RAILROAD CONSTRUCTION 273 



In loading dry holes the cartridge case is cut on one side, and 

 the cartridge lowered into the hole and gently pressed until it 

 completely fills the bore. This is repeated until a sufficient 

 amount of explosive has been placed. When the hole is wet the 

 cartridge case should not be cut. 



The hole is now ready for the primer and for tamping. 



Primers and Priming. — Most forms of dynamite are exploded 

 by the use of a fulminate detonator or cap, which is ignited either 

 by a safety fuse or an electric fuse. The former is used for 

 individual charges and the latter where many are to be fired 

 simultaneously. 



Safety Fuse and Caps. — There are several grades of safety 

 fuse offered on the market, some of which are waterproofed for 

 submarine work. The fuse used for blasting ordinarily burns at 

 the rate of 2 or 3 feet per minute, and is marketed in packages 

 containing two coils, each 50 feet long. 



The cap consists of a hollow copper cylinder | by i| inches in 

 size which is closed at one end. It is partly filled with from 

 three to twelve grains of fulminate of mercury. The open end 

 is sealed with shellac, collodion, thin copper foil, or paper. Caps 

 deteriorate very rapidly when exposed to moisture. Several 

 grades are made, but for general use a No. 6 is preferred. 



In making the primer for an ordinary blast a piece of safety 

 fuse of the required length is cut off and one end inserted into 

 the cap until it comes in contact with the filling. The fuse is 

 held in place by crimping the cap |-inch from the open end. 

 The fuse and cap are then ready for insertion in the primer, 

 which consists of a cartridge of dynamite of the same size and 

 quality as that used in the charge. 



There are two methods of inserting the cap into the primer. 

 A common method (Fig. 76, a) is to open the paper at the end 

 of a cartridge, and, with a sharpened stick about the size of a 

 lead pencil, make a hole |-inch deep in the dynamite. The cap, 

 with fuse attached, is then inserted in this cavity and should 

 project |-inch above the dynamite, otherwise the sputtering of 

 the fuse may ignite the dynamite before it does the cap. The 

 cartridge paper is then tied around the fuse with a string, care 



