62 EARTHQUAKES. 



represent distance from the origin in feet, and the vertical 

 measurements mean acceleration in thousands of milli- 

 metres per second. 



This curve approximates to an equi-angular hyperbola. 

 The area between the curve and its asymptotes is propor- 

 tional to the whole energy of the shock. The area of the 

 diagram is proportional to the energy gi^en up to the 

 ground by the explosion of three pounds of dynamite. If 

 we call the unit shock the effect produced by the explosion 

 of one pound of dynamite, the above artificial earthquake 

 had an intensity equal to three. 



The only other investigations which have been made 

 in this interesting branch of observational seismology are 

 those by Mr. Eobert Mallet,^ and those by General Henry 

 L. Abbot.2 



Mallefs Results. — The velocity with which earth 

 vibrations were transmitted as deduced by Mr. Mallet 

 were as follows : — 



Feet per second 



In sand 824-915 



In contorted stratified rock, quartz, and slate 



at Holyhead 1,088*669 



In discontinuous and much shattered granite 1,306-425 

 In more solid granite 1,664-674 



A striking result which was obtained by IMallet in his 

 experiments at Holyhead was that the transit velocity 

 increases with an increase in the intensity of the initial 

 shock. Thus with a charge of 12,000 pounds of powder 

 the transit rate was l,37cifeet per second, whilst with 2,100 

 pounds the transit rate was 1,099 feet per second. In 



* Report offhe British Association, 1851. 



2 'On the Velociiy of Transmission of Earth Waves,' by General 

 H. L. Abbot, Americari Joiirnal of Scienee and Arts, vol. xv. Mar. h 

 1878 ; * Shock of the Explosion at Hallet's Point,' by Bvt. Brig.-Gen. 

 Henry L. Abbot, read before the Essayons Club of the Corps of 

 Engineers, Nov. 1876. 



