Research in cosmic rays centered on Scott Forbush, indeed one could say with fair accuracy that it was a Department field that belonged to him alone. He was joined by visiting investigators, enjoyed associations with colleagues worldwide and had the help of a capable assistant, but he was the only staff member who devoted a significant part of his time to this discipline. Forbush joined the Department in 1926 as an observer at the Huancayo Observatory and served on the Carnegie for the last part of its final cruise, on which he made, among others, cosmic-ray measurements. Exciting things were happening in cosmic-ray research. Their nature had just been settled by establishing that they were particles, not gamma photons; a significant fraction had been demonstrated to have incredible energies, and there were startling bursts or showers that became more frequent with altitude. Where and how they originated was the great question, one that attracted adventurous minds. That their intensity was to some extent modulated by the geomagnetic field naturally aroused DTM interest; indeed the first evidence for their particulate nature came from the lower intensities observed near the geomagnetic equator, a property conflicting with a photon nature.

It was evident from the start that studying cosmic rays required global observation, and as the Department was preeminent in this kind of operation and the Mount Wilson Observatories preeminent in astronomy, the formation of the Committee on Coordination of Cosmic-Ray Investigations in December 1932 at the request of Robert A. Millikan and Arthur H. Compton by President Merriam came naturally.