EULOGY ON GAY-LUSSAC. 143 



ASCENSION OF GAY-LUSSAC ALONE — CONSEQUENCES OF THE OBSER- 

 VATIONS MADE ON MAGNETISM AND TEMPERATURE — IMPORTANCE 

 OF AERONAUTIC VOYAGES. 



This ascension took place September IG, 1804, at forty minutes after 

 nine in the morning. This time Gay-Lussac ascended to a height of 

 23,000 feet, 7,010 meters above the sea, the greatest well-autbenticated 

 height tbat man had then succeeded in attaining, and which, since that 

 epoch,. has been but once sbghtly exceeded by Messrs. Barrel and 

 Bixio. 



This second ascension has enriched physics with several important 

 results, which I will endeavor to explain in a few words. 



We find, for instance, that at the moment when Gay-Lnssac's ther- 

 mometer, at a height of 7,010 meters, indicated 9o.5 below the freezing 

 point, that of the Observatory of Paris, in the shade, and with a north- 

 ern exposure, stood at + 27°.75. Therefore 37° was the extent of the 

 thermometrical scale to which Gay-Lussac found himself exposed during 

 the interval from 10 o'clock in the morning till" 3 in the afternoon. It 

 was therefore no longer possible to attribute the perpetual snows exist- 

 ing on the summits of high mountains to any special action exerted by 

 ihose rocky summits on the surrounding strata of air, as no considera- 

 ble terrestrial elevation existed in the regions above which Gay-Lussac's 

 balloon had successively passed. 



Are these enormous variations of temperature connected in any way 

 by a simple mathematical law with the changes of height? 



By taking as exact the thermometrical observations, about which 

 Gay-Lussac himself raises some doubts, on account of the rapidity of 

 the ascensional motion of the balloon, and the time required by a 

 thermometer to indicate exactly the temperature of the mediums into 

 which it is immersed, we would arrive at this curious result, that the 

 temperature would vary less for a given change of height near the 

 earth than in the regions of the atmosphere of a mean elevation. 



But I must remark that the ordinary manner of discussing aerostatic 

 observations leads us into a vicious circle. The analytical formula, by 

 means of which the successive heights of the balloon are calculated, 

 absolutely supposes, in fact, an equal abatement of temperature in 

 every region of the atmosphere for the same change of height. Tlie 

 observations of 1801, and those subsequently made, will only give 

 results free from all objection when discussed according to the profound 

 method for which we are indebted to our ingenious and illustrious 

 associate M. Biot. 



The dififlculties might have been avoided if observers, furnished with 

 theodolites, and distributed at proper distances, had determined trigo- 

 nometrically by their combined observations the successive heights 

 of the balloon. Scientists and academies desiring to enter anew upon 



