104 



TERRESTRIAL MAGNETISM. 



indeed all remarked a decrease of magnetic force with increas- 

 ing elevation. It would seem, from the general discussion by 

 Bravais, as if the decrease were greater in the Pyrenees than 

 in the Alps.( 113 ) 



Quetelet's entirely opposite results, obtained in a journey 

 from Geneva to the Col de Balme and* the great St. Ber- 

 nard, render it doubly desirable that in order to obtain a 

 finally decisive reply to so important a question, recourse 

 should be had to the only effectual means, i. e. balloon 

 ascents, in which the surface of the earth is quitted alto- 

 gether, (a step taken so long ago as 1804, by Gay-Lussac, 

 first conjointly with Biot on the 24th of August, and then 

 alone on the 16th of September), and that a consecu- 

 tive series of experiments should be thus made. The time 

 of vibration of needles carried in aeronautic ascents to 

 elevations of eighteen or twenty thousand feet and upwards, 

 can only be made to afford just inferences respecting the 

 degree of the earth's magnetic force propagated through the 

 free atmosphere, when the temperature correction of the 

 needles employed is determined with great precision both 

 before and after the ascent. The neglect of any such correction 

 caused it to be erroneously inferred from Gay-Lussac's 

 experiments that the magnetic force remains unaltered to an 

 elevation of 21600 feet;( 114 ) whereas the experiment really 

 went to indicate the decrease of the force, since the colder 

 temperature of the higher region must be supposed to have 

 acted in accelerating the vibrations of the needle. ( 115 ) 

 Neither ought Faraday's brilliant discovery of the paramag- 

 netic force of oxygen to be left out of view in considering 

 the subject before us. That great physicist calls attention 

 to the consideration, that the cause of the diminution of the 



