ON THE FACTS OP EARTHQUAKE PHJENOMENA. 303 



The first gives a transit rate uncorrected of 1220*4>4 feet per second. 



The second gives a transit rate, uncorrected, of 1559"96 feet per second, 

 to which is to be applied the seismoscope and other corrections, which will 

 be reserved for future notice. 



It will at once occur to the reader, — Why thus divide one series of expe- 

 riments into two separate sets ? This requires explanation. If the medium 

 (granite) in which these or any similar experiments were made, were of 

 perfect homogeneity, the results of every separate experiment should closely 

 approximate within the limits of probable experimental error. The results 

 given in the preceding tables however are divisible at a glance into two 

 sets, each respectively very closely approximating ; viz. the sets A, C, D, E, 

 F, G on the one part, and those of H, I and K on the other ; while these re- 

 spectively differ, on the average, in the ratio of 4"3263 to 3*3847. 



This result had been anticipated before the experiments were made. 



The granite chosen for all the explosions was the most homogeneous that 

 could be commanded, still none of it was absolutely so ; nor probably does 

 a mass of rock exist upon the earth's surface that is homogeneous, i. e. free 

 from master joints, clefts and fissures, for a quarter of a mile in extent in 

 any direction ; and of the ten jumper-holes here employed, those from A 

 to G inclusive were of necessity placed in granite of a more fissured or shat- 

 tered character than the three others, viz. H, I and K. These three last 

 ones, as may be seen by the Map, were driven into one vast boss (roche 

 moutonee) of granite absolutely free, so far as it was exposed to view from 

 immediate fractures, and from its appearance extending down to a great 

 depth into the very foundations of the island before reaching any great fis- 

 sures. Thus it follows that the transit rate ascertained (or the experiments 

 A to H inclusive) belongs to an impulse wave transmitted through granite of 

 the ordinary degree of fissuring found in the great mass of European granites, 

 while that of H to K belongs to the same rock in a more solid condition, and 

 approaching much more nearly to the transit rate that would be due to the 

 same granite, if for the whole length of range it constituted one unbroken, 

 perfect and homogeneous mass of rock. We will return to this when finally 

 discussing the results of our experiments, and now proceed to describe some 

 subsidiary experiments, the propriety of making which was suggested by the 

 results obtained in those devised as principal. 



The large difference in transit time between the wave in loose sand, as 

 found at Killiney, and that in granite, as found at Dalkey, viz. between 

 906 feet per second in round numbers, and 1 220 feet per second the average 

 minimum in granite, might seem suflScient to establish at once that the 

 pulses observed in each case in the Seismoscope must have been truly due 

 to vibrations propagated through the earth, and could not be due to any 

 commotions of the air above, produced by the report or concussion of the 

 powder. But £is both these transit times do not differ enormously from the 

 admitted transit period of the ordinary sound-wave in air,it seemed desirable to 

 put beyond doubt any question as to this source of fallacy in the experiments. 



It is to be borne in mind that at Killiney Bay the noise or report of the 

 explosions was in every case so deafened by the dead sand, as to be scarcely 

 heard at the seismoscope ; and that at Dalkey, although the reports were 

 more distinct, there was no explosion heard at the seismoscope louder than 

 that of two or three muskets at, say 100 yards off, and not nearly as sharp 

 or ringing; nor this but from two of the blasts, the others producing as 

 little report as any mine at Killiney Bay. Aerial commotion is therefore 

 at the outset improbable. 



To set the matter experimentally at rest, however, eight jumper-holes 



