RAILROAD CONSTRUCTION 275 



material must be free from all forms of grit, and of such a nature 

 that it will pack firmly. The most satisfactory is moist clay or 

 loam. 



After the charge has been pressed tightly in the bore a paper 

 wad may be placed over the primer to keep it dry and from 2 

 to 3 inches of tamping material put in and firmly, but gently, 

 packed. Two to 3 inches more of tamping material are again 

 added and thoroughly tamped. After 5 or 6 inches of earth 

 have been placed in the bore the tamping can be carried on 

 without fear of premature explosion. The hole should be filled 

 to the surface and the material tightly packed, or it will blow out 

 and much of the force of the explosive will be lost. 



Low Explosives. — Low explosives belong to either the soda or 

 the saltpeter class and are known as black powder. The average 

 contain approximately 75 per cent of nitrate of soda, or India 

 saltpeter, 10 per cent of sulphur, and 15 per cent of carbon. 

 Dynamite of 75 per cent strength is usually rated as six times 

 stronger than average black powder. Soda powders can be made 

 cheaper than saltpeter powders but are more absorbent of 

 moisture and, therefore, deteriorate quicker. 



Black powders are especially suited for loosening hardpan, 

 shale, and other soft or rotten rock where a lifting action is 

 desired. It is much slower than high-grade dynamite and does 

 not shatter the rock as much. It is also used in redwood opera- 

 tions to blast open logs that are too large to be handled otherwise. 



Black powder is fired by a safety fuse, by a safety fuse and a 

 cap of low power, or by an electric fuse. In loading holes the 

 powder may be placed loose or in cartridges. When the holes 

 open downward the latter form is the only method possible. 



In priming holes it is customary to place the safety fuse or 

 safety fuse and cap at the top of the charge while electric fuses 

 are ordinarily placed in the center of the charge. 



Moist clay is the most satisfactory tamping material, 2 or 3 

 inches of dry earth being placed over the powder to prevent the 

 upper end of the charge from becoming moist. 



When blasting with black powder the holes may be ''sprung" 

 with dynamite before the powder is inserted, in order that a 



