92 COSMOS. 



source, to which I have already frequently referred, that is to 

 say, by a comparison with the intensity which I found at a 

 point of the magnetic equator in the Peruvian chain of the 

 Andes, which it intersects in 7 2' lat. and 78 48' W. long, 

 or, according to the earliest suggestions of Poisson and Gau&s, 

 by absolute measurement." If we assume the intensity at 

 the above indicated point of the magnetic equat or rr 1.000, 

 in the relative scale, we find from the comparison made be- 

 tween the intensity of Paris and that of London in the year 

 1827 (s-ee page 67), that the intensities of these two cities 

 are 1.348 and 1.372. If we express these numbers in ac- 

 cordance with the absolute scale they will stand as about 

 = 10.20 and 10.38, and the intensity, which was assumed to 

 be 1.000 for Peru, would, according to Sabine, be 7.57 in the 

 absolute scale, and therefore even greater than the intensity 

 at St. Helena, which, in the same absolute scale = 6.4. All 

 these numbers must be subjected to a revision on account of 

 the different years in which the comparisons were made. 

 They can only be regarded as provisional, whether they are 

 reckoned in the relative (or arbitrary) scale or in the absolute 

 scale, which is to be preferred to the former, but even in 

 their pres-snt imperfect degree of accuracy they throw con- 

 siderable light on the distribution of the magnetic force a 

 subject which, till within the last half century, was shrouded 

 in the greatest obs-curity. They afford what is cosmically 

 of very great importance, historical points of departure for 

 those alterations in the force, which will be manifested in 

 future years, probably through the dependence of the earth 

 upon the magnetic force of the sun, by which it is influenced. 

 In the northern hemisphere the stronger or Canadian 

 focus in 52 19' N. lat. and 92 W. long, has been most satis- 

 factorily determined by Lefroy. This intensity is expressed 

 in the relative scale by 1.878, the intensity of London being 

 1.372, while in the absolute scale it would be expressed by 

 14.21, 100 Even in New York, lat. 40 42', Sabine found the 



99 Phil. Transact, for 1850, pt. i, p. 201; Admiralty Manual, 1849, 

 p. 16; Erman, Magnet. Beob. s. 437 454. 



100 On the map of isodynamie lines for North America which occurs 

 in Sabine's Contributions to Terrestrial Magnetism, No. vii, we find, by 

 mistake, the value 14.88 instead of 14.21, although the latter, which ia 

 the true number, is given at page 252 of the text of this memoir. 



