SS TERRESTRIAL MAGNETISM. 



After assigning the positions of the four maxima of 

 magnetic force, it remains to give the ratio of their 

 respective degrees of intensity. These are stated either 

 according to the old relative and arbitrary scale which 

 has been so often alluded to, i. e. by comparison with the 

 intensity found by me at the above-mentioned point of the 

 magnetic equator in lat. 7 2' S., and long. 81 8' W. from 

 Paris, or in absolute measure, as first proposed by Poisson 

 and Gauss ("). In the relative scale in which the force 

 observed by me on the magnetic equator is taken as 

 = 1*000, the magnetic force in Paris and in London, (em- 

 ploying the determination of the ratio of the force in Paris 

 to that in London made in 1827, and mentioned in 

 p. 71 of the present volume), is 1*348 in Paris, and 

 1*372 in London. These numbers converted into the abso- 

 lute scale would be about 10*20 and 10*38 ; and the force 

 observed by me in Peru, and taken as =1'000 in the arbi- 

 trary scale, would be, according to Sabine, 7*57 ; this is 

 higher, therefore, than the force at St. Helena, which is 6*4 

 in the same absolute scale. All these numbers have still to 

 undergo further alterations on account of the different years 

 in which the comparisons were made : in both scales, the rela- 

 tive or arbitrary, and the absolute (the latter being the pre- 

 ferable scale), the above data are to be regarded simply as pro- 

 visional; but even with their present imperfect degree of 

 exactness they cast a clear light on the distribution of the 

 terrestrial magnetic force, an element concerning which, 

 half a century ago, the most entire ignorance prevailed. 

 They have also the very great cosmical value of presenting 

 historic points of commencement from whence to date those 



