Originally published in 1939, this book presents the revised content of three lectures delivered at the University of Virginia and Trinity College, Dublin during 1936 and 1937, respectively. The text provides an accessible discussion of cosmic rays, drawing on the results of research carried out at the Norman Bridge Laboratory of the California Institute of Technology. Illustrative figures and notes are included throughout. This book will be of value to anyone with an interest in cosmic rays and the history of science.