A.CTRORA BOKEAL1S. 187 



acquaintance ^ith the genetic conditions of magnetic pheno- 

 mena. As yet we can only boast of having opened a greater 

 number of paths which may possibly lead to an explana- 

 tion of this subject. In the physical science of terrestrial 

 magnetism, which must not be confounded with the purely 

 mathematical branch of the study, those persons only will 

 obtain perfect satisfaction who, as in the science of the 

 meteorological processes of the atmosphere, conveniently turn 

 aside the practical bearing of all phenomena that cannot be 

 explained according to their own views. 



Terrestrial magnetism, and the electro-dynamic forces com- 

 puted by the intellectual Ampere,* stand in simultaneous and 

 intimate connection with the terrestrial or polar light, as well 

 as with the internal and external heat of our planet, whose 

 magnetic poles may be considered as the poles of cold.| 

 The bold conjecture hazarded 128 years since by Halley,J 



* Instead of ascribing the internal heat of the earth to the transition 

 of matter from a vapour-like fluid to a solid condition, which accom- 

 panies the formation of the planets, Ampere has propounded the idea, 

 which I regard as highly improbable, that the earth's temperature may 

 be the consequence of the continuous chemical action of a nucleus of 

 the metals of the earths and alkalies on the oxidising external crust 

 " It cannot be doubted," he observes in his masterly Theorie ties Pheno- 

 mines Electro-dynamiques, 1826, p. 199, " that electro-magnetic currents 

 exist in the interior of the globe, and that these currents are the cause 

 of its temperature. They arise from the action of a central metallic 

 nucleus, composed of the metals discovered by Sir Humphrey Davy, 

 acting on the surrounding oxidised layer." 



t The remarkable connection between the curvature of the magnetic lines 

 and that of my isothermal lines, was first detected by Sir David Brewster. 

 See the Transactions of the Royal Society of Edinburgh, vol. ix. 1821, 

 p. 318, and Treatise on Magnetism, 1837, pp. 42, 44, 47, and 268. 

 This distinguished physicist admits two coid poles (poles of maximum 

 cold) in the northern hemisphere, an American one near Cape Walker 

 (73 lat., 100 W. long.), and an Asiatic one (73 lat., 80 E. long.) , 

 whence arise, according to him, two hot and two cold meridians, i. e., 

 meridians of greatest heat and cold. Even in the 16th century, Acosta 

 (Historia Natural de las Indias, 1589, lib. i. cap. 17), grounding his 

 opinion on the observations of a very experienced Portuguese pilot, 

 taught that there were four lines without declination. It would seem 

 from the controversy of Henry Bond (the author of The Longitudt 

 Found, 1676) with Beckborrow, that this view in some measure in- 

 fluenced Halley in his theory of four magnetic poles. See my Examen 

 Critique de fHist. de la Geographic, t. iii. p. 60. 



Halley, in the Philosophical Transactions, vol. xxix. (for 1714- 

 1716;, No. 341 



