RAILROAD CONSTRUCTION 271 



one hand and strikes the drill with a 2- or 4|-pound hammer held 

 in the other hand. These are used only for holes of small diam- 

 eter, 3 feet or less in depth. This drill may be used for hori- 

 zontal or inclined bores. 



Hand drill rods are made of octagon steel and range in size 

 from f of an inch in diameter, with a f- or i-inch bit, up to a 

 |-inch rod with a i|-inch bit. A i-inch drill rod is the maximum 

 size practicable. Chisel-shaped bits, similar to those for jumper 

 and churn drills, are used. 



B. EXPLOSIVES^ 



Explosives for blasting belong to two general classes: 



1. High explosives which require for explosion an inter- 

 mediate agent, such as a fulminate detonator. 



2. Low explosives which can be fired by direct ignition. 

 High Explosives. — For blasting purposes these are marketed 



in the form of dynamite, giant powder, gelatine, and some other 

 similar products. The more powerful forms are composed of a 

 mixture of nitro-glycerine and some absorbent, such as sawdust 

 and wood pulp, while the lower grades contain explosive salts 

 in addition. Nitro-glycerine undergoes no change when com- 

 bined with the absorbent, the latter acting only as a cushion and 

 as a means of solidifying the liquid. 



High explosives are made of varying strengths and are graded 

 on the percentage of nitro-glycerine they contain. The standard 

 grades range from 75 per cent down. Those most frequently 

 used are 40 and 60 per cent, the former being preferred for many 

 classes of work. 



High-grade dynamite explodes with great suddenness and will 

 shatter rocks and stumps into small fragments. It is especially 

 suitable for very hard rock or where small drill holes are necessary. 

 Medium grades are best for soft rock because their explosive 

 force is not so violent and sudden, and the tendency is to heave 

 up large masses of rock rather than to shatter them into smaller 

 fragments. 



^ The author is indebted to publications of the E. I. DuPont de Nemours Co. 

 for many facts regarding explosives. 



