Determination o/Temperature and Pressure in Balloon Ascents. 213 



must be the result ; consequently the question may be raised, 

 How great must the alteration of disgregation be in order to 

 bring about a given change of temperature, it being assumed 

 that all alterations of condition take place reversibly ? In this 

 case we must apply equation (II.), putting ^Q=0, whereby it is 

 transformed into 



ff+JWo. 



28) 



If we assume, for the sake of simplicity, that the temperature of 

 the entire body varies uniformly, so that T has the same value 

 for all parts, we may apply equation (26) to the determination 

 of the first of the two integrals; and we thus obtain, for the altera- 

 tion of disgregation sought, the equation 



Z-Z =wclog^ (29) 



If we desired to cool a body down to the absolute zero of tem- 

 perature, the corresponding alteration of disgregation, as shown 

 by the foregoing formula, in which we should then have T = 0, 

 would be infinitely great. Herein lies a chief argument for sup- 

 posing it to be impossible to produce such a degree of cold, by 

 any alteration of the condition of a body, as to arrive at the abso- 

 lute zero. 



XXX. On the determination of Temperature and Pressure in 

 Balloon Ascents. By M. V. Uegnault*. 



[With a Plate.] 



THE instruments usually employed to observe the tempera- 

 tures and pressures in the various atmospheric strata which 

 an aeronaut traverses, are difficult to observe and manage in the 

 exceptional conditions in which the observer is placed. The 

 mercurial barometer is fragile, its verticality doubtful, the 

 mercurial column continually oscillates, and only one of its 

 menisci can be* observed. The aneroid is less fragile, the ob- 

 servation is easier; but its indications are uncertain for great 

 variations of temperature, because the scale of the instrument 

 often undergoes a derangement. Lastly, these instruments only 

 give accurate indications when they are handled by a skilful 

 experimenter — and only then, provided he is not distracted by 

 the novelty of the spectacle offered to his view, and that he has 

 not to occupy himself with the manoeuvring of the balloon. 



It appeared useful to me to seek for a method which might be 

 practised with certainty by any aeronaut, and which required 



* Translated from the Annates de Chimieet de Physique, February 1862, 



