142 EULOGY ON GAY-LUSSAC. 



air with advantage. From his voyage made December 1, 178*3, in com- 

 pany with Robert the artist, iu a balloon thus inflated, date ascensions 

 infinitely less adventurous, and which iu our day have become a pastime 

 for idlers. 



It is to the original Academy of Sciences we must likewise go back, 

 if we wish to find one of the first scientifically useful ascensions made 

 with hydrogen-gas balloons. 



It seemed to result from the experiments made during an ascension by 

 Robertson and Lhoest at Hamburg, July 18, 1803, and renewed at St. Pe- 

 tersburg, under the auspices of the Imperial Academy of that city, by the 

 same Robertson and the Russian physicist, Saccharoff, June 30, 1S04, that 

 the maguetic force which directs the needle at the surface of the earth 

 grew considerably weaker in proportion as they rose in the atmos- 

 phere. This fact, which confirmed the diminution of this same force 

 that M. de Saussure supposed he had discovered in his celebrated jour- 

 ney to the Col du Geant, seemed to the priucipal members of the Insti- 

 tute, with good reason, to justify an especial experiment. This was 

 confided to the physicists, Messrs. Biot and Gay-Lussac, both young, 

 enterprising, and courageous. This last term may perhaps seem some- 

 what exaggerated to those who, in our day, have seen women, aping 

 in their costumes winged butterflies, placed entirely outside of the car 

 of the aerostat, rise from our public gardens, before the eyes of a won- 

 der-struck crowd. But such would forget that now balloons are con 

 structed with infinitely more care, and the means of safety have very 

 much increased. 



Our two physicists ascended from the garden of the Conservatoire 

 des Arts et Metiers August 24, 1801, furnished with all the instruments 

 necessary for investigation, but the small dimensions of their balloon 

 did not allow them to exceed a height of 4,000 meters. At this eleva- 

 tion they endeavored, with the aid of the oscillations of a horizontal 

 magnetic needle, to solve the problem which had been the chief object 

 of their ascension, but the rotary motion of the balloon presented unfore- 

 seen and serious obstacles. They succeeded, however, in partly sur- 

 mounting them, and they determined, in these aerial regions, the dura- 

 tion of five oscillations of the magnetic needle. It is known that this 

 duration must increase when the magnetic force which briugs back the 

 needle to its natural position has decreased, and that this duration must 

 be shorter, as the same directing force has increased. It is therefore a 

 case entirely analogous to that of the oscillating pendulum, although 

 the motion of the needle is performed iu a horizontal direction. The 

 consequences deduced from their experiments seem to me subject to 

 difficulties, which I shall point out after giving an account of the ascen- 

 sion made a few days later by Gay-Lussac alone. 



