94 HARBOUR CONSTRUCTION. 



many advantages which the others do not. They arc easier to 

 drill, load, and tamp ; and the line of least resistance is not, 

 unless in very short holes, that of the hole, which is manifestly 

 weaker than the solid rock, no matter how well the tamping 1 

 may have been done. 



At Holyhead, quarrying was conducted for a year or two 

 in the usual way by small-charge firing; but it was found 

 impossible to maintain a sufficient supply of stone for the break- 

 water works by this method. Shafts were consequently sunk, 

 or headings driven, and blasting on a very large scale was 

 resorted to. 



Headings were ultimately adopted, in preference to shafts, 

 in consequence of the height of the quarry face, and the greater 

 facility with which they could be driven. They were also more 

 easily kept dry, and the ventilation was better. 



As much as 130,000 tons of stone were, on one occasion, 

 thrown out by a single blast, the charge being 21,000 pounds of 

 powder. The yield of rock was, therefore, rather over 6 tons 

 per pound of powder. On an average, however, about 4 tons of 

 rock were thrown out per pound of powder used ; but in confined 

 corners, or where the rock was much " bound," a " shot " would 

 often not yield more than one-half of this quantity. 



In small-charge firing, a good deal of time is sometimes 

 wasted by the removal of men, horses, and plant from the quarry, 

 or into safe positions, while the charges are being fired. Delay 

 from this cause may be lessened, or altogether prevented, by 

 firing the " shots " during meal-hours, or when the quarriers have 

 left work for the day; also by the use of travelling cranes, 

 which can easily be moved out of danger. 



In mine-firing, the rock is sometimes dislodged in such large 

 pieces that the firing of small shots has to be resorted to in 

 breaking them up into manageable sizes. In consequence of 

 this, some of the inconveniences attendant on small-blast firing 

 are incurred. 



Quarry Plant. Since the use of large blocks of stone in sea- 

 works is often desirable, or indeed in some situations absolutely 

 necessary, the quany plant, as well as the waggons for conveying 

 the stone to the works, should be fully equal to deal with lumps 

 of from 10 to 12 tons, whenever the magnitude of the work and 

 financial considerations will admit of it. It is, however even 



1 See p. 102. 



