Robert Millikan originally set about to disprove Hess and Kolhörster’s discovery. He and Ira Sprague reached a height of 1500 m in a balloon over Texas where they recorded a radiation intensity of approximately one quarter of Hess and Kohörster's measurement. The difference was caused by a geomagnetic difference between Texas and Central Europe but was blamed on turnover in the intensity curve at high altitude.
Millikan and Harvey Cameron reported on experiments on high-altitude lakes in 1926. They measured ionization rates at various depths in lakes at altitudes of 1500 m and 3600 m. The underwater rate of the lower lake corresponded to the rate obtained 2 m deeper in the higher lake. The pair concluded that particles shoot through space equally in all directions. This demonstrated that two metres of water absorbed about the same as two kilometers of air, and convinced Millikan that rays do come from above.
Millikan was convinced that penetrating radiation entering the atmosphere was electromagnetic and coined the term ‘cosmic rays’ in a paper where he argued that cosmic rays were the ‘birth cries of atoms’ in the galaxy.
Read more: "The Origin of the Cosmic Rays" – R.A. Millikan, G.H. Cameron, Physical Review Letters, 32 (1928) 533