168 



ORIGIN OF TERRESTRIAL MAGNETISM. 



[1854. 



the Great Western of England. With reference to which, he 

 says, " I am not aware of any advantage whatever that it has. 

 It has I think several disadvantages. The first of course is the 

 additional expense of construction. It requires embankments 

 and cuttings four feet wider, in consequence of the guage. 

 * * * Their tunnels are of course necessarily increased 

 beyond what is sufficient for the narrow guage. The narrow 

 guage tunnels are twenty-four feet wide, that is six feet between 

 the rails, and four feet between the rail and the wall of the 

 tunnel; that makes twenty-four feet. Now of course to give 

 the same space between the rails, and the same space between 

 the outside rail and the wall, it requires the wide guage tunnel 

 to be four feet wider. * * * * 



( To be continued.) 



Views oil tile Origin of Terrestrial Magnetism.* 



The earliest view of terrestrial magnetism supposed the exis- 

 tence of a magnet at the earth's centre. As this does not 

 accord with the observations on declination, inclination, and 

 intensity, Tobias Meyer gave this fictitious magnet an eccentric 

 position, placing it one-seventh part of the earth's radius from 

 the centre. Hansteen imagined that there were two such mag- 

 nets, different in position and intensity. Ampere set aside these 

 unsatisfactory hypotheses by the view, derived from his discovery, 

 that the earth itself is an electro-magnet, magnetised by an 

 electric current, circulating about it from east to west, perpen- 

 dicularly to the plane of the magnetic meridian ; and that the 

 same currents give direction to the magnetic meridian, and 

 magnetise the ores of iron; the currents, being thermo-electric 

 currents, excited by the action of the sun's heat successively on 

 the different part of the earth's surface as it revolves towards the 

 east. 



A long time before the discovery of electro-magnetism, Biot 

 was occupied with this subject, and regarded the terrestrial 

 magnetism as the principal resultant of all the magnetic particles 

 disseminated in the earth. M. Gauss adops this view, as an in- 

 terpretation of the fact, without explaining it. An observation 

 which I made some years since along with one of my brothersf 

 has directed my attention to this subject. It related to the fall 

 of a cylindrical meteor whose position was sensibly in the plane 

 of the magnetic meridian. Many luminous meteors have been 

 observed in this same position or near it, if I may judge from 

 some of those described in the catalogue of Borguslawsky.J 



The special position of the meteor observed by my brother 

 and myself was not fortuitous ; it was determined by the mag- 

 netic action of the earth, an action which may be powerful in its 

 influence on meteorites consisting essentially of the magnetic 

 metals, iron, and nickel. In our view, the terrestrial magnet, the 

 earth, decomposed by its influence the normal fluid of the me- 

 teoric mass, and so gave the meteor thus polarized the direction 

 of a compass-needle. 



In generalising from this fact, and recalling the experiment of 

 Arago on the magnetism developed when a magnet acts upon a 

 turning disc, we ask whether the magnetic polarity of our planet 

 may not be due to a like cause. Considering it, as proved, 



* Silliman's Journal, correspondence of J. Nickles. 



fPoggendorff's Annalen, Ti. 1. 



X See Proceedings, Brit. Assoc., 1853, Sept. 7, Report of Col. Sabine. 



that the sun is polarized magnetically like the earth,§ the sun 

 will then be the inductor magnet, the agent which decomposes 

 the magnetic fluid of the terrestrial globe ; it will be to the 

 earth, what the earth was to the meteor. This explanation does 

 not resolve the difficulty, as it does not say whence comes the 

 magnetic polarity of the sun. It implies the intervention of a 

 magnet whose intensity is superior to that of the sun, acting on 

 this last by induction, and impressing a polarity which the sun 

 transmits to other planets of the system. It is the hypothesis 

 reversed of the central magnet, for it places in space the mag- 

 netic mass which some physicists have supposed to exist within 

 the earth. 



The real cause of the magnetic polarity of the planets, is in 

 my view the same for all, and Arago 's experiment conducts to 

 it in a straight line. It results even from the condition of their 

 existence. Each star turning around a central axis, and in de- 

 terminate curves, is influenced by the mass of these stars aud 

 their velocity at the circumference; in a word, the agent decom- 

 posing into two fluids the normal magnetism of the earth and 

 the other planets, is their rotation. A geometer examining 

 this opinion, would find, we believe, that the declination, in- 

 clination and the perturbations of the magnetic needle, are 

 explained on this hypothesis much better than on any other. 



Since my researches on circular electro-magnets and in general 

 on bodies in rotation, I have sought much for experimental de- 

 monstration of this theory, and have now the conviction that this 

 is impossible, as it is not possible for us while upon the earth to 

 remove ourselves from the action of its own magnetism. When- 

 ever a development of magnetism under the influence of rotation 

 is observed, it is common to attribute it to the inductive action 

 of the earth, rendered so striking by the experiments of Arago 

 and Mr. Barlow. 



Alongside of the different sources of magnetism mentioned in 

 Treatises on Physics, — friction, pressure, percussion, torsion, — we 

 should add rotation, a mechanical action of equal title with the 

 preceding, and whose effects, produced through a subdivision 

 like that of magnetic polarity, are found grouped at the extremities 

 of the axis in rotation ; in the same manner as the poles develope 

 at the extremities of a bar of iron when it is subjected to torsion. 



Ingenious application of Science and its Results« 



A very ingenious application of scientific principles to deter- 

 mine the point of fusion in a closed vessel, and a remarkable 

 result from high pressure on fluids, were incidently mentioned 

 by the President of the British Association in his inaugural 

 address. Experiments were instituted by Mr. Hopkins, Mr. 

 Fairburn, aud Mr. Jowle, to determine the effects of increased 

 pressure in raising the temperature of fusion. The substance 

 operated on was inclosed in a very strong metal chamber, and 

 the pressure was produced by water forced by a plunger acted 

 on by a long lever down au iron tube, three quarters of an inch 

 thick. Wax was the substance employed; and it was of course 

 essential to ascertain the exact moment that it became fluid when 

 heat was applied. As all the apparatus must necessarily be 

 opaque, the melting point could not be seen. The difficulty was 

 ingeniously surmounted in the following manner: a small magnet 

 was enclosed on the top of the wax, whilst outside the metallic 

 chamber containing it, and on the same level, a nicely-balanced 



§ Sur la chute d'ume bolide par II. N. Nickles and J. Nickles, Compt. Rend, de 

 I' Acad, xix. 1035. 



