ON THE PACTS OP EARTHQUAKE PHiENOMENA. 311 



logue of the present Report, indicates that as a fact, earth-waves transmitted 

 nearly vertically and hence probably from great depths, are most remarkable 

 for the intense violence and velocity of the shock. Humboldt's account oi: 

 the great earthquake at Riobamba (see preceding Report, and Humboldt's 

 Personal Narrative), may be especially noticed in confirmation, where the 

 velocity was stated to be such as to project the bodies of men and animals 

 many feet into the air. 



For this reason I have deemed it worth while to prepare a diagram, 

 in which, taking the extreme horizontal or surface limits within which 

 some of the greatest earthquakes on record have been observed to extend, 

 and assuming that the emergent wave at each of those limits had no deeper 

 origin than would be found by the chord of a great circle connecting 

 them, which is certainly below the truth, I have endeavoured to show the 

 depth within the earth's crust at which the origin might lie, equal in each 

 case to the versed sine of the arc cut off. On this supposition the depth 

 of origin of many of the great earthquakes may not have been very great 

 .(though it is likely in most of these cases the wave was an acutely emergent 

 one over the whole area shaken) ; but in some others the depth of origin, 

 even upon our most disadvantageous data, must have been very great ; while 

 in one extremely curious, though unfortunately doubtful case, there is some 

 ground for believing that one and the same shock of earthquake was felt on 

 Nov. 16, 1827, at places nearly antipodal, viz. at Ochotsk and at Columbia, 

 in South America ; and, if so, had its origin not very remote from the centre 

 of the earth. 



Coincident earthquakes in time at very distant places on the earth's sur- 

 face have been by no means rare, as at Iceland and Norway, Poland and 

 Constantinople, &c. (See Catalogue.) 



The preceding remarks as to the different velocity of the normal, and the 

 transversal wave in elastic solids, suggest future inquiries of great interest 

 and importance in relation to the so- often recorded double shock of earth- 

 quake. In frequently shaken regions popular belief attests the fact, that very 

 soon after each shock another of less intensity and of a somewhat different 

 character follows it, with however a very perceptible interval between, and 

 which is fancied to be greater in proportion as the first shock is more powerful. 



Admitting a sufficiently distant origin, the laws of normal and transversal 

 vibration, as interpreted by Poisson wpon his hypothesis, would be suffi- 

 cient to account for the perception of a double blow ; the probability of its 

 arising truly from the successive emergence, first of the normal and then of 

 the transversal wave, due to the one originating impulse, is rendered much 

 greater on taking into consideration the facts above noticed, as ascertained 

 by Wertheim and Breguet. 



Another source of double shock is highly probable, but remains to be as- 

 certained by further researches. In the more perfectly laminated and bedded 

 slates, it is likely that both the normal and transversal waves will be divided, 

 each into a pair having new paths of motion, each separating into what we 

 may denominate the ordinary and the extraordinary, normal and transversal 

 waves ; the peculiar molecular arrangement of such rocks producing effects 

 analogous to those of the ordinary and extraordinary rays of light in double 

 refracting crystals. 



Amongst the vast mass of recorded earthquakes that form the succeeding 

 catalogue, it is to be regretted that not half-a-dozen out of nearly as many 

 thousands, give any data by which the time of superficial transit of the shock 

 in passing from one spot on the earth's surface to another could be ascertained. 

 In a very few instances howeyer sufficient data have been collected to ascer- 



