594 



of Gauss's researches, which are contained in his various essays, 

 both theoretical and practical, on the magnetism of the earth. 



Of the three magnetic elements, the declination, the dip, and the 

 intensity, the two first were formerly the almost exclusive objects 

 of observation, though the methods which were employed for that 

 purpose were generally too rude for the requirements of accurate 

 science ; but the third, or magnetic intensity, of which no use had 

 been made in the business of navigation, was entirely neglected. 



Humboldt first called the attention of philosophers to the great 

 theoretical importance of this element, and he omitted no oppor- 

 tunity, in the course of his travels, of determining its value. 



It was during his Arctic voyages that the attention of Colonel 

 Sabine had been forcibly called to the consideration of this subject 

 by the remarkable magnetical phenomena observed when approach- 

 ing the magnetic pole ; and it was principally due to his influence and 

 example, and to the labours of Hansteen, Erman, and other eminent 

 travellers and navigators, that observations of the intensity were 

 rapidly multiplied in every part of the globe, and more especially 

 in Siberia, which had been generally believed to be the site of a 

 second northern magnetic pole. These observations, 753 in num- 

 ber, in 670 different localities, were collected, arranged, and dis- 

 cussed in an admirable report which was made by Colonel Sabine 

 to the British Association in 1837; and it is not one of the least 

 of the many claims of its author upon the gratitude of men of 

 science, more especially in connexion with magnetic researches, 

 that it not only suggested to Gauss as he himself declares his bold 

 attempt to grapple with the general theory of terrestrial magnetism, 

 but furnished him with the materials for testing the applicability at 

 least, if not for establishing the truth, of the theory which he 

 proposed. 



The observations of the terrestrial intensity which had hitherto 

 been made were comparative only, and it was with a view of con- 

 verting such comparative into absolute measures, with reference to 

 determinate units, that Gauss undertook the series of investigations 

 which are recorded in his memoir, entitled " Intensitas vis magneticae 

 terrestris ad mensuram absolutam revocata," which was published in 

 1832. He was assisted in these experiments, as in all others that he 

 made, by Weber, a philosopher who is well known by his " Wellen- 



