PHILOSOPHICAL TRANSACTIONS. 
XVII. Contributions to Terrestrial Magnetism. — No. VII. 
By Lieut. -Colonel Edward Sabine, R.A., For. Sec. R.S. 
Received April 30, — Read May 14, 1846. 
Containing a Magnetic Survey of a considerable portion of the North American Continent. 
FROM the moment that the fact was known, that the locality of the maximum of the 
magnetic Force in a hemisphere is not coincident, as was previously supposed, with 
the locality where the dip of the needle is 90°, researches in terrestrial magnetism 
assumed an interest and importance greatly exceeding that which they before pos- 
sessed ; for it was obvious that the hypothesis which then generally prevailed regard- 
ing the distribution of the magnetic Force at the surface of the globe, and which had 
been based on a too-limited induction, was erroneous, and that even the broad out- 
line of the general view of terrestrial magnetism had to be recast. 
The observations on which this discovery rested, (being those which I had had an 
opportunity of making in 1818, 1819 and 1820 within the Arctic Circle, and at New 
York in 1822,) were published in 1825* ; they constituted, I may be permitted to say, 
an important feature in the views, which led the British Association in the year 1835 
to request that a report should be prepared, in which the state of our knowledge in 
respect to the variations of the magnetic Force at different parts of the earth’s sur- 
face should be reviewed, and, as is customary in the reports presented to that very 
useful institution, that those measures should be pointed out which appeared most 
desirable for the advancement of this branch of science. In the maps attached to 
the report')', the isodynamic lines on the surface of the globe were drawn simply in 
conformity with observations, and unmixed with hypothesis of any sort. The obser- 
vations collected for that purpose were not those of any particular individual or of 
any single nation, but embodied the results obtained by all persons who up to that 
period had taken part in such researches, subjected to such amount of discussion 
* Pendulum and other Experiments, pp. 460-499. f Reports of the British Association for 1837. 
MDCCCXLVI. 2 I 
