156 DYNAMICAL GEOLOGY. 



primary importance, viz., that the shake does not occur 

 everywhere at the same time, but on the contrary appears 

 first at one place and spreads thence in all directions, 

 precisely like a system of waves when a stone is thrown 

 into the water. This point of first appearance is called 

 the " epicentrum," because it is immediately above the 

 origin. The violence of the earthquake is greatest there, 

 and thence decreases precisely like a system of widening 

 circular waves. 



Velocity of Shock and of Transit. The velocity of 

 the spread from the center or velocity of travel (transit) 

 must be carefully distinguished from the velocity of the 

 earth-movement (shock). There is no close relation be- 

 tween these. We may best illustrate this by water-waves. 

 Suppose we are in a boat on the surface of a bay traversed 

 by long, low swells. As each swell passes under us, we 

 are slowly heaved up and slowly let down again, but the 

 waves are here, there, and away with great velocity. The 

 velocity of oscillation is small, the velocity of transit 

 is great. But if the surface of the bay be agitated by 

 short, high waves, the oscillation or shaking is more rapid, 

 but the transit is comparatively slow. So in earthquakes, 

 the movement may be only a slow heaving up and down, 

 or swinging back and forth, and yet this movement may 

 travel from place to place with great velocity. Now, as 

 in water-waves generated by a stone thrown in still water, 

 so in earthquakes, the velocity and amount of movement 

 (which is equivalent to the wave-height) is greatest at the 

 center (epicentrum), and diminishes as it spreads, but the 

 velocity of the transit or travel is nearly or quite uniform. 



Now, the velocity of transit has been determined in 

 many earthquakes by noting the time of arrival at different 

 places. It varies with the kind of rock, being greatest in 

 the hardest, and also with the depth of the origin, being 

 greater for very deep earthquakes. In some cases it is 

 only ten miles per minute ; sometimes fifteen, twenty, 



