ON THE MAGNETIC INTENSITY OF THE EARTH. 69 
of great condensation would admit, the arrangement of the lines 
of intensity, and their systematic departure from parallelism 
with those of the dip, which, in his theory of four poles, founded 
on the assemblage and study of the earlier observations of the 
dip and variation, M. Hansteen had anticipated, previous to his 
own experiments. It is sufficient to show, as may be done by 
a single sentence written since his return from Siberia, that the” 
results of these have accorded with his previous views. ‘Thus 
is confirmed in the clearest and most satisfactory manner what 
I had earlier inferred from the two other magnetic phenomena ; 
namely, that in the northern hemisphere there are two magnetic 
centres, or poles ; and that the westernmost, in North America, 
has asensibly greater intensity than the easternmost in Siberia*.” 
Having thus shown the concurrent opinions which those 
who have most extensively engaged in the experimental in- 
quiry have been led to form, it remains to place the facts them- 
selves in a convenient manner before the general reader. The 
complete view of the systematic difference in the course of the 
two kinds of lines is best obtained, by comparing the map of 
the intensity lines in this Report with M. Hansteen’s map of 
the dip lines for 1780, in the Fifth Report of the British 
Association}. The lines of dip have undergone some changes 
since that period, but none which much affect their general 
configuration. All readers, however, may not have that 
‘volume at hand, and I have therefore traced in Plate I. 
the course of the line of equal intensity which passes through 
our own islands, for 160 degrees of longitude, and have 
exhibited it in comparison with the neighbouring lines of dip. 
The line of intensity, shown by the continuous line, is taken 
from the general map accompanying this memoir. The por- 
tions of dip-lines, marked by the dotted lines, are taken from 
M. Erman’s map drawn from his own observations, in_ the 
Annalen der Physik, vol. xxi. The intensity line, which in 
the meridians of 280° and 290° is in close juxtaposition with 
* Ann. der Physik, vol. xxviii. p. 579. 
+ I may take this opportunity of stating that the sea portions of M. Han- 
steen’s map of the dip in 1780 rest on the authority of between 900 and 1000 
observations of the dip made at sea between the years 1767 and 1788, and that 
these are tabulated in the Appendix of the Magn. der Erde. The observation 
of the dip at sea in favourable weather was the habitual practice of many of the 
“scientific navigators of that period, such as Le Gentil, La Perouse, Ekeberg, 
Lewenhorn, and our own countrymen Phipps, Hutchins, Abercrombie, and 
Pickersgill. It is much to be wished that it were a more frequent practice now. 
M. Erman, in his voyage from Kamtschatka to Europe, found a number of 
days sufficiently favourable to enable him to observe the dip in not less than 
167 geographical positions at sea. 
