About twenty years ago, the English theoretical physicist Dirac published some speculations on the possibility that there exists a particle endowed with a magnetic charge instead of an electric charge: in other words, an isolated magnetic pole, a monopole. This is a particle that no physicist has so far “seen” but whose existence is, at least theoretically, possible. Now my interest in this particle was revived, about a couple of years ago, by the arrival of the first (incomplete, fragmentary and largely erroneous) news about the discovery of heavy particles in cosmic radiation. To be precise, it was learned that, on photographic plates sent to great heights, particles had been observed whose ionizing power appeared to be much more intense than that of light particles or the usual heavy particles such as protons and \(\upalpha \) -particles; and at first, in some discussions with my colleague Edward Teller, the possibility was examined that these particles instead of heavy nuclei were Dirac monopoles.