84 EAETHQUAKES. 



Next, lie determined the column of limestone necessary 

 to balance such a pressure, which is about 8,550 feet in 

 height. As the least thickness of strata above this cavity 

 -was 16,700 feet, the pressure of 684 atmospheres was not 

 sufficient to blow away its cover, but if suddenly admitted 

 or generated in the cavity it might have produced the 

 wave of impulse by the sudden compression of the walls 

 of the cavity. 



The pressure of 684 atmospheres is equivalent to 

 about 4-58 tons on the square inch, and, as the total area 

 of the walls of the cavity is calculated at twenty-seven 

 square miles, the total accumulated pressure would be 

 more than 640,528 millions of tons. Mallet, however, 

 shows that it is probable that the temperature of the 

 focal cavity was much greater than that due to the 

 hypogeal increment, and that therefore the pressure may 

 have been greater. 



The capability of producing the earthquake impulse, 

 however, depends on the suddenness with which the steam 

 is flashed off. According to the experiments of Boutigny 

 and others. Mallet tells us that the most sudden produc- 

 tion of steam would take place at a temperature of 500°- 

 550° C, which is but a few degrees below that calculated 

 for the mean focal depth. 



Assuming the above calculated pressure to be true, 

 and knowing the co-efficient of compression of the 

 materials on which it acted, the volume of the wave at a 

 given moment near the instant of starting — that is, at the 

 focus — can be calculated, and from this the wave amplitude 

 on reaching the surface may be deduced. 



Proceeding backwards, if we have observed the wave 

 amplitude, calculated the depth of the focus, and know 

 the co-efficient of expansion, then the total compression 



