EAETHQUAKE MOTION DISCUSSED THEORETICALLY. 43 



seen in narratives of earthquakes that it had been often 

 observed that these tremblings and shakings continued 

 over a certain period of time. Although it had been 

 noticed that large areas were almost simultaneously 

 affected by these disturbances, no definite idea appears 

 to have existed as to how earthquake motion was pro- 

 pagated. Usually it was assumed that the disturbance 

 spread through subterranean channels. 



The first true conception of earthquake motion and 

 the manner of its propagation is due to Dr. Thomas 

 Young, who suggested that earthquake motion was vibra- 

 tory, and it might be 'propagated through the earth 

 nearly in the same manner as a noise is conveyed through 

 the air.' The same idea was moulded into a more definite 

 form by Gay Lussac. 



The first accurate definition of an earthquake is due to 

 Mr. Eobert Mallet, who, after collecting and examining 

 many facts connected with earthquake phenomena, and 

 reasoning on these, with the help of known laws con- 

 nected with the production and propagation of waves of 

 various descriptions, formulated his views as follows : — 



An earthquake is ' the transit of a wave or luaves of 

 elastic compression in any direction from vertically up^ 

 luards to horizontally, in any azimuth, through the crust 

 and surface of the earth, from any centre of impulse 

 or from more than one, and which may be attended with 

 sound and tidal waves, dependent upon the impulse and 

 upon circumstances of position as to sea and land.'' 



In brief, so far as motion in the earth is concerned, 

 INIallet defined an earthquake as being a motion due to 

 the transit of waves of elastic compression. In many cases 

 it is possible that this is strictly true, but in succeeding 

 pnges it will be shown that earthquake motion may also 

 be due to tlie transit of waves of elastic distortion. 



