C. Davison — Vorticose Earthquake Shocks. 263 



elliptical in the figure, for simplicity) consisting of a fissure approxi- 

 mately plane, and inclined to the horizon; 

 and let us consider the effect at a point 

 P nearly above its upper end. The shock 

 will consist of vibrations vi^hich will at 

 first be nearly vertical (or " sussultatore "), 

 but will gradually become more horizontal, 

 their direction changing in the order indicated 

 by the numbered arrowheads, so that the 

 shock will end by being slightly rotatory 

 in the direction of the curved arrow. If 

 P be taken on the other side of the line ab, 

 the rotary movement will be in the opposite 

 direction. Lastly, if P be taken on or near 

 ah, the shock will be at first vertical and 

 afterwards undulatory in the direction b to a. 



Sometimes vertical shocks of long duration 

 are observed, as, for instance, the earthquake 

 at Coquimbo on Feb. 25, 1853, which was 

 described as being vertical and lasting 75 

 seconds. The seismic focus was probably 

 a vertical plane fissure several miles in length, and the place of 

 observation almost directly above its middle point. 



Conversely, from the character of the vorticose shock as observed 

 at two or more places, we might make important inferences regard- 

 ing the nature and position of the corresponding seismic focus. 



Connexion between the three classes of Earthqualce-shoclcs. — The 

 same method may be employed for investigating the effect of the 

 earthquake at a point P, so distant that the seismic focus subtends at 

 it only a small solid angle. The equivalent centres will lie very 

 nearly on the straight line joining P to the centre of the focus, and 

 therefore the shock will vary but slightly, if at all, in direction, and 

 will be felt as a series of regular isochronous vibrations (varying, 

 perhaps, in intensity), provided the seismic focus be such that any 

 two points of it can be connected by a line l^'ing wholly within it. > 



Thus we are led to the following conclusion : that the three 

 different kinds of earthquakes, mentioned at the commencement of 

 this paper, are not independent, but are different manifestations of 

 the same phenomenon, the effect of the mere size of the seismic 

 focus. If the focus be very small, a single undulation will be felt 

 at all points of the disturbed area ; if of medium size, the move- 

 ment above it will be " sussultatore," in other parts undulatory ; if 

 it be large, the movement above it will be " sussultatore " and vorti- 

 cose, in the immediate neighboui'hood of the seismic vertex undula- 

 tory and vorticose, and in other places undulatory. 



Again, since, in any given earthquake, the undulatory movement 

 is felt over a very much larger area than the " sussultatoi'e " and 

 vorticose, it follows that the chance of its being felt as such is so 

 much the greater, and therefore that undulatory shocks should be 

 recorded very much more frequently than " sussultatore " and vorti- 

 cose shocks. 



