116 IGNEOUS AGENCIES. 



face, v' would become nearly equal to v at some point of its course ; but 

 as x approaches the center, C\ the limit of v' would be greater and 

 greater, until, if x is at the center, v' would become infinite everywhere ; 

 i. e., a shock at the center would reach the surface everywhere at the 

 same moment. 



Experimental Determination of the Velocity of the Spherical Wave. 

 — On the supposition that earthquakes are really produced by the emer- 

 gence on the surface of a series of elastic earth-waves, Mallet under- 

 took to determine experimentally the velocity of such waves. Two 

 stations were taken about a mile or more apart, and connected by tele- 

 graphic apparatus ; a keg of gunpowder was buried at one, and at the 

 other was placed an observatory, in which was a clock, a mercury mir- 

 ror, and a light, the image of which reflected from the mercury mirror 

 was thrown on a screen. The slightest tremor communicated to the 

 mercury surface of course caused the image to dance. The moment 

 of explosion was telegraphed; the moment of arrival of the earth- 

 tremor was observed. The difference gave the time of transit ; the 

 distance, divided by the time, gave the velocity per second. In this 

 manner Mallet found the velocity in sand 825 feet per second, or nearly 

 nine and one half miles per minute ; in slate 1,225 feet per second, or 

 fourteen miles per minute ; and in granite 1,665 feet per second, or 

 nineteen miles per minute. As an earthquake-focus is always several 

 miles beneath the surface, and as rocks at that depth are probably as 

 hard as granite, nineteen miles per minute may be taken as the aver- 

 age velocity of earth-waves as determined by these experiments. It 

 agrees well with the observed velocity of many earthquakes.* 



This result was unexpected, considering the law that all elastic 

 waves in the same medium run with the same velocity, for the velocity 

 of sound in granite or slate is probably not less than 10,000 or 12,000 

 feet per second. The explanation is to be found in the very imperfect 

 coherence and elasticity of rocks. The medium is broken by the pas- 

 sage of large and high waves of the explosion, but carries successfully 

 the small waves of sound. 



More recent experiments have given much higher velocities. In a 

 series of very careful experiments Fouque found in sand a velocity of 

 984 feet, in carboniferous sandstone about 7,400 feet, and in granite 

 about 9,200 feet per second. f The explosion at Hallett's Point gave 

 5,000 to 8,000 feet per second, and that of Flood Rock, 4,500 to 20,000 

 feet per second (Abbot). These agree well with the observed velocity of 

 some earthquakes. 



Character of the Earth-Wave. — For the sake of simplicity we have 



* Mallet, Second Report, Transactions of the British Association, 1851. 

 f Revue Scientifique, vol. xli, pp. 97, 161, 1888. 



