AbstractAbout a century ago, cosmic rays were identi ed as being a source of radiationon Earth. The proof came from two independent experiments. The Italianphysicist Domenico Pacini observed the radiation strength to decrease whengoing from the surface to a few meters underwater (both in a lake and ina sea). At about the same time, in a balloon ight, the Austrian VictorHess found the ionization rate to increase with height. The present articleattempts to give an unbiased historical account of the discovery of cosmicrays { and in doing so it will duly account for Pacini’s pioneering work,which involved a technique that was complementary to, and independentfrom, Hess’. Personal stories, and the pre- and post-war historical context,led Pacini’s work to slip into oblivion.1. IntroductionIt is generally well known, since Faraday’s early observations, that elec-troscopes spontaneously discharge. This phenomenon remained unexplaineduntil the beginning of the XX century: its explanation paved the way to oneof mankind’s revolutionary scienti c discoveries: cosmic rays.Since the early XX century cosmic rays were used to probe and under-stand the constituents of matter. Indeed many early discoveries in particlephysics (antimatter, mesons, muons, baryons, ...) were made while studyingcosmic rays. Cosmic rays are still being used in the framework of fundamentalphysics, as well as to investigate astrophysical properties of their sources.In 1896 the French physicist Henri Becquerel discovered the instability ofsome chemical elements. Some years later Marie and Pierre Curie discovered