102 TERRESTRIAL MAGNETISM, 



dynamic equator are complex, because they depend on forces 

 which have four points or foci of greatest intensity which 

 are unsymmetrically distributed on the surface of the globe, 

 and are of unequal strength. The most remarkable of these 

 inflections is the great convexity towards the southern pole, 

 in the Atlantic Ocean, between the coast of Brazil and the 

 Cape of Good Hope/' 



Does the intensity of the terrestrial magnetic force 

 diminish sensibly at accessible distances above the surface 

 of the earth ? or increase sensibly in descending below that 

 surface ? The problem which these questions propose for 

 solution is an exceedingly complicated one, in so far as it 

 has to be determined by observations made above or below 

 the ordinary level of the earth's surface. In attempting to 

 solve it by means of mountain journeys or descents into mines, 

 the facts that the upper and lower stations of observation are 

 seldom situated vertically in respect to each other, that the 

 observations are frequently influenced by the character of the 

 rocks and the presence perhaps of hidden mineral veins, and 

 that the fluctuations, regular or irregular, of the magnetic 

 force itself, where the observations are not strictly simulta- 

 neous, require to be allowed for ; are all circumstances which 

 are likely to affect the results( 108 ), and to cause to be ascribed 

 to differences of height or depth what may be by no means 

 due to them. Among the numerous mines which I have 

 visited, descending to considerable depths, in Europe, Peru, 

 Mexico, and Siberia, I have never found localities which were 

 suited to inspire me with confidence in reference to this ques- 

 tion :( 109 ) it must also be remembered, that considering the 

 normal plane to be that of the level of the sea (viewed as the 

 mean general surface of the terrestrial spheroid), many mines, 

 thouorh offeriner considerable denths below the surface of tlip. 



