288 OBSERVATIONS ON THE MAGNETIC DIP, 
for publication. ‘They extend, geographically, from Cambridge, in Massachusetts, to 
Towa territory, and from the middle of Kentucky to the north side of Lake Superior, 
including dip and intensity, and, in some places, declination also. 
“ Respectfully, Sir, your friend and obedient servant, 
“JOHN LOCKE,” 
“To Cotone J. J. Apert.” 
A letter containing, substantially, the same information as the above, was, about the 
same time, addressed to Colonel Sabine, from whom, in due time, was received a prompt 
and obliging answer, from which the following is an extract. 
“ Woolwich, (England,) November 22, 1843. 
“DEAR Sir: 
“T hasten to acknowledge the receipt of your very obliging and agreeable letter of the 
25th of October. Iam very glad to learn that the ‘Report on Magnetism,’ which I took 
the liberty of sending you in 1840, reached you in safety, and T cannot but be most 
highly gratified to hear that it in any degree contributed to induce you to undergo the 
labour and fatigue of making the series of magnetical observations over the extensive 
and important district of which you speak. I cannot doubt that so valuable a contribu- 
tion to magnetical science, bearing so immediately on one of the critical points of magnet- 
ical theory, will be most warmly welcomed by one of your own national institutions, and 
will be published, as it is so desirable it should be, among the scientific records of the 
nation, Permit me to express the hope that, not less for the interests of science than for 
your own sake, you will spare no endeavour to obtain a very early publication of your 
observations. Both what they accomplish and what they leave unaccomplished will be a 
guide to those engaged in similar undertakings. ‘The magnetic survey which is now in 
progress, under the direction of Lieutenant Lefroy, in British North America, is making 
a very rapid and successful advance, and promises to furnish ample materials for maps 
of the three magnetical elements north of the United States frontier. It is much to be 
desired that the survey should be met at the frontier by researches of citizens of the 
United States, conducted on their own grounds. I learn from your letter that you have 
yourself achieved already a very considerable part of this undertaking. The field of ope- 
ration is thus greatly narrowed on the one hand, while, on the other, a full knowledge of 
the results which are obtained may be expected to indicate the direction in which further 
researches can be most profitably made. Lieutenant Lefroy informs me that in his jour- 
ney from Toronto to York factory, on Hudson’s Bay, pursued by the usual route of the 
Hudson’s Bay Company boats, he found the magnetic force to increase until he arrived 
in the neighbourhood of Rainy Lake; at which spot it began to decrease, and continued 
to do so all the way to York factory. His observations would therefore seem to corre- 
spond in a remarkable degree with yours, in the indication of the locality of the maximum 
of intensity of the force ; and we may infer that the principal axis of the inner isodynamic 
ovals crosses the meridians of 88° to 94° west, in the parallels of 47° to 48° north. 
For a more exact determination an examination on a north and south line, ascending 
the Mississippi to the point from whence the country can be most conveniently crossed 
