TERRESTRIAL MAGNETISM. 177 



from the surface to the center, and is of opinion that all heat 

 has penetrated from without inward, and that the tempera- 

 ture of the globe depends upon the very high or very low 

 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 phys- 

 icists or geologists, or scarcely, indeed, any one besides its au- 

 thor. But, whatever may be the cause of the internal heat 

 of our planet, and of its limited or unUmited 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. Ter- 

 restrial magnetism, whose main character, expressed in the 

 three-fold manifestation of its forces, is incessant periodic va- 

 riabihty, 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.! 



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 



* William Gilbert, of Colchester, whom Galileo pronounced " great 

 vo a degree that might be envied," said " magnus magnes ipse est globus 

 terrestris." He ridicules the magnetic mountains of Frascatori, the great 

 cotemporary 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 given point on the surface of the Earth 

 to be invariable (variatio uniuscujusque loci constans est), and refers 

 the curvatures of the isogenic lines to the configuration of continents 

 and the relative positions of sea basins, which possess a weaker mag 

 netic force than the solid masses rising above the ocean. (Gilbert, de 

 Magnets, ed. 1G33, p. 42, 98, 152, and 155.) 



t Gauss, Allgemeine Theorie des Erdmagnetismus, in the Resultate aui 

 dfiti Beob. des Magnet. Vereins, 1838, s. 41, p. 56. 



H 2 



