By the time of the Bagnères-de-Bigorre conference, Rossi had already turned his attention toward the astrophysical implications of cosmic ray phenomena, particularly extensive air showers. After Rossi's recognition, in Eritrea, that these events exist, they were extensively studied by Pierre Auger, and by Williams. At this time, the extremely fast response of the newly developed scintillation counters offered a new way to study the structure of air showers. To do this, Rossi enlisted his student, George W. Clark, who completed a PhD in 1952, and Piero Bassi, who was a visitor from the University of Padua. Because solid scintillating material was unavailable, they decided to use terphenyl dissolved in benzine, which is an efficient liquid scintillator. With the aid of three counters deployed on the roof of the MIT Physics building during the winter of 1952/53, they found that shower particles arrived within only one or two meters of a disk, which travels at nearly the speed of light in the direction of the shower axis.