"Since the origin of the "penetrating rays" was still uncertain, Dr. Russell Otis and myself in the summer of 1923 went to the top of Pike's Peak for the sake of making absorption experiments upon these radiation at the highest altitude to which we could carry large quantities of absorbing materials. For if the rays were not of cosmic origin they did not need to be more penetrating than are the gamma rays from radioactive materials, while if they were of cosmic origin the sounding balloon experiments of Bowen and myself had shown that they must be very much harder (more penetrating) than anybody had thus far assumed. What was needed was absorption experiments to determine just what sort of rays they actually were."