EXCAVATINt; MA< H1NKS AND HOCK DU1LL8 25 



may be mounted either on carriages or on columns for tunnel 

 work. 



EXPLOSIVES AND BLASTING. 



When the holes are once drilled, either by hand or power 

 drills, they are charged with explosives. The principal explo- 

 sives employed in tunneling are gunpowder, nitroglycerine, and 

 dynamite. 



Gunpowder Gunpowder is composed of charcoal, sulphur, 



and saltpeter in proportions varying according to the quality of 

 the powder. For mining purposes the composition employed 

 is 65 % saltpeter, 15 # sulphur, and 20 $ charcoal. It is a black 

 granulated powder having a specific gravity of 1.5 ; the black 

 color is given by the charcoal ; and the grains have an angular 

 form, and vary in size from J in. to 3 in. Good blasting 

 powder should contain no fine grains, which may be detected 

 by pouring some of the powder upon a sheet of white paper. 

 The force developed by the explosion of gunpowder is not 

 accurately known ; it depends upon the space in which it is 

 confined. Different authorities estimate the pressure at from 

 15,000 Ibs. per sq. in. in loose blasts to 200,000 Ibs. per sq. in. 

 in gunnery. Authorities also differ in opinion as to the 

 character of the gases developed by the explosion of gun- 

 powder, a matter of vital concern to the tunnel engineer, since 

 they are likely to affect the health and comfort of his work- 

 men. It may be assumed in a general way, however, that the 

 oxygen of the saltpeter converts nearly all of the carlion of 

 the charcoal into carbon dioxide, a portion of which combine* 

 with the potash of the saltjK'ter to form carbonate of potaah, 

 the remainder continuing in the form of gas. The sulphur in 

 converted into sulphuric acid, and forms a sulphate of potn*h, 

 which by reaction is decomposed into hyposulphite and sul- 

 phide. The nit re lire n of the salt|K>tpr is almost entirely evolved 

 in a free state : and the oarlmn not having l*en whollv burnt 

 into carbonic acid, there is a proportion of carbonic oxide. 



