42 EARTHQUAKES. 



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 earth- 

 quake 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 earth- 

 quake 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 Eagusa. The method in which the pulses 

 were propagated he illustrated by experiments. 



Hooke, who, in 1690, delivered discourses on earth- 

 quakes before the Eoyal Society, divides these phenomena 

 according to the geological effects they have produced;" 

 thus there is a genus producing elevations, a genus pro- 

 ducing sinkings, a genus producing conversions and trans- 

 portations, 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. 



Stuckeley observed that an earthquake was ' a tremor 

 of the earth,' to be explained as a vibration in a solid. 

 The Eev. John Mitchell, writing in 1760, says that the 

 motion of the earth in earthquakes is partly tremulous 

 and partly propagated by waves. 



From these few examples, to which might be added 

 many more, it will be seen that an earthquake disturb- 

 ance has usually been regarded as a concussion, vibration, 

 trembling, or undulatory movement. Further, it can be 



