EAETHQUAKE MOTION DEDUCED FROM OBSERVATION. 85 



may be calculated and the temperature due to the pressure 

 producing this may be arrived at. In this way earthquakes 

 may be used as a means of calculating subterranean 

 temperature at depths that can never be attained experi- 

 mentally. 



A method of proceeding which is probably more 

 definite than that adopted by Mallet would be the 

 application of the method indicated when speaking of 

 the intensity of artificial disturbances. 



If for a given earthquake the origin of which is known 

 we have determined by seismographs the mean acceleration 

 of an earth particle at two or more stations at different 

 distances from that origin, we are enabled to construct 

 a curve of intensity the area between which and its 

 asymptotes was shown to be a measure of the total intensity 

 of the shock. Comparing this area with that of a unit 

 disturbance produced, say, by the explosion of a pound of 

 dynamite, one may approximately calculate in terms of 

 this unit the initial intensity of the earthquake. 



Radiation of an Earthquake. — The tremors preceding 

 the more violent movements of an earthquake may be 

 due, as Mallet has suggested,' to the free surface waves 

 reaching a distant point before the direct vibrations. 



The fact that earth vibrations produced by striking a 

 blow on or near the surface of the ground are wholly 

 obliterated in reaching a cutting or valley, there being no 

 underground waves of distortion to crop up on the oppo- 

 site side of the valley, indicates that the disturbance is 

 one that travels on the surface ; the same fact is illus- 

 trated when we endeavour to transmit vibrations through 

 the side of a hill into a tunnel. 



In the tunnel, although the distance may be small, 

 * Nea-poUtan Eartliqiiake^ ii. p. 300. 



