190 COSMOS. 



we must renounce all idea of a magnetic nucleus of the Earth, 

 All magnetism is certainly not lost until we arrive at a white 

 heat,* and it is manifested when iron is at a dark red heat , 

 however different, therefore, the modifications may be which 

 are excited in substances in their molecular state, and in the 

 coercive force depending upon that condition in experiments 

 of this nature, there will still remain a considerable thickness 

 of the terrestrial stratum, which might be assumed to be the 

 seat of magnetic currents. The old explanation of the horary 

 variations of declination by the progressive warming of the 

 Earth in the apparent revolution of the Sun from east to west 

 must be limited to the uppermost surface, since thermometers 

 sunk into the Earth, which are now being accurately observed 

 at so many different places, show how slowly the solar heat 

 penetrates even to the inconsiderable depth of a few feet. 

 xMoreover, the thermic condition of the surface of water, by 

 which two thirds of oftr planet is covered, is not favorable to 

 such modes of explanation, when we have reference to ap im- 

 mediate action and not to an effect of induction in the aerial 

 and aqueous investment of our terrestrial globe. 



In the present condition of our knowledge, it is impossible 

 to afford a satisfactory reply to all questions regarding the ulti- 

 mate physical causes of these phenomena. It is only with ref- 

 erence to that which presents itself in the triple manifestations 

 of the terrestrial force, as a measurable relation of space and 

 time, and as a stable element in the midst of change, that 

 science has recently made such brilliant advances by the aid 

 of the determination of mean numerical values. From To- 

 ronto in Upper Canada to the Cape of Good Hope and Van Die- 

 men's Land, from Paris to Pekin, the Earth has been covered, 

 since 1828, with magnetic observatories,! in which every regu- 



* Barlow, in the Philos. Trans, for 1822, Pt. i., p. 117 ; Sir David 

 Brewster, Treatise on Magnetism, p. 129. Long before the times of 

 Gilbert and Hooke, it was taught in the Chinese work Oio-thsa-tsou 

 that heat diminished the directive force of the magnetic needle. (Kla- 

 proth, Lettre a M. A. de Humboldt, aur V Invention de la Boussole, p. 9^.) 



t As the first demand for the establishment of these observatories ^_a 

 net-work of stations, provided with similar instruments) proceeded 

 from me, I did not dare to cherish the hop:) that I should live long 

 enough to see the time when both hemispheres should be uniformly 

 covered with magnetic houses under the associated activity of abl* 

 physicists and astronomers. This has, however, been accomplished, 

 and chiefly through the hberal and continued support of the Russian and 

 British governments. 



In the years 1806 and 1807, 1 and my friend and fellow-laborer, Herr 

 Oltmauns, while at Berlin, observed the movements of the needle, espe* 



