636 EARTHQUAKES. 
lieved that not only natural ravines and caves, but quarries, 
wells, and other human excavations, which break the continuity 
of the terrestrial strata and facilitate the escape of elastic va- 
pors, have a sensible influence in diminishing the violence and 
preventing the propagation of the earth-waves. In all coun- 
tries subject to earthquakes this opinion is still maintained, and 
it is asserted that, both in ancient and in modern times, build- 
ings protected by deep wells under or near them have suffered 
less from earthquakes than those the architects of which have 
neglected this precaution.* 
If the commonly received theory of the cause of earthquakes 
is true—that, namely, which ascribes them to the elastic force 
of gases accumulated or generated in subterranean reservoirs— 
it is evident that open channels of communication between 
such reservoirs and the atmosphere might serve as a harmless 
discharge of gases that would otherwise acquire destructive 
energy. The doubt is whether artificial excavations can be car- 
ried deep enough to reach the laboratory where the elastic fluids 
are distilled. There are, in many places, small natural crevices 
through which such fluids escape, and the source of them 
sometimes lies at so moderate a depth that they pervade the 
superficial soil and, as it were, transpire from it, over a consid- 
erable area. When the borer of an ordinary artesian well 
strikes into a cavity in the earth, imprisoned air often rushes 
out with great violence, and this has been still more frequently 
observed in sinking mineral-oil wells. In this latter case, the 
discharge of a vehement current of inflammable fluid some- 
times continues for hours and even longer periods. These facts 
seem to render it not wholly improbable that the popular belief 
of the efficacy of deep wells in mitigating the violence of earth- 
quakes is well founded. 
In general, light, wooden buildings are less injured by earth- 
quakes than more solid structures of stone or brick, and it is 
commonly supposed that the power put forth by the earth-wave 
is too great to be resisted by any amount of weight or solidity 
* LANDGREBE, Geschichte der Vulkane, ii., pp. 19, 20. 
