Fifty years ago seismology was still in early infancy although a seismoscope to indicate the direction of the maximum movement during earthquakes was known in China 2000 years ago, similar instruments were used in Europe over 200 years ago, and during the eighteenth century the effects of earthquakes were investigated in several instances. About 90 years ago Mallet started his investigations on the mechanics of earthquakes; he suggested better instruments to observe them, tried to determinethe wave velocity by using artificial explosions, prepared a catalogue of earthquakes, and developed a fair knowledge of the geographical distribution of epicenters. However,...