830 THE POPULAR SCIENCE MONTHLY. 



three conditions we have just been considering in the lower parts of 

 the dislocated regions : cavities, water, and a high temperature, all 

 constituting an agency capable of producing considerable dynamic 

 effects at any moment. Suppose a barrel of powder exploded in a 

 cavity situated a hundred metres underground. At the surface we 

 would hear a rumbling explosion, and feel a vertical shock within a 

 limited space, and an undulatory thrill over a wider circle. The phe- 

 nomenon will be much like that of an earthquake, except that the 

 essential element of repetition will be wanting ; for all will be over 

 with the first shock. But, in the majority of earthquakes, the shocks 

 come in succession, as if the cause of them were a regenerating one. 



Many ways may be conceived in which these enormous tensions 

 may end in reiterated shocks, according to the hypothesis on which we 

 place ourselves. Thus the water in a cavity having in time reached 

 the temperature of explosion, suddenly displaces some of the walls of 

 its prison. Hence, a first shock, followed by an expansion into the 

 cracks and adjoining cavities, which have lower temperature and ten- 

 sion. Then, the pressure in the original focus of exjulosion having 

 fallen off, the walls which had given way return upon themselves to 

 their former position, to give way again when the primitive reservoir 

 has regained its lost tension. This flow from cavity to cavity, which, 

 instead of being continuous, is made by bursts and starts, may con- 

 tinue to be reproduced time after time till the principal reservoir is 

 exhausted. But the mechanism is not destroyed then. During the 

 period of calm, following the seismic period, it can be charged again. 

 Something analogous to this takes place in volcanic eruptions, which 

 are separated by the lapse of time necessary to recharge their appa- 

 ratus by a slow alimentation. Furthermore, reservoirs of water may 

 be suddenly displaced under the effect of contractions of the crust, 

 and may thus be brought into contact with masses having a high tem- 

 perature. 



If we suppose a sea of melted matter to exist beneath the crust of 

 the earth, we should have analogous effects whenever hydrated rocks, 

 broken off from the shell, fall into the ignited masses. 



The theory of the agency of vapor is also supported by the rum- 

 blings and subterranean thunders which sometimes continue for 

 months and even for years without being attended by shocks, and for 

 which it is difficult to imasrine anv other causes than sudden condensa- 

 tion, or a flow of gaseous matter at very high tension through a nar- 

 row orifice. The vapor, having escaped from its prison, must in many 

 cases resume the liquid state very quickly, on account of the enormous 

 expansion it undergoes. It also has to traverse miles of relatively cold 

 rocks, more or less water-charged and full of cracks. It may thus con- 

 tribute to the production of thermal springs. Examples are on record, 

 too, of earthquakes, remote from any volcanic point, in which both hot 

 water and gaseous matter have been seen to issue from crevasses. 



