658 ME. MALLET ON THE TEANSIT-VELOCITY OE EAETHQTJAED WAVES. 
given in Plate XXI. Section I. The roughness of the ground and its inclination, how- 
ever, rendered direct measurement of the range of wave-path with sufficient accuracy 
impracticable, and it was found necessary to obtain it trigonometrically. For this pur- 
pose a base-line of 1432 feet in length was measured off along the rails of the tram- 
road which connects the quarry with the east breakwater, between the pomts A and B 
(Map, Plate XX.), where the road fortunately was found straight and nearly quite 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 6 72 ’05 feet. 
I was also enabled to connect the side OS with a trig point P, upon the w^estern 
breakwater, and another at R, the positions of which are defined upon the accurate sur- 
veys of the harbour in Mr. Dobson’s, C.E., possession, as a further means of verification ; 
we may therefore view the length of the constant part of the range between the observ- 
ing station and the flagstaff, its other permanent terminal, as equal to 4585 feet, neglecting 
fractions. 
The base of the staff at S was found to be 68' 78 feet above the level of the horizontal 
surface of the rock at Pen-y-Brin (the observing station O), and the base of the flag- 
staff at W is 5 ‘70 feet above the same point O. The levelled surface of rock at 0 is 
84 feet above the mean tide-level of the sea in the Asylum Harbour ; and the average 
rise and fall of spring tides at Holyhead is 18 feet; the line of rock, therefore, through 
which the range passes is, except as respects surface water, permanently dry to a con- 
siderable depth. The majority of the headings are driven into the face of the quarry 
cliff horizontally, at from 10 to 20 feet above the level of the floor of the quarry, which 
is on nearly the same level as the point W^. Hence, practically, the actual range of 
transmission through the solid rock, of the impulse from each heading when fired, to the 
seismoscope at the observer’s station, may be considered as a horizontal line, and no 
correction of distance is required for difference of elevation at the two extremities of the 
ob serving-range in the deduction of our results. 
