During the 1930s, cosmic rays were the most important source of data on the high‐energy behavior of both quantum electrodynamics and nuclear forces. In the period 1934–1938, with which this article is concerned, the dominant fundamental theory of nuclear forces was that of the Fermi field. In sorting out the various cosmic‐ray phenomena in the atmosphere, it was found that the less penetrating components were associated with electromagnetic cascade showers, and that the more penetrating component contained a new ‘‘elementary’’ particle, the mesotron. However, there remained puzzling features of the cosmic rays that left adequate room for other interpretations.