Vol. 6] Eeid: The Elastic-Rebound Theory of Earthquakes. 433 



this depends on the elasticity of the rock and on its density, on 

 exactly the same quantities, which, we shall see later, determine 

 the velocity of propagation of elastic waves; so that it is im- 

 possible for the fracture to extend more rapidly than the rate 

 of propagation of the fastest elastic waves, which are of the com- 

 pressional type, like the waves of sound ; and in general it would 

 extend less rapidly. It is clear that, if the strains were every- 

 where very close to the breaking limit, a very small movement 

 and therefore a very short time would be necessary to bring the 

 strain to that limit; whereas, if the strain were further from 

 the limit, a greater movement and a longer time would be needed. 

 This is probably one reason why some earthquakes of small 

 intensity last for a number of seconds. 



The progressive extension of the fracture is entirely in accord 

 with our general experience. When a bridge or structure 

 collapses, the break always starts at some particular point and 

 extends from there ; and the time taken for the complete down- 

 fall is far greater than would be necessary if the break occurred 

 everywhere simultaneously, and workmen often have time to 

 spring from the falling structure and save themselves. An 

 avalanche or landslide begins in a small way and gathers 

 material and momentum as it descends. When a chair breaks 

 it does not often happen that there is a sudden collapse, but 

 one part breaks after another, and the occupant usually has time 

 to spring to his feet. When an ice-jam in a river gives way, 

 one part always yields first. When a sheet of paper is torn the 

 tear begins at one side and passes across the sheet. Even when 

 gunpowder is fired, a measurable time is necessary for the ex- 

 plosion to spread from its starting point to other points of the 

 mass. 



Two instances have been found where the progression of a 

 crack in the ground, or rock, has apparently been seen. The 

 following is taken from the "Sun" newspaper, of Attleboro, 

 Mass., of January 23, 1903 : 



"The experience of the town of Whitman was repeated by 

 Attleboro yesterday, when an earthquake or frost crack or some- 

 thing of the kind made its appearance. There was a hollow 

 rumbling, a shaking of buildings, a small-sized panic among 



