ON THE TRANSIT-VELOCITY OP EARTHQUAKE WAVES. 207 



"We have also two different rocks, the one transmitting the impulse into 

 the other, yet neither so widely differing from the other in molecular and other 

 physical characters as to make any great or abrupt effect upon the wave at 

 the junction probable. In fact, widely, to the first glance, as the quartz 

 rock and the schist rock appear to differ, there is less real distinction 

 of physical character between them than would be supposed : both are 

 composed of the same siliceous sand in about the same size of original 

 grains, variously enveloped, in the one in chlorite and mica, and in the 

 other in white or grey clay and mica; both have, in ancient geological 

 epochs, doubtless derived their materials by degradation and transport from 

 a common source, as respects their main constituent, the silex ; both have 

 been submitted to approximately similar pressures, and probably like tem- 

 peratures ; and the agglutinating flux has probably been mainly the same for 

 both, viz. the minute proportions of alkalies derived from the waters of an 

 ancient ocean. The main difference in physical structure, viewed upon the 

 broad scale, between the quartz rock and the slate is this (as regards our 

 experiments) : — that the great joints and planes of separation on the whole 

 approximate to verticaliiy in the former, while in the latter, with the ex- 

 ception of some larger faults and dykes, the planes of separation are twisted 

 and involved in all directions, but tend more to approach horizontality . 



More interesting conditions could thus scarcely be found for experimental 

 determination of the transit-rate of earth-waves, or more desirable for future 

 comparison with that of earthquake-waves themselves ; much more instructive, 

 indeed, were the actual conditions than if the means of experiment presented 

 by these vast quarry operations had been in the most regular, undisturbed, 

 and horizontal stratified rock, like some of the mountain limestone of Ireland, 

 or the finest and densest laminated roofing-slates of Wales. In such ranges 

 we can predict that the transit-velocity would at least be high. In the 

 medium chosen for these experiments it was impossible even to guess what it 

 might be found. 



I proceed to describe the instrumental arrangements made for the observa- 

 tion of the impulse-wave transmitted from the blasts chosen, and for the de- 

 termination of the transit-time along the range of wave-path. Over the 

 surface of solid rock that had been chiselled down to a level tabular surface 

 at (O) Pen-y-Brin, a timber-shed was erected, of sufficient size to place the 

 observer, an assistant, and all the instruments proper to that spot, under cover 

 and secured from the wind. The side to the N.W. was open, to permit of 

 observation along the line of range, with the means of partially closing it in 

 high winds. 



Along the line of the boundary-wall of the railway next Pen-y-Brin, 

 and thence along up to the highest and most distant point of the quarry 

 cliffs, a line of telegraph-posts was planted, and upon these two properly 

 insulated iron wires were hung, in such a manner that at any point along 

 their length over the quarry cliffs, a pair of branch wires (covered' with 

 gutta percha) could be led off, and in like manner another pair to the appa- 

 ratus in the observing-shed at Pen-y-Brin, thus giving the means of galva- 

 nically connecting the extremities of the range in any way that might be 

 required. 



The mines in use at the quarries frequently consist of two, three, or four 

 separate chambers and charges, which are all fired simultaneously (see PI. IV.); 

 but each charge is fired by a distinct pair of wires, igniting a fine platinum 

 wire interposed in the circuit and immersed in one of the powder-bags. The 

 arrangement of this platinum wire in its hollow wooden frame to prevent 

 disturbance, and its connexion with the large conducting wires, are practi- 



