QUARRYING. 103 



The best materials for tamping are those which are cohesive, 

 impervious, and moderately hard such, for instance, as good dry 

 clay, powdered brick slightly moistened, and some kinds of 

 decayed rock. 



Sand, stone-chips, and such-like, are not good materials for 

 tamping, and should not be used when other and more suitable 

 material can be procured. Stone-chips should on no account be 

 used near the charge. The tamping material should be put 

 down the bore-hole in small pieces, which should be well 

 rammed with a copper or wooden rod ; iron must on no account 

 be used. 



For a length of, say, 12 inches from the charge, it is well not 

 to ram too hard for fear of disturbing the fuse, or breaking its 

 connection with the charge; moreover, nothing should be used 

 in the. tamping which would be likely to cut or injure the fuse, 

 as a miss-fire might thus be caused. 



When using dynamite or other quick explosives, the same 

 care in selecting material for tamping is not necessary, fine sand 

 being quite effective. With nitro-glycerine compounds, even 

 water may be used. 



In the mine-blasts at Holyhead, the powder was usually 

 built in for a few feet with dry rubble, the remainder of the 

 headings being rammed with clay. 



Firing by Electricity. Large blasts are usually fired by means 

 of electricity. 



The batteries used for this purpose are of various designs, 

 but the methods adopted for igniting the charges are somewhat 

 similar. This is usually done by passing a sufficiently strong 

 electrical current through an exceedingly fine platinum wire, 

 whereby it is made red-hot or fused. This fine wire, which is 

 contained in a specially constructed fuse, is included in the 

 circuit, and is surrounded by a highly inflammable substance, 

 which the heated wire ignites. Fuses of this description are 

 called "low tension" or "quantity" fuses. 



In what are termed " high-tension " fuses, the ends of two 

 copper wires are brought to within about -^ or ^ of an inch of 

 each other, there being thus a break in the circuit. In this 

 break the priming composition is placed. It is fired by the 

 spark which is generated by the passage of the electric fluid 

 from one wire to the other. 



When a charge is to be fired one or other of the above fuses 



