AERONAUTIC VOYAGES. 335 



It is impossible to frame a programme whiob will embrace all tlie points 

 wortbj of examination; wc are constrained to admit tbat tlie unforeseen will 

 always play a principal part in aeronautic expeditions. We know little at 

 present of tbe constitution of clouds, of tbe pbenomeua of refrigeration produced 

 by tbcir evaporation, of tbe mixture of strata of air differently saturated witb 

 humidity and derived from very different sources, of tbe action of electricity 

 wbich traverses great aerial spaces, &c. In every case it is desirable tbat, during 

 tbe progress of aerial voyages, tbere should be made, at least from hour to hour, 

 in the principal terrestrial observatories, observations analogous to those which 

 the aeronauts propose to undertake. This was advised in 1S41 by a committee 

 of the British Association, in a report relative to the advantages which science 

 might derive from aerostatic ascensions, a report signed by Brewster, Herschel, 

 Lubbock, Robinson, Sabine, Whewell, and Miller, and the advice was observed 

 by MM. Barral and Bixio, who were thus enabled to connect the phenomena 

 noticed in the higher regions of the air with those which occurred at the same 

 time on the surface of Europe. 



Barometric observations in connexion with those of temperature }-ield, by 

 means of a formula which we owe to the genius of Laplace, the measure of the 

 elevation to which balloons ascend above tbe level of the sea. This formula has 

 been reduced into the usual tables which are found in the Annualre du bureau 

 des longitudes. The considerations on which the illustrious geometer founded 

 his analysis led him to employ in \\.U admirable formula a coefficient whose 

 determination Ramond bad arrived at, by comparing a great number of the 

 measurements of the height of mountains taken with tbe barometer with their 

 trigonometric measurements. Now, as Kamond operated chiefly under the par- 

 allel of 45^, and upou mountains whose elevation scarcely reached 3,000 meters, 

 there is nothing to prove that the undetermined coefficient of Laplace's formula 

 is susceptible of being applied to the measurement of much more considerable 

 heights, and made in other latitudes. It would not be superfluous to measure 

 directly, by observations made from several astronomical stations situated at 

 known distances, the heights to which aeronauts attain, and to compare the 

 results obtained Avith the barometric determinations. No doubt these operations 

 will present numerous difficulties, and may be not uufrequcntly tried without 

 success, because the balloons may disappear in the clouds or be carried in 

 directions which will not permit the terrestrial telescopes to follow them with 

 any advantage. But the problem to which I here call attention merits by its 

 importance the sacrifices which may bo encountered in giving it a satisfactory 

 solution. 



III. — AERONAUTIC VOYAGES OF LHOEST, ROBERTSON, AND SACHAROFF. 



The first aeronautic voyage to which science was indebted for some useful 

 indications was that performed at Hamburg, July 18, 1803, by the physicist, 

 Robertson, accompanied by Ids countryman, Lhoest. They remained suspended 

 in the air five hours and a half, and descended at Hanover, twenty-five leagues 

 distant from the place of departure. 



At tbe moment of the ascension the barometer on the earth stood at 28 inches, 

 and the thermometer at+16^ Reaumur; at the greatest height to which they 

 attained the barometer showed 12.4 inches, and the thermometer — 5^.5 Reaumur. 

 These observations, reduced to metric and centigrade measurements, give 758 

 millimeters for the barometric height, and + 20^ for the temperature at starting ; 

 336 millimeters and — 6^.9 at the highest point reached. Hence, according to 

 the formula of Laplace, we deduce 6,831 meters as the maximum height to 

 which the balloon ascended. 



The two aeronauts thought that at that height they observed the oscillations 

 of the magnetic needle to be much less rapid than at the surface of the earth, 

 and that consequently the magnetic intensity diminishes rapidly as the elevation 



