2?he Causes and Phenomena of Earthquakes. 59 



vents. He supposes, therefore, that by the expansive force of the 

 gases large surfaces of the earth's crust may be upraised and 

 fissured, that slow elevation may take place during long-continued 

 feeble action, and sudden elevation by paroxysmal action ; whilst 

 vibratory motions may be originated according to proximity or 

 distance, and quantity of force employed. 



JSTow, it must be remarked, again, that vibrations must also 

 depend on various conditions ; and that velocity, direction, extent 

 of motion, and elasticity of substance, must be taken into the 

 account of earthquake shocks. 



All supposable cases are illustrated in Hopkins's Memoirs by 

 diagrams, and the depth of the foci of disturbance he deduces 

 from formulae connected with the velocity ascertained between 

 points well determined as to the instant of a shock along a given 

 line of direction. 



He assumes three conditions : — 1. When the earthquake vibra- 

 tions pass through a homogeneous mass. 2. "When the forma- 

 tions traversed are variable. 3. When the sea overlies them. 



He then shows that where a shock traverses a coast line at 

 moderate depth, the velocity of motion is greatest in the solid 

 crust, least in the sea, and intermediate in the subjacent molten 

 fluid, and that the wave of motion is refracted in passing from one 

 medium to another. 



[Supposing this to take place in a wide area, with considerable 

 force, then permanent elevation might occur ; but if the intensity 

 be small, and only vibrations ensue, all points of the surface 

 would be reached nearly at the same time, and the apparent 

 horizontal propagation would be great, and, therefore, contem- 

 poraneous shocks over a large area afford the strongest proof of 

 the original disturbance, although a stronger shock, acting ver- 

 tically, and coining from a greater depth and a single centre might 

 produce a greater effect immediately over the point of disturbance. 

 Thus, also, a permanent elevation of the surface may take place 

 where the area of original disturbance is great. To understand 

 these views one must refer to the diagrams by which they are 

 explained. 



A stone thrown into water produces undulations which advance 

 from the focus of disturbance in concentric sweeps, which may 

 represent the passage of a shock, as that of waves rolling onwards 

 all round. Now the front of such waves must be circular or 

 elliptical, whether in land or water. 



Suppose, then, a sudden elevation of the sea bottom ; the front 

 of the wave produced will be steep, and the fposterior slope 

 gradual, and this wave will diverge all round the point of dis- 

 turbance, and be accompanied in water by a current. The 

 velocity will depend on the depth ; therefore if the wave pass 

 through different depths of water, the front will not be circular, 



