CHAP. VII., 8.] 



ELECTRICITY (.MAGNETISM). M. HANSTEEN. 



193 



Dist. from 

 Pole of Earth. 



N, or strongest northern pole (N. America), for 1838 25 22? 



n, or weakest northern pole (Siberia), for 1829 7 57 



S, or strongest southern pole (S. from Australia), for 1845 21 9 



, or weakest southern pole (S. from Tierra del Fuego), for 1842. 13 53 



Long. E. from 

 Greenwich. 



280 C 



131 

 216 



19' 

 33 



28 

 26 



Annual Variation 

 in E. Longitude. 



+ 15'-5 



+ 43-4-0'-153(f-1608)* 



4-0 



16-5 



* t denotes the current year. This result appears very uncertain. It gives about + 6' -5 of annual change at the present time. 

 The latitudes of these " points of convergence" are also varying. 



Variation maps of a different kind have been con- 

 structed by the French Admiral Duperrey, in which 

 the actual direction of the needle is represented by 

 arrows. The above positions of the magnetic poles 

 were deduced by Mr Hansteen from the apparent 

 convergency of the magnetic needle towards those 

 four points. The earlier observations appear to be 

 more precise than might have been expected. 



Professor Hansteen also constructed charts of the 

 lines of equal dip. In certain positions between the 

 tropics the dip is nothing, or the freely suspended 

 magnetic needle remains horizontal. The line con- 

 necting these places is called the magnetic equator. 

 It is an undulating line inclined somewhere near 12 

 or 13 to the terrestrial equator, and cutting it in 

 two points not exactly opposite, but in about 3 20' 

 and 174 30' of east longitude from Paris, according 

 to Admiral Duperrey's observations in 1825. The 

 position of these nodes is, however, variable. The 

 north end of the needle (as is well known) dips 

 more and more in the northern hemisphere, until 

 in a certain place it becomes vertical, where there- 

 fore the horizontal component of the magnetic 

 force is nothing, and the common compass loses 

 altogether its directive power. Similar phenomena 

 occur in the southern hemisphere. Lines of dip 

 of 10, 20, &c., may be drawn, and where the dip 

 is 90 there is a true magnetic pole. The best obser- 

 vations seem to show that there is but one such true 

 pole in each hemisphere. 



If we define as " pole" a spot where the needle 

 points vertically (which is the signification adopted 

 by Gauss and others), these points do not coin- 

 cide with those of greatest intensity. On the 1st 

 June 1831, Commander Ross (now Sir James 

 Clark Ross) attained the true magnetic pole in the 

 North American continent in lat. 70 5' 17" N., and 

 long. 96 45' 48" W. The dip of the needle was 

 sensibly 90. 



A third element not less important than Variation 

 and Dip, though more lately brought into notice, is 

 the Intensity of the earth's magnetism, and to this Pro- 

 fessor Hansteen directed special attention. That the 

 intensity of the earth's directive force may be mea- 

 sured by counting the oscillations of a suspended 

 magnet in the same way that a pendulum measures 

 gravity was known to Graham, Lambert, and others, 



towards the middle of the last century ; but the ex- 

 ploration of its variation on the earth's surface was 

 first attempted by the officers of Laperouse in 1785, 

 and later by De Rossel. But it was Baron Hum- 

 boldt who, at the instigation of Borda, 1 undertook 

 the earliest observations which have had any perma- 

 nent influence on this branch of science, and who in 

 the first years of this century determined the rela- B aron 

 tive intensity of the earth's magnetism at Paris and Hum- 

 at the magnetic equator in South America, to be in boldt ' 8 

 the ratio of 1-3482 to 1-0000 ; 2 a result which has unit ' 

 become, so to speak, classical, and which the author 

 considered as the most important result of his jour- 

 ney. 3 M. Hansteen promoted the same enquiry ex- 

 tensively ; he devised a neat and convenient apparatus 

 for counting the oscillations of the needles, and he 

 investigated the effects of time and temperature in 

 altering their magnetism.* He made numerous ob- 

 servations in the north of Europe, and finally under- 

 took, between 1828 and 1830 (by the liberality of 

 the Norwegian parliament or Storthing), an adven- 

 turous journey into Siberia, for the purpose of exa- 

 mining the " region of convergence" of the needle in 

 that quarter. His account of his journey has most M. Erman. 

 unfortunately not been published; but his compa- 

 nion, Professor Adolphe Erman, has given the main 

 results, together with extensive observations entirely 

 his own. 5 This was the first magnetical expedition 

 of any magnitude, and of a national character. It 

 yields in importance to none which have succeed- 

 ed it. 



The magnetic intensity then increases from the (886.) 

 neighbourhood of the equator towards the Arctic Distribu- 



Regions, and there it has two foci of greatest inten- * 10 i n . 



. o ' . intensity, 



sity. A similar arrangement occurs in the southern 



hemisphere, and these four points of maximum at- 

 traction or convergence of the needle constitute the 

 " poles" of Halley and of M. Hansteen. To M. 

 Hansteen we are indebted for the first good ap- 

 proximation to a general chart of the lines of equal 

 intensity. 



The supposed diminution of the magnetic inten- (887.) 

 sity as we recede from the earth's surface has been Diminu- 

 repeatedly made the subject of experiment. The m g etic 

 insulated experiments of Gay-Lussac in a balloon intensity 

 (630), and of M. Kupffer at Mont Elbroutz, led to with 

 no conclusive result. From a numerous series of heig 



3 See Kosmos, ubi. sup. 



1 See Kosmos, vol. i., note 159. 



3 On the same scale the intensity at London is 1-3720. 



* De Mutationibus Virgce Magneticoe, 1842. 



6 See his Lines of Equal Variation, reproduced in the Royal Society's " Report on Physics and Meteorology, 1840." 



