I 



AERONAUTIC VOYAGES. 337 



Under these circumstances it was evident that a new ascension ought to be 

 undertaken. This time Gay Lussac ascended alone. He rose from the garden 

 of the Conservatory, September 16, 1804, at 9 hours 40 minutes in the morning. 

 Ho alighted at 3 hours 45 minutes, between lloucn and Dieppe, 40 leagues from 

 Paris, near the village of Saint (lourgon. 



The distinguished savant had furnished his aerostat with long cords, designed 

 to moderate its movement of rotation, and he could consequently count more 

 easily the oscillations of the magnetic needle; he obtained the following results: 



Heights. Duration of 10 oscillations. 



meters 42.I6 seconds. 



3,371 " 41.5 



3,857 " 42.0 



4,551 " 42.5 " 



4,294 " 41. « 



4,367 " 43.0 



4,765 ^' '42.2 



4,848 " 42.8 



5,277 " 42.2 " 



5,671 " 42.5 



6,146 " 42.0 



6,182 " 41.0 



6,923 " 41.7 " 



From these observations, which do not present sufficiently appreciable dif- 

 ferences, Gay Lussac drew the conclusion that the magnetic force does not 

 undergo sensible variations up to the greatest heights which we can attain. In 

 regard to this he thus expresses himself: "The consequence which we have 

 drawn from our experiments may seem a little too precipitate to those who 

 remember that we have not been able to make observations on the inclination 

 of the magnetic needle. But when it is remarked that the force which causes 

 a horizontal needle to oscillate is necessarily dependent on the intensity and 

 direction of the magnetic force itself, and that it is represented by the cosine 

 of the angle of inclination of this last force, the conclusion which we have 

 arrived at cannot fail to be drawn, that, since the horizontal force has not 

 varied, the total force cannot have varied, unless one chooses to suppose 

 that the magnetic force may vary precisely in an opposite direction, and with 

 the same relation to the cosine of its inclination, whfch is not at all probable. 

 We have, moreover, in support of our conclusion, the experiment of the incli- 

 nation which was made at the height of 3,902 meters, and which proves that 

 at that elevation the inclination did not vary in a perceptible degree." This 

 conclusion was logical at an epoch when it was not generally known that at a 

 given place and under given circumstances the duration of the oscillations of a 

 magnetic needle is influenced by its temperature. Now, the depression of the 

 thermometer of Gay Lussac had been sufficiently considerable to produce 

 noticeable changes in the magnetic needle. We see that, in the imperfect state 

 of the instruments and the science in 1804, it was impossible to arrive at an 

 exact solution of the problem which the Institute had in view. Even at 

 present this problem is still unsolved. 



The principal result of the aeronautic voyage of Gay Lussac relates to the 

 constant composition of the atmospheric air to a height of 7,000 metres. The 

 illustrious physicist had the good fortune to bring the first air from those high 

 regions, and to give an analysis of it, whose accuracy has been uniformly veri- 

 fied by new experiments conducted with the improved processes which science 

 has discovered during half a century. 



Another fact, no less important, is the wide difference which Gay Lussac 

 found between the temperatures below and at the great height to which Ikj 

 22 s 



