AURORA BOREALIS. 193 



trial 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 oi the 

 meteorological processes of the atmosphere, conveniently turn 

 aside the practical bearing of all phenomena that can not 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 cormection with the terrestrial or polar light, as well 

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

 nagnetic poles may be considered as the poles of cold.f The 

 oold conjecture hazarded one hundred and twenty-eight years 

 since by Halley,| that the Aurora Borealis was a magnetic 

 phenomenon, has acquired empirical certainty from Faraday's 

 brilliant discovery of the evolution of light by magnetic forces. 

 The northern light is preceded by premonitory signs. Thus, 

 in the morning before the occurrence of the phenomenon, the 

 irregular horary course of the magnetic needle generally indi- 

 cates a disturbance of the equilibrium in the distribution of 



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

 of matter from a vapor-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 oxydizing external crust. 

 " It can not be doubted," he observes in his masterly VMorie des Pheno- 

 menes Electro-dynaviiques, 1826, p. 199, "that electro-magnetic cur- 

 rents exist in the interior of the globe, and that tlie&s cun-ents are the 

 cause of its temperature. They arise from the action of a central me- 

 tallic nucleus, composed of the metals discovered by Sir Humphrey 

 Davy, acting on the surrounding oxydized layer." 



t The remai'kable 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, p. 42, 44, 47, and 

 268. This distinguished physicist admits two cold poles (poles of maxi- 

 mum 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 merid- 

 ians, i. c, meridians of greatest heat and cold. Even in the sixteenth 

 century, Acosta {Historia Nattiral de las Indias, 1589, lib. i., cap. 17), 

 grounding his opinion on the observations of a very experienced Portu- 

 guese pilot, taught that there were four lines without declination. It 

 would seem from the controversy of Henry Bond (the author of The 

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

 ure influenced Halley in his theory of four magnetic poles. See my 

 Examen Critiqiie dc V Hist, de la Oeographie, t. iii., p. 60. 



X Halley, in the Philosophical Transactions, vol. xxix. (for 1714-1716), 

 No. 341. 



Vol. T.— r 



