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LXI. Account of an Aerostatic Voyage performed ly 

 Messrs. Guy-Lussac and Biot. Read in the Ma^he* 

 matical and Physical Class of the French National Insti- 

 tute, August 27, 1804. 



Oince the use of aerostatic machines has become easy and 

 simple, philosophers have been desirous" that they might be 

 employed for making observations which require a great 

 elevation at a distance from terrestrial objects. The mi- 

 nister M. Chaptal afforded a favourable opportunity for re- 

 alizing these projects so useful to the sciences; and Messrs. 

 Berthollet and Laplace having been pleased to interest them- 

 selves in this affair, the minister complied with their re- 

 quest, and M. Guy-Lussac and myself* offered to under- 

 take the expedition. We have performed our first voyage, 

 and we now give an account of it to the class. 



Our principal object was to examine whether the mag- 

 netic property experiences any appreciable diminution on 

 removing from the earth. Saussure, according to experi- 

 ments made on the Col du Geant, at the height of 3435 

 metres, thought he could perceive a very sensible decrease, 

 which he estimated at one-iifth. Some philosophers even 

 "have announced that this property vanishes entirely on 

 ascending from the earth in an aerostatic machine. As this 

 fact is naturally connected with the causes of magnetism, 

 it was of importance to clear up and ascertain the truth of 

 it. At least, such was the opinion of several members of 

 the class, and of the illustrious Saussure himself, who 

 strongly recommends this observation, to which lie several 

 times paid attention in his excursions to the Alps. 



To determine this question a very simple apparatus is 

 sufficient. Nothing is necessary but a magnetic needle, 

 suspended by a very fine silk thread. The needle is turned 

 a little from its magnetic meridian, and suffered to oscillate. 

 The more rapid the oscillations, the greater the magnetic 

 force. This excellent method was invented by M. Borda, 

 and Coulomb gave the means of estimating the force ac^ 

 cording to the number of oscillations. Saussure employed 

 this apparatus during his excursion to the top of the Col 

 du Geant. We carried one of the same kind with- us in our 

 balloon. The needle we employed had been carefully con- 

 structed by that excellent artist Fortin ; and Coulomb was 

 SjD kind as to magnetize it himself by the method of ^Epi- 



* M. Biot. 



nus. 



