ABOUT twenty years ago we first noted the presence of a γ-type radiation of high penetrability, probably of cosmic origin, passing through the atmosphere. The first fundamental experiments carried out by Hess and improved on by Kolhörster, in which an air-tight electrometer registered the ionisation current at different altitudes, showed us that the current increased with the altitude; at not more than a few thousand metres the current was several times larger than at sea-level. These results indicated the presence of an extremely penetrating radiation, which seemed to be markedly absorbed by the atmosphere above us. Great difficulties were met with in studying the nature of this remarkable radiation, due principally to its extremely weak intensity, high penetrability, and lack of properties affected by external influences. Our knowledge progressed slowly, and only after many years of experimentation were we able to say that the penetrability is about ten times greater than the hard γ-rays of radium, and that the ionisation in air at sea-level is slightly more than 1 ion per second per cubic centimetre. We were able to add that in the explorable regions of the atmosphere the radiation was diffuse, and, further, there seemed to be no relation to the daily movement of the sun. Even at the present time, we cannot say with certainty that the variations observed in the intensity of the radiation are wholly of atmospheric origin or partly an intrinsic property.