TERBESTRIAL MAGNETISM. 169 



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 ^J^th of a degree of Fah- 

 renheit.* 



This invariability of form presupposes also a great invari- 

 ability 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 from the surface to the centre, and is of opinion that all 

 heat has penetrated from without inward, and that the tem- 

 perature 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 

 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. Ter- 

 restrial magnetism, whose main character expressed in the 

 threefold manifestation of its forces is incessant periodic 



* Laplace, Jfap. du Syst. du Monde, pp. 229 and 263; Mecanigue 

 ctteste, t. v., pp. 18 and 72. It should be remarked, that the fraction 

 3 'g 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 

 clays of Hipparchus, rests on the assumption that the dilatation of the 

 substances of which the Earth is composed is equal to that of glass, that 

 is to guy, Tg '-_ for 1. Regarding this hvcoth^sis see Arago, in the 

 Annuaire for 1834, pp. 177-190. 



