MAGNETISM. 



25 



it can arrive at the southern situation in 

 which it has been met with in longitude 

 156.}. These inflexions will, therefore, 

 assume a figure, with relation to the 

 terrestrial equator, somewhat like that 

 represented in fig. 40, where the dotted 

 line m m m, is the magnetic, and the 

 continuous line, e e, the terrestrial 

 equator. 



Fig. 40. 



3. Variations in the Intensity of 

 Terrestrial Magnetism . 



(100.) Besides the variation and the 

 dip, which together constitute the mag- 

 netic position, and which differ much in 

 different situations, there is also a third 

 circumstance highly deserving our atten- 

 tion in connexion with this subject, 

 namely, the intensity of the force which 

 directs the needle towards this position. 

 Extensive observations of the relative in- 

 tensities of the magnetic force of the earth 

 in different parts of its surface, are of 

 greater value in enabling us to under- 

 stand the general system of terrestrial 

 magnetism than those in the dip or va- 

 riation. We know that this force varies 

 greatly in different latitudes ; but our 

 information with regard to the exact 

 amount of this variation is exceedingly 

 scanty, both from its importance not 

 having been felt, and the consequent 

 omission of the proper observations 

 with regard to it, and also from the 

 greater difficulty there is in conducting 

 the experiments which are required to 

 ascertain it. - 



(101.) The best mode of estimating 

 the comparative intensities of the mag- 

 netic action in the same needle in two 

 different places, is to count the number 

 of oscillations which it makes in a given 

 time, a minute for example, on its being 

 disturbed from its position of equili- 

 brium, while it is resuming that position. 

 The movements of the needle being re- 

 gulated by the same dynamical la\vs 

 which govern the oscillations of the pen- 

 dulum, it is a necessary consequence of 

 those laws, that the intensity of the force 

 producing the oscillations, is propor- 

 tional to the square of the number of 

 oscillations performed in a given time. 

 Mr. Graham appears to have been the 

 first who devised this method of mea- 

 suring the magnetic intensities. 



(1 02.) The first accurate observations 

 of this kind were those made by Hum- 

 boldt, and by De Rossel : who have 

 completely established the general fact, 

 that the intensity of the force of terres- 

 trial magnetism increases as we recede 

 from the equator, where it is weakest, 

 till we approach the poles : at the mag- 

 netic poles themselves, it is probably 

 greater than at any other spot. We 

 have every reason to expect that great 

 light will be thrown on this department 

 of the science from the labours of Pro- 

 fessor Hansteen of Christiana, who is 

 now travelling at the expense of the 

 King of Sweden, and with the permis- 

 sion of the Emperor of Russia, for the 

 purpose of observing the magnetic dip, 

 variation, and intensity, over the whole 

 of the North of Europe and of Asia. He 

 has especially directed his attention to 

 trace the course of the lines of equal in- 

 tensity, or isodynamic lines as they have 

 been called: that is, the lines connecting 

 those places where a needle freely sus- 

 pended in the magnetic direction, and 

 drawn a certain number of degrees from 

 this position, makes the same number of 

 vibrations round the point of rest in an 

 equal time. 



$ 4. Hypothesis of the Magnetism of 

 the Earth. 



(103.) From a consideration of the 

 general facts that have now been stated 

 with respect to the influence of terres- 

 trial magnetism, it will be sufficiently 

 evident that the earth acts upon mag- 

 netised bodies in the same way as if 

 it | were itself a magnet ; or rather as 

 if it contained within itself a powerful 

 magnet lying in a position nearly coin- 

 ciding with its axis of rotation. This 

 hypothesis was originally proposed by 

 Dr. Gilbert in his work entitled 

 " Physiologia nova de Magnete, et de 

 Tellure magno magnete," published in 

 the year 1 6 00; and Kepler ranks this 

 hypothesis among the greatest disco- 

 veries in the annals of science. 



(104.) In order to make this hypo- 

 thesis agree with facts, we must assume 

 that that pole of the terrestrial magnet 

 which is situated in the northern regions 

 of the earth, attracts the north pole of 

 the compass needle, and consequently 

 that it has the same properties as the 

 south pole of an ordinary magnet. The 

 opposite pole of the earth, or that situ- 

 ated in the antarctic regions, has the con- 

 trary properties, for it attracts the south 

 pole of the compass ; and therefore cor- 



