30 



MAGNETISM. 



mean annual diminution is about 3'. 

 Mr. Barlow finds that these observa- 

 tions accord much more nearly with 

 what would take place on the supposi- 

 tion of a uniform motion of revolution in 

 the magnetic pole round the pole of the 

 earth. From the most authentic ob- 

 servations on the dip and variation 

 of the needle in London, he calculates 

 that the longitude of the northern ex- 

 tremity of the magnetic polar axis 

 which it obeys was, in 1818, 67 41' 

 west, and its latitude 75 2' north. If 

 we suppose that the motion of this pole 

 has been uniform since the year 1660, 

 when, from the disappearance of varia- 

 tion, its longitude must have been zero, 

 and 1 hat it has preserved the same dis- 

 tance from the terrestrial pole, its an- 

 nual motion of revolution must have 

 been about 25'. 4. It would, therefore, 

 require eight hundred and fifty years 

 to make an entire revolution of 360. 

 Computing from these data, it would 

 follow that the variation ought to reach 

 its maximum when the longitude of the 

 magnetic pole is 70 23' west. It would 

 have arrived at this situation about the 

 year 1 823 ; about which time, as it would 

 appear, the variation was stationary, 

 having attained its real maximum, and 

 having, since that period, actually re- 

 troceded. 



(118.) On calculating what the dip 

 should be in 1823, according to Mr. 

 Barlow's hypothesis, a very near agree- 

 ment with actual observation is found 

 to take place. It follows, however, from 

 this hypothesis, that the dip has not an 

 uniform decrease, but that it is changing 

 much more rapidly at this present time 

 than it has ever done since magnetical 

 observations have been made. Its de- 

 crease, during the five years preceding 

 1824, has been nearly half a degree, and 

 it ought to have diminished to an equal 

 extent during the following five years. 

 Mr. Barlow* has computed that in 



The Variation should be and the Dip 



1828 . . 24 29' . , 69 43' 

 1833 . . 24 26 . . 69 21 

 The near accordance of these results 

 with what has actually been observed, 

 is considered by him as a strong confir- 

 mation of the truth of his hypothesis. 



(119.) It would appear, then, both 

 from observation and from theory, that 

 the dip is at present changing more 

 rapidly than the variation; and the 



-P * ? ee his Essay on Magnetic Attractions, 2nd 

 Edition, p, 218, 



theory leads to the expectation that it 

 will continue to decrease together with 

 the dip, for about two hundred and 

 fifty-five years, at the end of which 

 period, that is in 2085, the longitude of 

 the magnetic pole will be 180; the 

 variation will then be nothing, and the 

 dip only 56, which will be its minimum ; 

 they will then both increase together for 

 the next two hundred and sixty years, 

 when the needle will have its greatest 

 easterly variation, and will then again 

 return towards the north, the variation 

 decreasing, but the dip still increasing, 

 for one hundred and sixty-five years 

 longer, namely, till about the year 2510, 

 when the magnetic pole will be again in 

 the meridian of London ; the variation 

 will then be zero, and the dip will amount 

 to 77 43'. It is to be observed, how- 

 ever, that Mr. Barlow advances this 

 merely as an hypothesis, the truth of 

 which remains to be determined by 

 future experience. 



(120.) A curious hypothesis was ad- 

 vanced by Dr. H alley, and supported 

 with some ingenuity, in order to explain 

 the progressive changes that take place 

 in the variation of the compass. He 

 supposes the globe we inhabit to be a 

 mere external shell, enclosing, towards 

 its centre, a detached magnetic nucleus, 

 of a spherical shape, which revolves with 

 the external shell on a similar axis, with 

 nearly the same velocity. He supposes 

 both these spheres to be magnets, having 

 each two poles ; but the poles of 

 the one not exactly corresponding in 

 situation with the poles of the other. 

 The difference of the periods of rotation 

 of the two spheres, he conceives to be 

 exceedingly small, yet sufficient to be- 

 come sensible after the lapse of years, 

 and to occasion a change in the relative 

 situation of the two sets of magnetic 

 poles ; and hence would arise changes in 

 the direction of their resulting actions, 

 and corresponding changes in the varia- 

 tion of the magnetic needle. However 

 ingeniously this hypothesis may have 

 been framed, it was too bold and fanciful 

 to have been ever generally adopted. Its 

 author, indeed, has the candour to ac- 

 knowledge that it is beset with nume- 

 rous difficulties, which further experience, 

 extended over a long period of time, can 

 alone enable us to remove. He con- 

 cludes his paper in the Philosophical 

 Transactions in which he has developed 

 his theory, with the following sentence : 

 * But whether these magnetical poles 

 move altogether with one motion, or 



