AND THE INTENSITY OF MAGNETICAL FORCE. 287 
course of these tables, rules are given by which these results may be reduced to the 
standard of unity adopted by Colonel Sabine, in his Report on Magnetism in 1838. 
Correspondence.—After reducing my observations, in 1848, and putting them into the 
tabular form, I then compared them with the charts in Colonel Sabine’s Report, above 
named, and was agreeably surprised at the coincidence of my actual observations with 
the chart lines which he had projected by means of surrounding observations, made so 
distant as the Atlantic, Baffin’s Bay, and the Pacific. I immediately addressed letters to 
my friend Sears C. Walker, of Philadelphia, to the superintendent of the Macnetical 
Observatory, at ‘Toronto, to the National Institute, at Washington, and to Colonel 
Sabine, R. A., at Woolwich, England, announcing briefly the result of my labours, and 
soliciting information as to what others had been accomplishing. Most of these letters 
were promptly responded to, and from Mr. Walker, Professor Bache, and Colonel Abert, 
I received valuable information, communicated in terms of great kindness. I have 
finally received a reply from Colonel Sabine, containing precisely the information which 
I desired, and encouraging me to further exertions in the same department of research, 
most obligingly offering me such assistance, in procuring instruments, as I might need. 
I quote, below, my own letter to the National Institute, as published by Colonel J. J. 
Abert, to whom it was addressed, and who, in conjunction with Mr. F. Markoe, commu- 
nicated it to the National Intelligencer. 
* Medical College of Ohio, Cincinnati, October 16, 1843. 
“DEAR SIR: 
“Task the favour of you to announce to the National Institute, that, in my late tour 
to Lake Superior, I have discovered what is probably the place of maximum intensity of 
terrestrial magnetism. Major Sabine, in his Report to the British Association, in 1838, 
gives a chart of isodynamic lines in the northern hemisphere, on which he describes the 
line of intensity, 1.70, as an ellipsoid, extending, in latitude, from 35° to 69° north, and 
from Nova Scotia to the Pacific, almost, if not quite returning into itself at Behring’s 
Straits; within this ellipsoid he has given a small portion of the line of 1.80, commencing 
at New York, and curving southward and westward to the Ohio River, and thence 
across the middle of Lake Erie. He suggests that this must be a closed curve, and adds, 
“as must also be that of 1.90, supposing such to exist.” 'This would suggest that, some- 
where within this curve, there would be a point of maximum intensity, a kind of pole of 
greatest attraction, separate from the pole of greatest dip, and from that of the convergence 
of the magnetical meridians. Major Sabine observes, that “the intensities in Baffin’s Bay 
and in the polar sea, have alla much lower value than at New York, and the general 
configuration of the line of intensity would rather point to a maximum in the vicinity of 
the shores of Hudson’s Bay.” ‘These suggestions of Colonel Sabine are, in general, sub- 
stantially supported by my observations, but the place of maximum intensity is much 
farther south than Hudson’s Bay, being on Kewenan peninsula, on the south shore of 
Lake Superior. At this place, the intensity of one locality reached 1.97, and that of a 
mean of a group of observations, made in various neighbouring localities, was as high as 
1.918. On the north side of the lake, at Isle Royal, it was found to be 1.908. I have 
just now completed the reduction of all my observations since 1838, and prepared them 
VOL. IX.—-76 
