Recent Developments in Cosmic Rays *

I N order to place the results of the recent studies of cosmic rays in appropriate perspective, let us recall very briefly their early history. It is well known how at the beginning of the present century C. T. R. Wilson,l and Elster and GeiteJ,2 showed that normal air is slightly ionized, and how McClennan and Burton3 at Toronto, and Rutherford and Cooke4 at Montreal showed by use of absorbing screens that a considerable part of this ionization is due to a penetrating radiation coming from outside the ionization chamber. This radiation was generally supposed to be the gamma-rays from radioactive materials known to occur in the ground and the air. About 1910, however, observations on high towers by WuIf6 and a series of balloon flights by Gockel6 showed that the intensity of these ionizing rays decreases less rapidly with altitude than could thus be explained. These striking results led to further balloon observations by Victor Hess7 • 8 and W. Kolh6rster,9 who established the fact that the penetrating radiation actually increases in intensity with increasing altitude, which, of course, it should not do if it emanted from the ground. From these studies Hess8 drew the bold conclusion in 1912 that a penetrating type of radiation enters our atmosphere from without, and .