IN 1925, when it was still believed that a potential of n × 109 V was necessary to cause a lightning flash1a, C. T. R. Wilson put forward the suggestion that the soft cosmic rays up to about 109 eV might be accounted for by runaway electrons in thundercloud fields1b. When it was pointed out that a lightning flash can occur with a total thundercloud potential of n × 107 V (refs. 2a,b), the suggestion ceased to have much significance as applied to thunderclouds. However, in presenting a paper on these long leader strokes at the Belfast meeting of the British Association in 1952, I pointed out2c that the Wilson mechanism might apply in the much longer cosmic electrical discharges, in which relatively large electric fields must be built up by the short-circuiting of these much more extensive electric fields by the leader stroke of such discharges as solar flares.