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Penetrating Radiation on the Sea

At the beginning of the twentieth century, two scientists, the Austrian Victor Hess and the Italian Domenico Pacini, developed two brilliant lines of research independently, leading to the determination of the origin of atmospheric radiation. Hess measured the rate of discharge of an electroscope that flew aboard an atmospheric balloon. Because the discharge rate increased as the balloon flew at higher altitude, he concluded in 1912 that the origin could not be terrestrial. For this discovery, Hess was awarded the Nobel Prize in 1936, and his experiment became legendary. At the same time, in 1911, Pacini, a professor at the University of Bari, made a series of measurements to determine the variation in the speed of discharge of an electroscope (and thus the intensity of the radiation) while the electroscope was immersed in a box in a sea near the Naval Academy in the Bay of Livorno (the Italian Navy supported the research). Pacini discovered that the discharge of the oscilloscope was significantly slower than at the surface. Before his conclusive measurements, Pacini performed a series of experiments between 1907 and 1910, in particular by comparing radioactivity on ground and on the sea surface a few kilometers off the coast. These experimennts gave marginal indications on the presence of a non-terrestrial component, encouraging Pacini to continue his research till the conclusive results. Such first experiments on the sea are presented in the article by D. Pacini in Le Radium VIII (1911) 307, which is essentially a translation into French of another article published in Italian in Ann. Uff. Centr. Meteor. XXXII, parte I (1910); this is a translation in English of the article by Pacini in "Le Radium", article which was quoted by Hess in his famous paper.