322 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



solidified but also cooled to such an extent as to be capable of 

 being magnetised to the most powerful degree. On the contrary, 

 we must suppose them to be still very considerably heated, and 

 consequently to possess but feeble magnetic properties. Accord- 

 ing to Humboldt, " 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." * The feeble magnetic power of the metallic 

 globe would however be amply compensated for by its enormous 

 size. 



The possibility of the existence of such a metallic centre 

 having been once admitted, the field opened for further reasoning 

 as to the influence which cosmical bodies may exert upon its 

 position, and consequently upon that of the earth's centre of 

 gravity, is very wide indeed. That these changes may affect the 

 phenomena of volcanic eruptions, I shall endeavour to shew in 

 Part II. of this paper. In the meantime it may be remarked 

 that there exists a decided connection between magnetic, and 

 volcanic phenomena. In the year 1767, Bernouilli observed 

 that during an earthquake the inclination decreased half a degree, 

 and Father de la Torre remarked that during an eruption of 

 Vesuvius the declination varied several degrees. On the 18th 

 April, 1842, at ten minutes past nine, Kreil in Prague observed 

 that the needle received a very sudden stroke, and the same 

 oscillation in the same direction was observed at the same instant 

 by Cella in Parma, and Lamont in Munich. Shortly afterwards 

 it was ascertained that exactly at the same minute a violent earth- 

 quake had been felt in Qreece.f From the irregularities in the 

 course of the magnetic curves, Lamont regards it as in the high- 

 est degree probable that the seat of terrestrial magnetism is to be 

 sought in a compact nucleus which lies under the earth's crust* 

 Miiller is of opinion that the magnetic variations and oscillations 

 can be most simply explained by considering terrestrial magnet- 

 ism as dependent on electric currents which pass through the 

 nucleus in ever varying direction and intensity. J 



That the magnetic variations stand in connection with the 

 movements of certain of the heavenly bodies is a well ascertained 

 /fact. Sabine came to the conclusion that the disturbances belong 



* Humboldt's Cosmos, English Edition, I, 183. 

 t Mailer's Kosmische Pbysik, p. 497. 

 t Ibid, p. 498. 



