ON THE PHENOMENA OF TERRESTRIAL MAGNETISM. 03 
the changes in that century by the more exact knowledge we 
possess of those which took place in the succeeding century, 
we may still trace the general order of the changes between 
1600 and 1700, though without the precision in point of dates 
which is subsequently attainable. 
In the map of 1600 the line of no variation in the Atlantic 
quarter of the globe forms two branches, an eastern and a 
western. These bend towards each other at the point of their 
nearest approach in 20° to 30° north latitude; and we may for 
convenience divide this line of no variation into four portions; 
a north-eastern from Lapland to the middle of Africa; a south¬ 
eastern from the middle of Africa to Cape Lagulius; a north¬ 
western from Labrador to 30° north latitude ; and a south¬ 
western from latitude 30° north, to the Pacific, passing across 
the northern part of South America. In comparing the maps 
of 1600 and 1700, the region of easterly variation included in 1600 
between the two northern branches of the line of no variation ap¬ 
pears to have moved subsequently in a north-easterly direction, 
towards the North of Asia, where it is seen in the maps of 1770 and 
1787*. The westerly variation, which in 1600 occupied the 
coasts of Iceland and Greenland, moving south-westward, ap¬ 
pears to have taken the place, in Europe and the adjoining seas, 
of the easterly variation which prevailed there in 1600. On the 
other hand, the region of easterly variation comprised between 
the two southern branches of the line of no variation of 1600 
appears to have moved in a south-westerly direction towards 
the southern point of South America ; and the westerly variation, 
which in 1600 occupied the Indian Ocean, to have moved cor¬ 
respondingly towards the Cape of Good Hope. Whilst the 
systems in each hemisphere were thus moving in opposite di¬ 
rections, the eastern and western branches of the line of no va¬ 
riation in 1600, approaching each other more and more in lati¬ 
tude 30° north, united previously to 1700, probably in the 
neighbourhood of the Cape Verd Islands, and appear in the 
map of 1700 as a continuous line, which they have ever since 
preserved. This is the line in Halley’s chart entitled “Line 
of no variation in the Atlantic.” The two northern branches, 
united between the 20° and 30° north latitude, and still com¬ 
prising between them the small region of easterly variation, ap¬ 
pear to have moved towards Siberia, where they are seen in the 
maps of 1770, 17^7? and 1800 ; whilst the lines of westerly va¬ 
riation in the north-western Atlantic, following the eastward 
* The Maps, which accompany this notice ofM. Hansteen’s work, are those 
of the Variation in 1000, 1700, 1744, and 1787 : and of the Dip in 1600, 1700, 
and 1780. 
