122 TERRESTRIAL MAGNETISM. 



excellent comparative horizontal force observations (pub- 

 lished in great detail) at 35 stations, including the 

 summits of Mont Blanc (14809 Trench feet), the Great 

 St. Bernard (7848 French feet), and the Faulhorn (8175 

 French feet) ; contemporaneously with which were observa- 

 tions of the inclination on the Grand Plateau of Mont 

 Blanc (12097 French feet), and at Chamounix (3201 

 French feet). If the comparison of these results would 

 indicate a diminution of inclination with increased ele- 

 vation, observations on the Faulhorn and at Brienz 

 (1754 French feet) on the other hand, would give the 

 contrary indication. Thus no satisfactory solution of the 

 problem was obtained by either class of experiment. 

 (Bravais sur Fintensite du Magnetisme terrestre en France, 

 en Suisse et en Savoie, in the Ann ales de Chimie et de 

 Physique, 3eme Serie, T. xviii. 1846, p. 225.) In a 

 manuscript of Borda's on the subject of his expedition to 

 the Canaries in 1776, (preserved in the Depot de la Marine 

 at Paris, and for the communication of which I am indebted 

 to Admiral Rosily), I have found the proof that Borda made 

 the first attempt to examine the influence of height on this 

 magnetic element. He found the inclination on the summit 

 of the Peak of Teneriffe 1 15' greater than in the port of 

 Santa Cruz, a difference, no doubt, in great part at least, 

 the consequence of the local attraction of the lavas, similar to 

 that which I have myself often observed on American 

 volcanoes and on Vesuvius. (Humboldt, Yoy. aux Beg. 

 equinox. T. i. pp. 116, 277, and 288.) 



With the view of trying the analogous question of the 

 influence of depth below the earth's surface, being at Frei- 

 berg in July 1828, I made observations of the inclination 



