  During a series of experiments performed between 1907 and 1911, Domenico
Pacini (Marino 1878-Roma 1934), at that time researcher at the Central Bureau
of Meteorology and Geodynamics in Roma, studied the origin of the radiation
today called "cosmic rays", the nature of which was unknown at that time. In
his conclusive measurements in June 1911 at the Naval Academy in Livorno, and
confirmed in Bracciano a couple of months later, Pacini, proposing a novel
experimental technique, observed the radiation strength to decrease when going
from 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 in
Physics in 1936, two years after the death of Pacini, who had become a full
professor of Experimental Physics at the University of Bari. The discovery of
cosmic rays -a milestone in science- involved several scientists in Europe and
in the United States of America and took place during a period characterized by
nationalism and lack of communication. Historical, political and personal
facts, embedded in the pre- and post-World War I context, might have
contributed to the substantial disappearance of Pacini from the history of
science. This article aims to give an unbiased historical account of the
discovery of cosmic rays; in the centenary of Pacini's pioneering experiments,
his work, which employed a technique that was complementary to, and independent
of that of Hess, will be duly taken into consideration. A translation into
English of three fundamental early articles by Pacini is provided in the
Appendix.
