LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
35 
When these results are transferred to the map of the portion of the Atlantic to 
which they refer, and are examined in detail, their systematic character becomes 
much more obvious than in the Table, where, in consequence of the successive alter- 
nations of increasing- and decreasing latitude, their consistency is not so easily fol- 
lowed by the eye. On attentive examination of the map it is not difficult to trace within 
small limits the course of an ideal line, which should connect the points in the several 
meridians, where the intensity was weakest at the epoch of Captain Ross’s voyage. 
The determination of the position of this line is easier, and in some respects more 
sure, than that of an isodynamic line, because it is independent of the permanency 
of the magnetism of the needle employed, for more than the few days occupied in 
the immediate research ; and it is also independent of the correctness of an assumed 
intensity at a base station. It is therefore to be expected that the position of this 
line will become in future years the subject of frequent examination, serving to mark, 
from time to time, the progress of the secular change in its position. This may be 
done with the more interest and advantage, because there is reason to believe that 
its position is changing rapidly in the space referred to, particularly in the eastern 
meridians ; and that the southern magnetic hemisphere, in so far as its boundary 
may be indicated by this line, is in that quarter of the globe gaining rapidly upon 
the northern. In the first of the present series of “ Contributions”*, the line of least 
intensity was drawn from observations corresponding nearly to the epoch of 1825, 
and this line of 1825 is lightly retraced in the present map for the purpose of com- 
parison. It will be seen, that whilst its general direction is consistent with the ob- 
servations of Captain Ross in 1840, its earlier position is everywhere three or four 
degrees south of that which would be now inferred. It is readily admitted that many 
of the observations from which the line of 1825 was drawn are inferior in precision 
to those of Captain Ross ; and I rejoice in the late improvement in this class of ob- 
servations, for which we are mainly indebted to the method and instrument devised 
by Mr. Fox, and to the zeal and unwearied patience of our naval officers. To an 
observer, however, who is proceeding in a nearly north and south direction, very 
little uncertainty attends the determination of the time and place at which he finds 
the weakest intensity ; and if we compare the observations of Dunlop, Erman, and 
Sulivan, with those of Ross and Crozier, we invariably find that the earlier ob- 
server makes the place of the minimum a little more southerly than later determi- 
nations. 
A glance at the map suffices to show where determinations are now most wanted, 
and to point out the track where additional observations would be most valuable : 
it would be nearly that of a vessel making the eastern passage to the Cape of Good 
Hope. 
* Philosophical Transactions, 1840, Plate V. 
