applica- 

 tion of the magnetic needle, wholly unknown 

 in the w^st, gave the Chinese geographers over 



