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ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 8 



planes. The crucial experiments, which showed the cosmic nature 

 of the rays, were those of Hess in 1911 and 1912, who took ionization 

 chambers to a height of 5,000 m in balloons. Hess found that the 

 ionization due to the rays was larger at a great height than at sea level. 

 This showed conclusively that the rays causing the ionization must 

 have come downward from the top of the atmosphere, and not upward 



10 20 30 40 50 60 



Figure 1. — Atmospheric pressure (Piccard, Kipfer & Cosyns) . 



• \Pressur e 



70 80 



from the earth; for if they had come upward from the earth, they would 

 decrease in intensity by absorption as one went up. Hess also found 

 that the rays were equally intense both during the day and at night, 

 and also during an eclipse of the sun. This showed that the rays could 

 not come from the sun, because if they had, their intensity would be 

 much less at night, or during an eclipse. Thus it was these experi- 

 ments which led to the rays acquiring the name of "cosmic." 



