296 REPORT — 1851. 



charges more so than the first. The interval in time between the explo- 

 sion and the disappearance of the cross wires of the Seismoscope was quite, 

 perceptible, notwithstanding the verj' short range, and gave the first indica- 

 tion of what experiment afterwards revealed as to the comparatively slow 

 rate in which the pulse-wave moves even in granite rock. 



These experiments led me to conclude that a single pound of powder 

 would have given more than suflScient shock to have been perceptible in the 

 Seismoscope at 300 feet range ; and as the charge for equal pulses may be 

 assumed (as before at Killiney) to vary as the square of the distance, then- 

 lib. : 300ft. : : X : 1500ft., 



or the charges should be as 5- : l-=25 : 1, or for 1500 feet range, 25 lbs. 

 of same sort of powder, as used at Glasthule, viz. from Ballincollig Mills. 

 This however might be reduced at Dalkey for the government powder, in 

 the ratio of 9 : 13, i.e. in the inverse ratio of their explosive powers, or the 

 sufficient charge would be 17'3 lbs. 



It was therefore determined to form the jumper-holes of a depth and size 

 suitable to receive a charge of 20 lbs. of powder, and 12 feet in depth was 

 fixed upon for each, with a diameter of cylinder of 3^ inches nearly, being 

 that made by a jumper of 3-inch gauge or width of edge. 



The next operation was to mark out the positions most suitable for the 

 several holes to be jumped. This was done, and contracts made through 

 Mr. Foot, foreman of works at Kingstown Harbour ; boring tools being lent 

 by the contractor for the harbour, Mr. M. B. Mullins, C.E., to whom I am ' 

 indebted for this assistance. 



On the 30th of September, 1850, the jumper-holes were finished; they 

 were all nearly vertical, not many feet different in level from each other, and 

 situated as in the map of the northern end of the island (Plate XIV.) and 

 in section of range (Plate XV.). 



The datum stone A remained a fixed point of solid rock to measure from, 

 and an ordinate, A b, having been stretched out by a cord in the line of range, 

 the position of each hole was found and referred to it by measured abscissae ; 

 so that when, after the explosions, the whole surface of the rock constituting 

 that part of the island should have been shattered and destroyed, the distances 

 and positions at which the holes had been, should still be known. 



From the 1st to the 25th of September was occupied in landing instru- 

 ments and stores of various sorts upon the island, in which some difficulty 

 was found, owing to the period of the year, the stormy state of the weather, 

 and the powerful run of tide and heavy sea that sets through Dalkey Sound. 



The arrangements for firing and for noting the times were precisely simi- 

 lar to those used at Killiney Strand. Two distinct batteries, one of Bergin's 

 of 12 pair of plates, being used for firing, and 5 tioughs, or 30 pair of plates 

 of Smee's batterj', for releasing the chronograph at the observer's end. The 

 batteries were placed at the ruined church (see Map) upon a scaffold covered 

 by a temporary splinter proof, and placed nearly at the top of the old church 

 upon its walls, thus : — 

 Position of holes. 400 feet. Old church. 766 feet. Point of obsenration . 



'^ Diag. 10. 



The distance from the congeries of jumper-holes to the church was about 

 400 feet. On the 2nd of October, 1850, a preliminary experiment was made 

 as to the firing power upon priming cartridges, when it was found that the 

 intensity of 12 pairs of Bergin's battery (reduced by the low temperature of 



