condensation of water and the formation of the clouds, no longer exist. 

 Thus the stratosphere is rightly termed the region of perpetual good 

 weather. It is because it commences at 7^ miles, as an average in our 

 regions, that aviators in their everyday usage give this altitude as its 

 lower limit. 



It was to this high region, to be more precise to an altitude of 10 

 miles, that I wished to ascend to meet the cosmic rays in order to 

 observe them in mass, where their initial properties would not yet 

 have been too modified by collisions with the molecules of our 

 atmosphere. 



For a number of investigations, use had been made of sounding- 

 balloons, the classic free balloon scarcely allowing man to do useful 

 work beyond 3I- to yf miles. Beyond this range, in fact, the air is 

 too rarefied for our organism, and even if the aeronauts have an 

 equipment permitting them to breathe pure oxygen, they cannot 

 stay for long above about 7^ miles. The sounding-balloon was thus, 

 for meteorologists, the sole means of exploring the high atmosphere. 

 A generation had laboured to devise automatic instruments for record- 

 ing pressure, temperature and humidity. But the measurement of 

 cosmic rays was a delicate operation very different in nature, and 

 could not be effected at the time with the necessary precision by these 

 automatic instruments. That is why I decided to ascend myself to 

 10 miles. Luckily I was licensed as a free-balloon pilot and I had 

 already made a dozen ascents. May I here relate how I became an 

 aeronaut ? 



Like most young men of my time, I had a passion for everything 

 related closely or remotely to this new science. It was the epoch when 

 the heavier-than-air machine was making its first essays and when 

 only optimists foresaw the future development of aviation: the 

 lighter-than-air machine was still king of the sky. As a young physicist 

 I naturally read all the aeronautical journals within reach. A question 

 was being discussed in them by specialists : that of the distribution of 

 the gas temperatures in the interior of spherical balloons. Now, I did 

 not agree with the published results. These seemed to me to be in 

 contradiction with theory, and this was explained by the fact that the 

 method of measurement chosen was not suitable. It was necessary to 

 take the measurements again in better conditions. I addressed myself 

 to the Swiss Aero-Club (Aéro-Club Suisse) which, understanding the 

 importance of the problem, permitted me to make several ascents with 



