CONTEMPORARY ADVANCES IN PHYSICS 



163 



The observations had thus been extended over the range, of metres- 

 of-water beneath the "top of the atmosphere," from 8.45 to 69. 

 During the three following years Millikan and Cameron made yet 

 another and more-closely-spaced aggregate of readings, over the same 

 range with slight extensions at both ends, in the same two lakes, with 



Fig. 6 — Electroscope used in later under-water measurements of cosmic rays by 



Millikan and Cameron. 



an electroscope (Fig. 6) into the 1622 cubic centimetres of whose 

 volume air was forced to a pressure of 30 atmospheres. The reader 

 can find the new curve in the issue of the Physical Review for February 

 1931; it is the one from which the next-cited deductions are drawn; 

 but as it does not differ markedly from its two predecessors, and is 

 much more trying to the eyes than these, I do not reproduce it here. 

 (In studying it, or the curve here pictured as Fig. 5, one must remem- 

 ber that the ordinate is proportional to the number of ions appearing 

 per second per cubic centimetre, not of the atmosphere at the level in 

 question, but of air of a standard density.) 



Is the whole of the curve, abstraction being made of the Restgang, 

 an exponential curve such as would be found if the cosmic rays were a 

 beam of gamma-rays of a single frequency descending vertically from 



