two counters. Rossi himself proposed that very extended showers occasionally arrived upon the instruments. Rossi’s hypothesis was confirmed at the end of the 1930s by the work of Pierre Auger and Roland Maze, who analysed for the first time the structure and dimension of extensive air showers [@AugerMaze1939]. It is worth mentioning, stopping just for a moment the chronological narration, that the very extended air showers were to be fundamental in the study – carried out by Rossi himself and his collaborators in the USA by the end of the 1940s – of the highest energy part of the cosmic radiation spectrum: actually, the measurements on extensive showers were precious in giving indirectly quantitative details on the primary cosmic-ray particles of very high energy. As Rossi wrote severalas a secondary radiation produced from the penetrating rays, which were regarded as the primary radiation. However, further experiments carried out in Padua in collaboration with several young scientists led Rossi to change some his views.[^14] In particular, with De Benedetti, Rossi measured the intensity of both kind of radiations at increasing altitudes,[^15] and he observed that the intensity of the soft radiation increased much more rapidly than the intensity of the penetrating hard radiation, so that the two radiations seemed to be regarded both as primary radiations, independent one from another.

A few years later, in October 1937, a Conference was held in Bologna on the occasion of the second centenary of Luigi Galvani’s birth, and several of the most important physicists of those years were invited to