THE more important agencies which may conceivably cause the ionisation of the upper atmosphere of the earth are the ultra-violet light, α- and β-particles, all of solar origin, the penetrating radiation of cosmic origin, and the ionising radiations from terrestrial sources. The last mentioned may perhaps be ruled out immediately because of the fact that the conductivity of the lower atmospheric strata increases rapidly with the height for the first few kilometres. The possible effects of these ionising agencies have been considered in papers by Chapman and Milne, Benndorf, Elias, Lassen, and others. Recently, experiments with the electromagnetic waves of wireless telegraphy, together with theories of the propagation of these waves over the surface of the earth, have led to more definite information about the ionisation in the upper atmosphere, and it has been of interest to examine again the causes of the ionisation.