28 TUNNELING 



have the objection for tunnel work of burning with a bad odor, 

 especially when they are coated with tar, and to remedy this 

 many others have been invented. Those of Rzika and Franzl are 

 the best known of these. The former has many advantages, but 

 it burns too quickly, about 3 ft. per second, and is expensive ; 

 the latter consists of a small hollow rope filled with dynamite. 



Blickford matches cannot be used to explode dynamite, the 

 use of a cartridge being required. These cartridges are small 

 copper cylinders containing fulminate of mercury. They may 

 be attached to the end of the Blickford match, which being 

 ignited the spark travels along its length until it reaches the 

 copper cylinder, where it explodes the fulminate of mercury, 

 which in turn explodes the dynamite. Blasts may also be fired 

 by electricity, which, in fact, is the most common and the 

 preferable method, because several blasts can be fired simulta- 

 neously, and because the current is turned on at a great dis- 

 tance, thus affording greater safety to the workmen. 



The method of electric firing generally employed in America 

 is known as the connecting series method, and consists in firing 

 several mines simultaneously. The ends of the wires are 

 scraped bare, and the wire of the first hole of the series is 

 twisted together with the wire of the second hole, and so on ; 

 finally the two odd wires of the first and last holes are connected 

 to two wires of a single cable or to two separate cables extend- 

 ing to some safe place to which the men can retreat. Here the 

 two cable wires are connected by binding screws to the poles of 

 a battery, or sometimes to a frictional electric machine. The cur- 

 rent passes through the wires, making a spark at each break, and 

 so fires the fulminate of mercury, which explodes the dynamite. 



Simultaneous firing by electricity by utilizing the united 

 strength of the blasts at the same instant secures about 10 ^ 

 greater efficiency from the explosives. Another advantage 

 of electric firing is that in case of a missfire of any one of the 

 holes there is slight possibility of explosion afterwards, and the 

 place can be approached at once to discover the cause. 



