ON THE PACTS OF EARTHQUAKE PHENOMENA. 295 



cimens of this granite of different size of grain had the following specific 

 gravities : — • 



sp. gr. 



No. 1. Coarse grain -. 2*592 



No. 2. Medium grain 2'65b 



No. 3. Fine grained 2-676 



Mean specific gravity =2*64-l. 



In its greatest length from north to south, the island admits of a tolerably- 

 level range of above 1600 feet. The general run of the beds, or rather fis- 

 sures of the granite (or master joints), is about N.W. and S,E., while the best 

 position presented for the experimental range is about N.N.W. and S.S.E, ; 

 thus giving as much solidity and freedom from the effects of discontinuity as 

 the case admitted of (see map, PI. XIV., and section, Plate XV.). 



The island is owned by Government, and under the control of the Board 

 of Ordnance, the grazing of the surface being rented from that Board. 



By application through the favour of General Sir John F. Burgoyne, 

 K.C.B., I obtained permission to make my experiments, subject to the ap- 

 proval of the local engineer officers at Dublin. The rentor of the grazing, 

 a farmer, was next to be dealt with, and his permission was only had by 

 paying him a sum of money and undertaking to make good all surface 

 damage. 



I then applied to the Board of Ordnance to permit me to purchase a suffi- 

 cient quantity of the best government cannon gunpowder, finding it stated by 

 Sir John Burgoyne, in his ' Treatise on Blasting,' that government powder 

 is stronger and better than the best merchant powder, in the ratio of about 

 13 : 9. 



Through the kind assistance of Sir John Burgoyne this permission was 

 given, and the powder obtained from the Ordnance magazine. Phoenix Park. 



I then roughly measured by the chain the best range I could lay out upon 

 the island. I found I could get one excellent range of 1166 feet, or the 

 choice of another and shorter one of 900 feet. In the former one, the end was 

 not visible from the other extreme; but a ruined old church on the island, 

 within about 4<00 feet of the firing end and visible from both extremities, 

 gave the means of elevating the batteries for firing, &c., and the signal man, 

 so as to answer as well as if the whole range were in view together. 



Arrangements were now made for getting the holes jumped in the granite 

 rock wherein to place the charges ; and in order to form some notion of the 

 quantity of powder that would be necessary to fire in each, a few preliminary 

 experiments were made with blasts fired in the ordinary way in the granite 

 of Glasthule Quarry, at Sandycove, near Kingstown Harbour, wrought by 

 the Commissioners of Public Works, with a view to determine the effects upon 

 the Seismoscope of various moderate charges. Three blasts were fired of 

 the following depths of hole and charges. The two smaller holes were 2 

 inches diameter, the largest 2^ inches diameter : — 



6 feet depth of hole 6 lbs. powder, 



8 feet depth of hole 7 lbs. powder, 



10 feet depth of hole 8 lbs. powder, 



tamped with clay in the ordinary way and fired with Bickford's patent fuse. 



The Seismoscope was placed upon the solid granite rock, about 15 feet 

 above the level of the holes, and at an average distance from them of ] 50 

 measured yards. On the 3rd of August, 1850, they were fired. 



The wave of impulse through the granite produced in each of the explo- 

 sions was distinctly visible in the seismoscope, but those of the two latter 



