AERONAUTIC VOYAGES. 341 



renounce the proposed ascension. I made, in tte presence of the two aeronauts, 

 the observation that it might be very useful to know the decrease of the atmos- 

 pheric temperature with the height when a continuous screen of clouds shuts 

 from us the view of the sky.* Now it sometimes happens that the sky 

 becomes clear of a sudden ; in this case there must remain iu the atmosphere 

 traces more or less marked of the abnormal deci;ease of temperature of which 

 the presence of the cloud had been the cause. The observations made iu 

 aerostatic ascensions, performed during clear weather, are not completely 

 applicable to this special case. Besides, there are numerous occasions when 

 we observe through openings in the clouds. When MM. Barral and Bixio 

 arrived at the conclusion, from these considerations and others which it Vi'ould 

 be superfluous to mention, that their voyage might prove useful, they placed 

 themselves in the car and launched into the air. 



" All the details of this ascension are scrupulously given iu the journal 

 written at the time by the aeronauts, and the calculations were compared by 

 M. Regnault with the indications of the sealed instruments carried in the expe- 

 dition. I shall only advert here to the fact that at their greatest elevation our 

 explorers experienced no uneashiess or embarrassment in their respiration ; and 

 that M. Bixio, who had suffered iu his first voyage from acute pain in the ears, 

 guarded against that annoyance by simply counterfeiting from time to time 

 the act of deglutition, by Avhich the air within and without the organ was 

 maintained in a state of equal pressure. It may be added, that they encoun- 

 tered a mass of cloud of more than 5,000 meters iu thickness, that they did not 

 succeed in rising entirely above it, but at the height of about 7,000 meters 

 (22,960 feet) were forced to commence an involuntary descent, the effect of a 

 rent in the lower part of the ballocn. They might, perhaps, by throwing out 

 the last of their ballast, have prolonged their stay at the height which they 

 bad readied, but circumstances no longer permitting them to gather useful indi- 

 cations for science, they thought best not to struggle against the downward 

 tendency of the apparatus. * 



" Let us speak novv^ of the observations which they had an opportunity of 

 making. When they had attained their highest station in the immense bed of 

 cloud, an opening took place in the vaporous mass Avhich surrounded them, 

 through which the blue sky was apparent. The polariscope, directed towards 

 this region, showed an intense polarization ; on the contrary, there was none at 

 all, when the instrument was pointed aside beyond the opening. This should 

 not be regarded as a repetition of the experiment made in the first voyage, for 

 then they observed the light reflected by the clouds, while now it was in the 

 tran,smitted light that they verified the absence of all polarization. 



"An interesting optical phenomenon was exhibited during this ascension. 

 Before attaining the highest limit, the bed of cloud which enveloped the bal- 

 loon, having diminished in thickness or become less dense, the sun appeared 

 weak and quite white ; at the same time there appeared, below the horizontal 

 plane of the car, at an angular distance from that plane equal to the angle 

 formed by the sun's height, a second sun similar to one which might have been 

 reflected from a sheet of water situated at that elevation. It is natural to sup- 

 pose, with our aeronauts, that the second sun was formed by the reflection of 

 the luminous rays on the horizontal faces of crystals of ice floating in that 

 vaporous atmosphere. 



" We now come to the most striking and wholly unexpected result furnished 

 by the thermometrical observations. Gay Lussac, in his ascension in clear or 

 rather slightly vaporous weather, had found a temperature of 9°. 5 below zero 



* The refractions at moderate heights depend on the law according to which this decrease 

 is effected. 



