C. Davison — Vorticose Earthquake Shocks. 259 



direction, and attended by a movement np and down of much 

 greater range, to which the word ' sussultatore ' is often applied." ^ 

 We may further state in support of this fact : 



a. That vorticose shocks often consist in part of vertical move- 

 ments. This is noticeable in most of the instances given above, and 

 in nearly all of those mentioned by M. Perrey. 



b. That they are felt near those places which we gather from other 

 evidence to be vertically above the centre of disturbance, the other 

 evidence being : earlier occurrence in point of time, occurrence near 

 the centre of the disturbed area, observations on the direction of the 

 shock. Thus, in the Neapolitan earthquake of 1857, all the points 

 where vorticose shocks were felt were near the place indicated by 

 Mr. Mallet as the superficial position of the seismic focus. 



c. That Humboldt remarks, when treating of rotatory shocks, that 

 walls were observed to be twisted, but not thrown down by them 

 (Cosmos, vol. i.). This implies a nearly vertical angle of emergence, 

 and therefore proximity to the place (which we may call the seismic 

 vertex) vertically above the centre of disturbance. 



d. That vorticose shocks are sometimes preceded or followed by 

 very feeble shocks felt only at or near the same places. Thus, the 

 earthquake (at first vertical, afterwards undulatory) felt at Laybach, 

 on Nov. 9, 1856,^ was followed, about nine hours afterwards, by a 

 feeble shock, which is not mentioned (as M. Perrey expressly states) 

 as having been noticed at any other place. Now if a shock be so 

 feeble that it can only just be felt within a very limited area at the 

 surface of the earth, the centre of that area must be vertically above 

 the seismic focus. Hence, assuming that both earthquakes originated 

 at the same spot, this second shock corroborates our inference that 

 Laybach was near the seismic vertex. 



3. Vorticose shocks are usually of rather long duration, the 

 average of over thirty cases in which the duration is explicitly stated 

 being about 20 seconds. From this we must infer either that the 

 cause producing the earthquake does not act instantaneously, or, 

 more probably, that the seismic focus is of some magnitude. In the 

 latter case, the velocity of the earthquake wave being about 800 feet 

 per second, it follows that, if the duration of a shock be 20 seconds, 

 the seismic focus must have been at least three miles long in one 

 direction, for the time of vibration of each particle of rock is very 

 small, probably only a fraction of a second. We may also add that, 

 according to Mr. Mallet, the seismic focus of the Neapolitan earth- 

 quake of 1857 was a fissure nine geographical miles in length and 

 three in height. 



Theory of Vorticose Shocks. — Several explanations have been offered 

 to account for vorticose shocks, and may here be briefly described, 

 before proceeding to suggest what we believe to be their most pro- 

 bable cause. The first two, it should be remarked, were proposed 

 before the publication, in 1846, of Mr. Mallet's paper on " The 



1 Mr. R. Mallet. " The Neapolitan Earthquake of 1857," vol. i. p. 76. 

 ^ M. Perrey. "Note sur les tremblements de terre en 1856, etc." : Mem. Cour. 

 de I'Acad. Eoy. de Belgique (1859), Collection in 8vo. vol. viii. 



