COSMIC RADIATION — BLACKETT 177 



Since that time a very great number of experiments have been made 

 to find out how the cosmic radiation varies in intensity in the atmos- 

 phere. Perhaps the most beautiful of these, and the most successful, 

 have been the balloon flights of Regener and lately those of Millikan, 

 who send very light recording apparatus up to great heights by means 

 of small hydrogen-filled rubber balloons. A small ionization chamber, 

 which is made to record automatically the intensity of the rays on a 

 photographic plate, together with the barometric height and the 

 temperature, is made to weigh only a few pounds. This is enclosed in 

 a light frame covered with cellophane, which acts like an ordinary 

 greenhouse; the sun's rays coming in through the cellophane get 

 degraded into heat and cannot get out again. In this way the appara- 

 tus is kept warm at about room temperature, even though the tem- 

 perature outside the apparatus is —50° centigrade. The rubber 

 balloons which are used to take the apparatus up have to be examined 

 very carefully for small pinholes, otherwise they are apt to burst 

 prematurely, before the desired height is reached. Professor Regener 

 sometimes has to stop up 100 or so tiny holes with rubber solution be- 

 fore using a balloon. With such an apparatus, heights of 30 km above 

 the surface of the earth have been reached, where the pressure of the 

 atmosphere is only about 1 percent of that at sea level. It is found 

 that the intensity of cosmic radiation is about 200 times as great as 

 on the ground, confirming, of course, the view of Hess that wherever 

 it is the rays come from it is at least outside the earth's atmosphere. 

 Figure 1 shows a typical curve of the variation of the ionization with 

 the pressure of the- atmosphere. 



With similar, but much larger apparatus, the ionization due to 

 cosmic radiation has been studied under water, down to great depths. 

 Regener, again, has measured the ionization down to depths of 280 m 

 below the surface of Lake Constance, and finds that their intensity is 

 only about 1 percent of that at sea level. So from the bottom of 

 Lake Constance to the top of the stratosphere the intensity of cosmic 

 rays increases by a factor of over 10,000 to 1. The enormous pene- 

 tration of the rays, a penetration quite unexpected in the region of 

 atomic physics, leads naturally to the conclusion that the rays must 

 be of immense energy. The most penetrating atomic rays previously 

 known, the beta rays from radium, can only penetrate a few centi- 

 meters of water. Thus, the cosmic radiation is many hundred times 

 more penetrating, and is, therefore, likely to be very much more 

 energetic. In fact, it can be easily estimated that, to explain the very 

 great penetrating power of some of the rays, it is necessary to assume 

 energies up to 10 11 electron volts. 



The next great series of experiments consisted in the carrying of 

 ionization chambers all over the world. Expeditions to the equator, 

 to near the north magnetic pole, expeditions on mountains and in 



