CAUSE OF EARTHQUAKES. 105 



ing even under the bottom of the sea, and leading 

 deep into the bowels of the earth. 



Any violent blow struck upon the ground causes 

 the place, as you know, to tremble. The fall of a 

 wall, or of any building, the blasting of a rock, 

 even the rolling of carriage-wheels over a paved 

 street, may be felt at some distance, through a 

 slight motion of the ground : in the same manner 

 must underground movements become perceptible. 

 Thus the shock given by the discharge of the 

 Geyser is felt especially just around the spring, 

 and during an outburst of Vesuvius the whole 

 mountain, and often even the country around at 

 some distance, is affected with a trembling motion. 

 But such shakings of the earth are the usual fore- 

 runners of volcanic discharges ; and if we believe 

 that we are justified in ascribing the latter to the 

 powerful expansion of elastic gaseous bodies, it 

 would be illogical to seek for another cause for the 

 explanation of the former. We may consider them 

 as the endeavours of the vapours confined in the 

 depths below to force for themselves an escape 

 upwards; endeavours by which they enlarge the 

 channels under the earth, and set in motion the 

 moveable masses within them, water and melted 

 rocks, and, here and there, perhaps, heave up and 

 rend the solid crust of the earth, and at last break 

 their way out from their confinement. 



The frequent occurrence of earthquakes, espe- 



