During a series of experiments performed between 1907 and 1911, DomenicoPacini (Marino 1878-Roma 1934), at that time researcher at the Central Bureauof Meteorology and Geodynamics in Roma, studied the origin of the radiationtoday called "cosmic rays", the nature of which was unknown at that time. Inhis conclusive measurements in June 1911 at the Naval Academy in Livorno, andconfirmed in Bracciano a couple of months later, Pacini, proposing a novelexperimental technique, observed the radiation strength to decrease when goingfrom the surface to a few meters underwater (both in the sea and in the lake),thus demonstrating that such radiation could not come from the Earth's crust.Pacini's work was largely overlooked. Hess was awarded the Nobel Prize inPhysics in 1936, two years after the death of Pacini, who had become a fullprofessor of Experimental Physics at the University of Bari. The discovery ofcosmic rays -a milestone in science- involved several scientists in Europe andin the United States of America and took place during a period characterized bynationalism and lack of communication. Historical, political and personalfacts, embedded in the pre- and post-World War I context, might havecontributed to the substantial disappearance of Pacini from the history ofscience. This article aims to give an unbiased historical account of thediscovery of cosmic rays; in the centenary of Pacini's pioneering experiments,his work, which employed a technique that was complementary to, and independentof that of Hess, will be duly taken into consideration. A translation intoEnglish of three fundamental early articles by Pacini is provided in theAppendix.
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