353 



tural earthquakes, through the stratified and highly convoluted, lami- 

 nated, and shattered slate and quartz rocks of the neighbourhood of 

 Holyhead, where the recurrent explosion of very large mines of gun- 

 powder in the Government quarries (for the obtaining of rock for 

 construction of the Asylum Harbour) afforded a valuable opportu- 

 nity in the production of the requisite impulses for generation of the 

 wave. 



The author had previously determined experimentally (at Killiney 

 Bay, in Ireland) the transit velocity of such waves in wet sand, and 

 in highly-shattered and more solid granite ; media presenting, pro- 

 bably, the extremes of slowness and of fastness of wave transit. It 

 was still desirable to determine this for rock, not only minutely 

 crystalline, but also stratified, convoluted, and generally highly per- 

 plexed and heterogeneous in internal structure. The instrumental 

 means employed were generally similar to those adopted previously 

 at Killiney Bay, with suitable modifications consequent upon the 

 great charges of powder fired, which at these quarries have reached 

 as much as nine tons at a single blast or mine. The seismoscope 

 (see Trans. Brit. Assoc. 1851, Second Report on Facts of Earth- 

 quakes, R. Mallet, p. 277, &c.) was placed upon a levelled table 

 of solid rock at a suitable station (shown on the map and sections 

 that accompany the paper), and with it the chronograph and gal- 

 vanic apparatus, by which, on making contact, by the author's 

 pressing his hand upon the lever of the latter instrument, the mine 

 at the quarry, distant in all cases about a mile, was ignited, and the 

 time that elapsed between the starting of the elastic wave from the 

 impulse of the explosion to its arrival at the observer and visibility 

 in the field of the seismoscope was recorded. This registered time 

 was subject to three principal corrections, the respective coefficients 

 of which are also determined experimentally. The instruments ad- 

 mitted of time determinations to within nearly y- Vc7 of a second. 

 The range over which the wave traversed was accurately obtained in 

 length for each separate experiment. A constant distance from the 

 observing station = 4584*80 feet, up to a fixed point near the 

 quarries, was obtained with precision, in the first instance, by trigo- 

 nometrical operations, upon a measured base of 1432 feet. The 

 distance of the mean centre of each mine or heading was subsequently 

 measured in a right line to this fixed point, and the angle made by 



