Ideas of the ancients (the views of Travagini, Hooke, Woodward, Stukeley, Mitchell, Young, Mallet)—Nature of elastic waves and vibrations—Possible causes of disturbance in the Earth’s crust—The time of vibration of an earth particle—Velocity and acceleration of a particle—Propagation of a disturbance as determined by experiments upon the elastic moduli of rocks—The intensity of an earthquake—Area of greatest overturning moment—Earthquake waves—Reflexion, refraction, and interference of waves—Radiation of a disturbance. _Ideas of Early Writers._—One of the first accounts of the varieties of motion which may be experienced at the time of an earthquake is to be found in the classification of earthquakes given by Aristotle.[8] It is as follows:— 1. Epiclintæ, or earthquakes which move the ground obliquely. 2. Brastæ, with an upward vertical motion like boiling water. 3. Chasmatiæ, which cause the ground to sink and form hollows. 4. Rhectæ, which raise the ground and make fissures. 5. Ostæ, which overthrow with one thrust. 6. Palmatiæ, which shake from side to side with a sort of tremor. From the sixth group in this classification we see that this early writer did not regard earthquakes as necessarily isolated events, but that some of them consisted of a succession of backward and forward vibratory motions. He also distinguishes between the total duration of an earthquake and the length of, and intervals between, a series of shocks. Aristotle had, in fact, some idea of what modern writers upon ordinary earthquakes would term ‘modality.’ The earliest writer who had the idea that an earthquake was a pulse-like motion propagated through solid ground appears to have been Francisci Travagini, who, in 1679, wrote upon an earthquake which in 1667 had overthrown Ragusa. The method in which the pulses were propagated he illustrated by experiments. Hooke, who, in 1690, delivered discourses on earthquakes before the Royal Society, divides these phenomena according to the geological effects they have produced; thus there is a _genus_ producing elevations, a _genus_ producing sinkings, a _genus_ producing conversions and transportations, and a _genus_ which produces what, in modern language, we should term metamorphic action. Woodward, in his ‘Natural History,’ written in 1695, speaks of earthquakes as being agitations and concussions produced by water in the interior of the earth coming in contact with internal fires. Stukeley observed that an earthquake was ‘a tremor of the earth,’ to be explained as a vibration in a solid. The Rev. John Mitchell, writing in 1760, says that the motion of the earth in earthquakes is partly tremulous and partly propagated by waves.