336 AEEONAUTIC VOYAGES. 



iu the atmnsplicro increases. They also reported that they had experienced 

 much pliyt^ical t^uffering, and observed phyt>iological phenomena, f^uch as the 

 Bwelliiijj of the lips and veins, the bleeding of the eyes, tScc, which have not 

 been uniformly verified in subsequent expeditions. 



However this might be, the Academy of Sciences of St. Pctersburgh deter- 

 mined on a repetition of the experiment to be made by Robertson himself, assisted 

 by Sacharoff. one of its own members, distinguished both as a physicist and 

 chemist. This second expedition took place June 30, 1804. The aeronauts 

 ascended from St. Pctersburgh at 7 hours 45 minutes p. m., and descended at 

 10 hours 45 minutes, near Sivoritz, at a distance of about 20 leagues. At the 

 moment of departure the barometer stood at 30 inches, and the thermometer at 

 19^ llcaumur; at the greatest elevation the two instruments indicated respect- 

 ively 22 inches and 4^.5 Reaumur. We conclude from these observations that 

 the barometric pressure and the temperature Averc, at the point of departure, 812.1 

 millimeters, and + 23°.7; at the greatest elevation, 595.5 millimeters and + 5^.6; 

 and from this it results that the highest point reached was 2,703 meters. MM. 

 Robertson and Sacharoff were not able to make re";ular mag-netic observations, 

 but they felt authorized to affirm that the needle of declination had ceased to 

 be horizontal, and that its north pole Avas elevated about 10 degrees, its south 

 pole having an inclination of the same amount towards the earth. 



IV. — VOYAGES OF BIOT AND GAY LUSSAC. 



Saussure, after a scries of observations made on the Col du fleant at a height 

 of 3,435 meters, conceived it to be ascertained that at that height the magnetic 

 intensity undergoes a sensible diminution, which he estimated at about one-fifth. 

 This result appeared to bo verified by the aeronautic voyages of Robertson, 

 Lhoest, and Sacharoff, just spoken of. But the proof's of the fact were not 

 given in a sufficiently decisive manner to secure it a definitive reception into 

 science, and the question appeared important enough to the principal members 

 of the Institute, Laplace, Berthollet, Chaptal, to jusl^ify a special experiment. 

 This Avas intrusted to MM. Eiot and Gay Lussac, who ascended from the garden 

 of the Conservatoire clcs arts ct metiers, August 24, 1804, provided with all the 

 necessary instruments. The small dimensions of the balloon did not allow the 

 two aeronauts to reach the height of more than 4,000 meters, and at that eleva- 

 tion the temperature, Avhich had been+17°.5 on the earth, had only sunk to 

 + 10^.5. Leaving at 10 o'clock in the forenoon, they descended, about half 

 after one, 18 leagues from Paris, in the department of Loiret. Taking advantage 

 of the moments when the movement of rotation of the balloon in one direc- 

 tion stopped, being about to be resumed in the opposite direction, the learned 

 physicists were able to determine the duration of five oscillations of the magnetic 

 needle iu different aerial strata, and they obtained the following results : 



Heights. Duration of 5 oscillations. 



meters 35.25 seconds. 



2,8G2 " 35 



2,897 " 35 



3,038 " 35 " 



3,589 " 34 



3,6G5 "* 35.5 " 



3,742 " 35 " 



3,845 " 36 



3,977 " 35 



Thus the observations agree in giving 35 seconds for the duration of five 

 oscillations, or at least the observed difiercuces arc too small to allow of any 

 conclusiou being drawn from them. 



