QUARRYING. 95 



from an economical point of view well to provide good plant at 

 the outset of a work. 



Although the power of the cranes to be used in a quarry 

 must greatly depend upon the weight of the stones which they 

 will be required to lift, this is not, in all cases, the only 

 consideration. After the firing of a "shot," when the "output" 

 is lying in a confused heap, a powerful crane will often be found 

 of the utmost service in dragging out the larger lumps and 

 distributing them to the stone-cutters for splitting up into such 

 sizes as may be required. It is not always easy to estimate 

 what power is required to dislodge such stones from a heap ; 

 but where work of this kind is anticipated, it would scarcely 

 be prudent to provide cranes of less power than I have named, 

 smaller auxiliary cranes being provided, if necessary. 



The cranes which are most convenient if of the travelling 

 type, inasmuch as they can follow up the work and move out 

 of danger, as has already been pointed out should command 

 two or more lines of way, by which arrangement one line at each 

 crane will always be clear to receive the empty waggons on their 

 return. 



In order to protect cranes from injury when they are not 

 able to run back under shelter, strong shields, of sufficiently 

 large area, should be provided. 



These may be made of timber, or other suitable material. 

 They are usually hauled up in front of the cranes by the cranes 

 themselves. 



Fig. 17, p. 96, is copied from a woodcut which illustrates 

 Mr. Harrison Hayter's paper on the Holyhead new harbour 

 (Min. Proc. Inst. C.E., vol. xliv.). It shows the arrangement of 

 galleries and charges in a blast which produced 120,000 tons of 

 stone. 



For much of the information which this chapter contains 

 relative to the quarrying operations at Holyhead, I am in- 

 debted to the above paper, and also to some pleasant con- 

 versations which I had with Mr. Hayter my old friend and 

 chief while visiting the Holyhead quarries in the autumn of 

 1889. 



In driving galleries, or headings, they should always be 

 made to take one or more rectangular turns, in order to prevent 

 any possibility of the tamping being blown out. 



The average rate of progress in driving the headings at 



