In 1937, with Walter Heitler, he co-authored a paper, "The passage of fast electrons and the theory of cosmic showers" in the Proceedings of the Royal Society, Series A, in which they used their theory to describe how primary cosmic rays from outer space interact with the upper atmosphere to produce particles observed at the ground level. Bhabha and Heitler then made numerical estimates of the number of electrons in the cascade process at different altitudes for different electron initiation energies. The calculations agreed with the experimental observations of cosmic ray showers made by Bruno Rossi and Pierre Victor Auger a few years before. Bhabha and Heitler postulated that the penetrating component of cosmic radiation comprised "heavy electrons", most of which "must have masses nearer to hundred times the electron mass". The paper was announced in a letter in Nature.