CHAPTER IIL 



EAKTBQUAKE MOTION DISCUSSED THEORETICALLY. 



Ideas of the ancients (the views of Travagini, Ilooke, Woodward, 

 Stuckeley, 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 accelera- 

 tion 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.^ It is as follows : — 



1. Epiclintse, or earthquakes which move the ground 

 obliquely. 



2. Brastse, with an upward vertical motion like boil- 

 ing water. 



3. Chasmatise, which cause the ground to sink and 

 form hollows. 



4. Ehectse, which raise the ground and make fissures. 



5. Ostse, which overthrow with one thrust. 



6. Palmatiae, which shake from side to side with a 

 sort of tremor. 



From the sixth group in this classification we see that 



' De Mvndo, c. iv. 



