28 



MAGNETISM. 



and the compass traverses slowly. The 

 absolute intensify of the terrestrial force 

 is, indeed, greater in the latter case than 

 in the former ; but the increase is not 

 sufficient to compensate for the greater 

 obliquity of its action. If we could 

 place ourselves exactly over the north 

 or south magnetic pole of the earth, 

 the dipping-needle would take a vertical 

 position, and the horizontal compass 

 would no longer be sensible to the in- 

 fluence of terrestrial magnetism, but 

 would remain at rest in any position in 

 which it might happen to be placed. 



(112.) All these consequences of the 

 hypothesis which ascribes terrestrial 

 magnetism to the influence of a magnetic 

 power in the central regions of the earth, 

 and of which the direction nearly coin- 

 cides with its axis of rotation, may be 

 experimentally illustrated by placing a 

 strong magnet in the centre of an arti- 

 ficial globe. The points on the surface 

 which are opposite to the poles of the 

 magnet, are to be marked as the terres- 

 trial magnetic poles. A great circle 

 being traced equidistant from these 

 poles, will be the magnetic equator, 

 dividing the globe into the northern and 

 southern magnetic hemispheres. Great 

 circles passing through the poles, and 

 crossing the equator at right angles, 

 will be magnetic meridians ; of which 

 the one which also passes through the 

 poles of the earth's rotation will t>e the 

 lines of no variation. Smaller circles 

 parallel to the magnetic equator, will 

 indicate situations when the dip is the 

 same in all. The lines of equal varia- 

 tion will be curves of particular forms 

 less easily determinable. The accord- 

 ance of fact with theory may now be 

 verified by placing in different situations, 

 on the surface of a globe so prepared, a 

 small needle suspended as freely as pos- 

 sible by a fine thread, which holds it 

 balanced as nearly as possible at its 

 centre of gravity, and observing the 

 positions it assumes in each situation. 



(1 13.) But when we come to compare 

 the regular lines thus traced from theory, 

 on the supposition of a single central 

 magnet, with the lines which obser- 

 vation points out as those indicating 

 the actual variations of the magnetism 

 of the earth, we meet with very remark- 

 able discordances. Many have been the 

 attempts made to explain the irregula- 

 rities and anomalies in the course of the 

 magnetic lines by suppositions of various 

 kinds. There is reason for believing 

 that the northern and the southern 



magnetic poles do not occupy points on 

 the globe diametrically opposite to each 

 other, which would be the case if the 

 magnetic influence emanated from the 

 centre of the earth. It has been sup- 

 posed, in consequence, that the terres- 

 trial magnet, or centre of magnetic 

 force, was eccentric. But this suppo- 

 sition alone will not suffice ; for there 

 are various indications of the influence 

 of more than one pole in each hemi- 

 sphere of the earth ; and the probability 

 is that these poles are of very unequal 

 intensities. Other irregularities exist 

 which appear to owe their existence to 

 the influence of causes entirely local and 

 of limited extent, such as might be sup- 

 posed to be derived from large masses 

 of iron situated at different depths be- 

 neath the surface of the earth. 



(114.) The observations best calcu- 

 lated to decide the important question 

 of the existence of secondary magnetic 

 poles, appear to be those of the varia- 

 tions of magnetic intensity, from which 

 we derive the knowledge of the isodyna- 

 mic lines already adverted to ( 102) ; for 

 these lines will necessarily arrange them- 

 selves in regular order around the point 

 or points in each hemisphere when the in- 

 tensity is greatest, that is around each 

 respective pole. If these poles were 

 single, and placed opposite to each other 

 in the globe, one in the northern and the 

 other in the southern hemisphere, the 

 lines of equal intensity would form pa- 

 rallel circles, analogous to those of geo- 

 graphic latitude. Captain Sabine re- 

 marks * that the observations on this 

 subject, made previously to those of Pro- 

 fessor Hansteen, appeared to corrobo- 

 rate such an hypothesis ; for, although 

 they extended widely over the magnetic 

 parallels in the northern hemisphere, 

 namely, from the least almost to the 

 greatest intensity, yet they were confined, 

 in respect to longitude, to a space little 

 more than a quarter of a hemisphere ; 

 and to that quarter which is immedi- 

 ately opposite to the countries visited by 

 Professor Hansteen. Within the space 

 that had been thus examined, the isody- 

 namic curves appeared to arrange them- 

 selves with comparative insignificant 

 deviations, in parallel circles around a 

 point situated in the north-eastern part 

 of Hudson's Bay, and, as nearly as could 

 be judged, about the intersection of the 

 sixtieth degree of geographical latitude 



t * Quarterly Journal of Science, Sept. 1829, p. 3. 



