AND THE INTENSITY OF MAGNETICAL FORCE. 309 
COMPARISON OF THE INTENSITIES, AS GIVEN IN THE PRECEDING TABLES, WITH THOSE 
REPORTED TO THE BRITISH ASSOCIATION IN 1838, BY MAJOR SABINE, R.A. 
In Major Sabine’s Report the intensity of some place near the equator, as observed by 
Humboldt, in 1802, has been assumed as unity, and, on page forty-three, we find that of 
New York to be 1803.0. By the unit assumed in the above tables, the intensity at the 
same locality, Columbia College, New York, is expressed by 994.3. ‘Then we have 
(Locke,) 994.3 : (Sabine,) 1803.0 : : (Locke,) 1000 : (Sabine,) 1813.3, the last being the 
intensity at Cincinnati, according to Colonel Sabine’s unit. Hence to reduce any total 
intensity, a, given in the preceding tables, where that at Cincinnati is assumed as unity, 
to a comparison with the standard or unit assumed by Major Sabine, we have the fol- 
lowing formula: 12133 @ = 2, the required value. By the application of this formula 
to the intensity observed on Porter’s Island, in Lake Superior, 1086.58, we obtain 1970.29. 
Situated, as this locality is, nearly in the centre of the great ellipsoid described by Major 
Sabine in his plates, Nos. 3 and 4, by the lines representing the intensities 1.700 and 
1,800, and being the highest magnetical intensity yet observed on the earth, so far as 
known to me, we deem the observations worthy of peculiar attention. But the more 
correct result will be obtained, not by an extreme instance, but by the mean of a group, 
as that of the five stations given on a previous page, “3158.90,” which, reduced to the 
Cincinnati standard of unity, becomes 1058.02, and, reduced again by the above formula, 
to Colonel Sabine’s standard, is equal to 1918.5, or, as it is pointed on Colonel Sabine’s 
charts, 1.9185. By a similar reduction of the temperature at Isle Royale, we have 
1.90808. As the intensity at Copper Harbour is evidently high, compared with that 
which shows itself at the other localities along the south shore of the lake, it is proper, 
perhaps, to make a group of all the others, excluding those of the Harbour. 
It appears, by the intensity at Cincinnati and Williamstown, in Kentucky, that the 
isodynamic line of New York passes between those two places, about twenty-five 
geographic miles south of Cincinnati, and five north of Williamstown. By a calculation 
of the change of intensity proportional to change of latitude, from Davenport to St. Louis, 
it appears that the same isodynamic line crosses the Mississippi at a point thirty-two 
geographic miles south of St. Louis, namely, in latitude 38° 04’, north. 
From Lexington, in Kentucky, to Isle Royale the difference of latitude is exactly ten 
degrees; and the difference of intensity, according to my unit, is 67.18, or, 6.718 to each 
degree. According to Colonel Sabine’s unit, the whole difference will be 0.12122, or, 
0.012122 to each degree, 
GENERAL SUMMARY. 
I. SERIES, FOR OHIO, 1838. 
Horizontal 
Total intensity |Horizontal) ‘Total Intensity | 
Dip. | Intensity, 
Longitude, or. being 1000,| Intensity. |by Sabine’s 0g 
No. Date. Place. | Latitude, 
» + |89°06'N,|84°22’W./70°28’08"11000. 2991.1  |1000. 1.8138 
- + (89 44 (8417 1/71 22.42 | 958.20] 3001.0 {1003.2 | 
39 54 «(88 5171 27.23 | 949.4 | 2984.3 | 997.8 | 
- - 140 05 |83 48 |71 3945] 953.26} 3027.9 {1012.2 | 
- - |39 57 I|s3 02 |71 0452 | 966.64! 2981.3 | 996.72) 
1 |March 20,/Cineinnati, - 
2; “ 26; Dayton, . - 
3; “ 29,/Springfield, - 
4; % 80; Unbbha, a2 & 
5 jApril 2, {Columbus, - 
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