ON THE TRANSIT-VELOCITY OF EARTHQUAKE WAVES. 203 



from Holyhead by their respective vessels from Kingstown Harbour. After 

 much fruitless correspondence I regret to say that both these Companies 

 refused to render any assistance whatever, a boon the refusal of which 

 greatly increased the expenditure for these experiments. Lastly, I placed 

 myself in communication with Messrs. Rigby, the contractors for the vast 

 works of the Quarries and Harbour, and in August 1856 received from them 

 the assurance of every assistance that they could afford consistently with the 

 prosecution of the works. To them, to Mr. R. L. Cousens, C.E., the acting 

 engineer for their firm on the works, and to Mr. G. C. Dobson, C.E., chief 

 engineer on the work under Mr. Rendel (since under Mr. Hawkshaw), my 

 thanks are due for the best and most cordial assistance upon all occasions. 



The position for the observer's station and seismoscope upon the levelled 

 floor of rock at Pen-y-Brin having been fixed upon, the first operation neces- 

 sary was to obtain an accurate section of the surface in the line between that 

 and the quarries, a geological section of the rock formations along the same 

 line, and with precision the exact distance in a straight line, from some fixed 

 point adjacent to the quarries, to the observer's station. The fixed point 

 chosen at the quarries was the flagstaff at the bell, which is rung whenever 

 a blast is about to be fired, this being so placed that from it measurements 

 and angular bearings, with the line of range O W (Map), from the various 

 sites of future explosions could readily be made, and thus the exact distance 

 of each focus of explosion (to be hereafter experimented on) from the seismo- 

 scope at O ascertained, the flagstaff always remaining undisturbed as a 

 fixed terminal at the quarry end of the range. The whole surface, O to W, 

 was carefully levelled over, and the distances chained, as given in the diagram, 

 PI. III. section 2. fig. 1. The roughness of the ground and its inclination, 

 however, rendered direct measurement of the range of wave-path with suf- 

 ficient accuracy impracticable, and it was found necessary to obtain it trigo- 

 nometrically. For this purpose a base line of 1432 feet in length was mea- 

 sured off along the rails of the tramroad which connects the quarry with the 

 east breakwater, between the points A and B (Map, PI. II.), where the road 

 fortunately was found straight and nearly level. 



This was measured with two brass-shod pine rods, each of 35 feet in length, 

 of the same sort, and applied in the same manner, as I used in 1849 for 

 measuring the base of one mile on Killiney Strand, for the particulars of 

 which the " Second Report on Earthquakes," &c.. Report Brit. Assoc. 

 1851, p. 274, &c., may be referred to. The base was measured forwards and 

 backwards, with a result differing by less than 3 inches. The flagstaff at the 

 spot marked W in the Map is not visible from the observer's station, owing 

 to some intervening houses and other objects ; a staff was therefore set up at 

 S, upon the hill-side. The point O was connected by angular measurements 

 with the extremities of the measured base A and B ; the triangles OBS and 

 OSW were then obtained, whence that OBW was arrived at, from which 

 finally the distance OW (the constant part of the range) was ascertained to 

 be=4584*80 feet. The triangle OBW was used as a check upon that OSW, 

 as the angles at O, S, and W had to be taken, owing to local circumstances, 

 smaller than is desirable. The lengths of the side OW obtained from the 

 two triangles separately closely agreed ; and as a further check, the side 

 SW, which gave, trigonometrically, a length of 671 '07 feet, when actually 

 measured as a base of verification, gave 672*05 feet. 



I was also enabled to connect the side OS with a trig-point P, upon the 

 western breakwater, and another at R, the positions of which are defined 

 upon the accurate surveys of the harbour in Mr. Dobson's possession, 

 as a further means of verification. We may therefore view the length of the 



