In Milan cosmic-rays studies are going on since 1932 [1] when, just a few years after the birth of the local University, Giovanni Polvani got the chair of Experimental Physics at the Science Faculty. His assistant G. Cocconi, just arrived from Rome, began observation using counters and cloud chambers installed in Milan and at mountain altitudes (Passo Sella on the Dolomites). With the help of V. Tongiorgi and G. Loverdo, Cocconi intended to study the composition of the secondary radiation produced in the Earth atmosphere and the nature of the primary radiation which arrives at the top of the atmosphere from the external space.