UNDULATIONS OF THE INTERNAL FLUID. 



affected, as a floating body is by the waves of the ocean. If the 

 height of the waves of the subterranean fluid be greater than the 

 elasticity of the solid shell which confines them can bear, that 

 shell must be fractured to a greater or less extent, and through 

 the openings thus produced in it, the internal matter, in a state of 

 igneous fusion, may issue, producing volcanic phenomena. Or in 

 fine, the fracture may be only external, in which case the con- 

 sequences will be limited to local derangement and disturbance of 

 the surface. 



Let us first consider the case in which an undulation is propa- 

 gated to the surface without fracture. 



12. The part of the surface of the earth thus affected, and all 

 bodies placed upon it, suffer in this case the same sort of disturb- 

 ance and dislocation as does a ship floating on water upon which 

 a system of waves is formed. The waves have a progressive 

 motion in some certain direction, passing successively under the 

 ship, which is alternately raised to the crest and lowered to the 

 hollow of each successive wave as it passes. Neither the water nor 

 the ship partakes of this progressive motion. If they did no 

 alternate rise and fall would take place ; the ship once placed on 

 the crest of a wave would be carried forward by the water on 

 which it floats, and would still remain on the crest of the same 

 wave, a circumstance which never takes place. In the same 

 manner exactly the undulations imparted to the surface of the 

 earth by the fluid confined within it, have a certain progressive 

 motion which causes every part of the surface over which they 

 pass, alternately to rise and fall, through a height equal to the 

 difference of the levels of the crest and hollow of the wave. 



13. But besides this, another effect of much importance is pro- 

 duced upon solid structures, which are placed, as buildings are, in 

 a position perpendicular to the level surface of the ground. That 

 surface when it is affected by the wave, ceases to be level. As 

 the wave passes it, it is first inclined in one direction, and after 

 being raised to the summit of the wave it is inclined in the 

 opposite direction. Any solid structure, having a vertical position, 

 would, therefore, while the wave passes, be inclined from the 

 vertical, first to the right,- and then to the left, according as it is 

 successively at one side or other of the wave. 



Such superficial undulation of the ground, would therefore pro- 

 duce a twofold displacement of all such objects ; first an alternate 

 motion upwards and downwards in the vertical direction, and 

 secondly, a sort of rocking or oscillating motion like that of a 

 pendulum, leaning alternately right and left of its true vertical 

 position. 



This will be better understood by reference to fig. 2, in which 



151 



