Position of 
magnetic 
centres or 
poles. 
Cuar. VIL, § 8.] ELECTRICITY (MAGNETISM).— M. HANSTEEN. 991 
‘N, or strongest northern pole (N. America), for 1838 ....... 
Dist. from Long. E. from Annnal Variation 
Pole of Earth. Greenwich. in E. Longitude. 
+» 25° 22 280° 19° + 155 
n, or weakest northern pole (Siberia), for 1829. ........e+«..+ se Ahi 114 33 + 434-0153 (¢~1608)* 
S, or strongest southern pole (S. from Australia), for 1845,........ 21 9 131 28 — 40 
s, or weakest southern pole (S. from Tierra del Fuego), for 1842, 13 53 216 26 —16°5 
* ¢ denotes the current year. This result appears very uneertain. It gives about +6’-5 of annual change at the present time. 
> 
The latitudes of these “ points of convergence 
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 
eonvergency 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 18° 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 
oceur 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, 
are also varying. 
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 Lapérouse 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- garon 
tive intensity of the earth’s magnetism at Paris and Hum- 
at the magnetic equator in South America, to be in boldt’s 
the ratio of 1:3482 to 1:0000;? a result which has gs 
become, so to speak, classical, and which the author 
considered as the most important result of his jour- 
ney.> 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 1827 and 1829 (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.® 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 (gg¢6.) 
neighbourhood of the equator towards the Arctic Distribu- 
Regions, and there it has two foci of greatest inten- tio of 
sity. A similararrangement occurs in the southern pial 
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 ats 
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 5*8%* 
= 
1 See Kosmos, vol, i., note 159. 
2 On the same scale the intensity at London is 1:3720. 
‘ De Mutationibus Virga Magnetic, 1842. 
3 See Kosmos, ubi. sup. 
See his Lines of Equal Variation, reproduced in the Royal Society’s ‘‘ Report on Physics and Meteorology, 1840.” 
