ON THE PHENOMENA OP TERRESTRIAL MAGNETISM. 
65 
to have moved in 1800 towards Corea and the adjoining seas; 
still evidencing the general progress eastward of the lines of va¬ 
riation in the northern hemisphere. 
The second chapter is entitled £c On the Lines of Dip, and on 
the Magnetic Force.” The commencement of this chapter is 
occupied in discussing the materials existing for the construc¬ 
tion of the maps of dip in 1600, 1700, and 1780. For that of 
1600 the authorities, though few, are shown to be entitled to much 
confidence. Those for the map of 1700 are much more numer¬ 
ous; but that of 1780 is the first tolerably complete system of 
the lines of dip warranted by observations. A large space is 
occupied in examining the observations upon which the line of no 
dip, or, as it is frequently called, the magnetic equator, is laid 
down on this map. In the hypothesis of a single magnetic axis 
this line should be a great circle; it however differs much from 
the simplicity of figure which would correspond with that hy¬ 
pothesis. As much importance is attached to the correct deli¬ 
neation of this line, M. Hansteen inserts a table of the several 
observations, seventy in number, from which its course is laid 
down, and makes each-observation the subject of a particular 
discussion. 
The extreme southern latitude in which the line of no dip is 
found is 13j°, which it reaches in from 20° to 26° west from 
Greenwich. From that point it slowly but uninterruptedly ap¬ 
proaches the geographical equator to the east and to the west, 
until it cuts it in Africa in or about 25° east, and in the Pacific 
in or about 110° west. These points of intersection, or nodes, 
are thus not more than 135° apart, whereas if the magnetic equa¬ 
tor were a great circle, they ought to be 180° apart. The inter¬ 
section with the geographical equator in Africa is at an angle of 
21^°, but in the Pacific at an angle of only 7i°* The greatest 
northern latitude it attains is in 12j-° east of Africa in or about 
65° east. Following its course from that point eastwardly, it 
slowly declines towards the south to the longitude of Malacca, 
where it coincides very nearly with the parallel of 9° north. 
Here, however, it bends again to the north, being found in 9J° 
north in the longitude of the Philippines ; whence it finally de¬ 
scends without interruption till it cuts the geographical equator, 
as before stated, in the Pacific, in longitude 110° west. The 
greatest northern latitude attained by this line is in or about 
65° east, and the southern extreme is in 23° west. These points 
are only 88° apart, furnishing additional evidence that the line 
of no dip does not correspond with a great circle on the earth’s 
surface. 
Each of the lines of dip and variation drawn in the maps 
1835. f 
