338 EARTHaUAKES. 



The relief of external pressure and the increase of 

 internal pressure, it will be observed, both tend in the 

 same direction — namely, to an elevation of the earth's 

 crust. 



This explanation of the increased activity of earth 

 tremors, which has also been suggested by M. S. di Kossi, is 

 here only advanced as a speculation, more probable per- 

 haps than many others. 



We know how a mass of sulphur which has been fused 

 in the presence of water in a closed boiler gives up in 

 the form of steam the occluded moisture upon the relief 

 of pressure. In a similar manner we see steam escaping 

 from volcanic vents and cooling streams of lava. We also 

 know how gas escapes from the pores and cavities in a 

 seam of coal on the fall of the barometrical column. We 

 also know that certain wells increase the height of their 

 column under like conditions. The latter of these pheno- 

 mena, resulting in an increase in the rate of drainage of 

 an area by its tendency to render such an area of less 

 weight, facilitates its rise. If we follow the views of Mr. 

 Mallet in considering that the pressures exerted on the 

 crust of our earth may in volcanic regions be roughly esti- 

 mated by the height of a column of lava in the volcanoes 

 of such districts, we see that in the neighbourhood of a 

 volcano like Cotopaxi the upward pressures must be enor- 

 mously great. Further, the phenomena of earthquakes 

 and volcanoes indicate that these pressures are variable. 

 Before a volcano bursts forth we should expect that there 

 would be in its vicinity an upward bulging of the crust, 

 and after its formation a fall. Further, it is not difficult 

 to conjecture other possible means by which such pres- 

 sures may obtain relief. 



Should these pressures then find relief without ruptur- 

 ing the surface, it is not difficult to imagine them as the 



