MAGNETISM. 



29 



with the meridian of 80 west of Green- 

 wich. 



But M. Hansteen was led by a more 

 careful consideration of the slight ap- 

 parent deviations which had been no- 

 ticed, and of the general disposition on 

 the globe of the lines of dip and varia- 

 tion, to infer the existence of a second 

 point of principal magnetic action in 

 the northern hemisphere. This fact 

 may now, indeed, be regarded as fully 

 established by his recent observations ; 

 the isodynamic curves being found to 

 arrange themselves systematically round 

 two poles, the one in Hudson's Bay and 

 the other in Siberia ; and to be governed 

 in the courses which they follow, partly 

 by their distances respectively from 

 those points, and partly by a disparity 

 in the absolute attractive force at the 

 points themselves : the maximum in- 

 tensity in Siberia appearing to be weaker 

 than that in Hudson's Bay, and existing 

 at a point situated in longitude 102 east 

 of Greenwich, which is as nearly as can 

 be judged, 180 from the present posi- 

 tion of the corresponding point in Hud- 

 son's Bay, and in a latitude somewhat 

 to the north of 60, but which, it is to 

 be hoped, will soon be more particularly 

 determined. 



5. Progi'essiue Changes of Varia- 

 tion and of Dip. 



(115.) The most singular and unac- 

 countable circumstance relative to ter- 

 restrial magnetism remains yet to be 

 noticed ; namely, that it does not remain 

 constantly the same in the same place, 

 but undergoes a slow and progressive 

 change. The variation of the compass 

 is itself variable, not merely in different 

 regions of the globe, but at different 

 periods of time. Thus, the needle in 

 London, in the beginning of the seven- 

 teenth century, was inclined a few de- 

 grees to the eastward of the true north. 

 In 1659 or 1660, it pointed exactly 

 north ; or in other words, the variation 

 was reduced to zero; and, of course, 

 London was at that time one of the 

 points of the line of no variation. After 

 this, the variation became westerly, and 

 has continued so to the present time. 

 The line of no variation, therefore, has 

 been progressively, but slowly moving 

 in a westerly direction, and has now 

 passed over to North America. 



Similar changes have taken place at 

 Paris ; but the line of no variation ap- 

 pears to have passed over that city 

 rather later than it did over London: 



for it was not till the year 1G64 that the 

 magnetic coincided with the true meri- 

 dian. In 1814, it was 22 34' west. In 

 October, 1829, the variation at Paris 

 was ascertained, by M. Arago, to be 

 22 12' 5" west*. 



At London, the westerly variation 

 continued to increase till the year 1818, 

 when it amounted to 24 30.' This ap- 

 pears to have been its maximum ; for 

 since that time it has somewhat dimi- 

 nished, and is at present about 24. 



It appears, from the table given by 

 Mr. Gilpin f, that the annual change in 

 the variation has diminished, in each 

 successive period, since the beginning of 

 the last century. In the preceding cen- 

 tury, that is from 1622 to 1692, the an- 

 nual change was about 1 0' ; from 1 723 

 to 1773, it was about 8'; from 1787 to 

 1795, about 5'; from that time to 1802, 

 only l'.2: in 1818 it was reduced to 

 zero. 



(116.) The dip has also undergone 

 corresponding changes, though less con- 

 siderable ones than the variation. In 

 1680, the dip in London was 73 30'; 

 in 1723 it was 74 42': since which time 

 it has been observed to diminish pro- 

 gressively, though, as it would seem, 

 not quite regularly. 



Authorities and Localities. 



In 1773 it was 72 19' Dr. Heberden. 

 1786 72 8} Gilpin, Royal 

 1805 70 21 J Society'sRooms 

 Phil. Trans, for 

 1806, p. 419. 



1818 70 34 Capt. Kater, Re- 

 gent's Park. 

 1821 70 3 Captain Sabine, 



i C his wick. 



1828 69 47 " Ditto. 

 1830 69 38 Capt. Segelcke, 



Woolwich.^ 



On the continent of Europe the dip 

 has undergone a similar diminution of 

 late years. The dip at the observatory 

 at Paris, in the year 1814, was 68 36', 

 according to the determination of 

 M. Bouvard. In June, 1829, it was as- 

 certained by M. Arago to be 67 41'. 3. 



(1 1 7.) Captain Sabine, by comparing 

 the present dip with that observed for 

 the last fifty years, concludes that the 



Annuaire pour 1'An 1830. 



i Phil. Trans, for 1806, p. 395. 



t For the information relative to the last of these 

 determinations, we are indebted to the kindness 

 of Professor Barlow, of Woolwich, who states that 

 Captain Segelcke, of the Norwegian Navy, and 

 a friend of Professor Hansteen, employed in tiiis 

 determination of the dip, the same needle which 

 the latter had with him in hia recent tour in 

 Siberia. 



