Page:St. Nicholas (serial) (IA stnicholasserial402dodg).pdf/482

928 to hasten the borings and enable two drill gangs to work at the same time, It is comparatively easy to blast away the “bench,” because the “shot” holes are drilled downward from the flat, upper surface, and when the dynamite is set off, it splits away the rock in slabs; but in the case of the “heading,” a special arrangement of “shot” holes is necessary, because the rock can be attacked only from the face. The first thing to be done is to remove a wedge, or “cut,” from the center of the “heading”; two rows of holes are driven in at an angle so that they will meet, or nearly meet, at a depth of about eight feet back from the face of the rock, Alter this wedge has been blasted out, it is not so difficult to split out the rest of the heading. At each side of the “cut” a row of “relief,” or side, holes is drilled, and finally a set of “liners,” or rim holes, that slant outward to some extent, so that the rock will be shattered to the full diameter of the tunnel. After the holes are drilled, the cut is blasted out first, then the “relief” holes, and, finally, the “liners.”



The bench-drills were mounted on tripods so that they could drill vertically, but the drills at the heading were carried on two posts or columns that were tightly wedged between the bench and the rock roof overhead by means of jack-screws. There were three “engines” on each column, so mounted that they could be turned in any desired direction, We watched one of them starting a hole on a shelving piece of rock. The steel was pounding with short, quick strokes, trying to hammer out a seat for itself, while sparks were dancing around the drill point. After a while, when a sufficient hollow had been pounded in the rock, the steel began to strike with longer and longer blows, until it reached a full seven-inch stroke. The exhausts of the drills were coated with something white and glistening. One of the men scraped off a bit of the stuff and handed it to me. It was frost! I stared in astonishment! We could n't, comfortably, do any talking down there, but when finally we got back to the surface, the superintendent explained it to us as follows:

“These drills are run by air compressed to one hundred pounds per square inch, When that air is compressed, up at the pumping station, it gets so hot that it blisters all the paint off the compressors, where they are not protected by water-jackets; in fact, it gets so hot that we cannot bring it to the full one hundred pounds at once but have to compress it in two stages, and cool it off between stages. You know how it is with a bicycle pump, don’t you? It gets so hot that you can scarcely bear your hand on the cylinder, just from the heat that is developed in compressing the air, Well, this compressed air we use has to be cooled, before we bring it down into the tunnel, by passing it through radiators and water, or air-cooled cylinders; but if air gives off heat when it is compressed, very naturally it has to absorb heat again when it is expanding, so as to regain what it lost before. It absorbs so much heat out of its surroundings, that any moisture it contains is condensed, and settles as frost around the exhaust-port. In fact, if we don’t watch carefully, it is likely to freeze the parts fast.”

We went down the shaft again later, to watch the charging of the holes after the drilling wascompleted. The drill boss began first with his “cut” holes. The dynamite cartridges were about eight inches long and an inch and a half in diameter, wrapped in paper tubes. The man would take a stick of the dynamite, or “powder,” as miners call all explosives, place it in the hole, and press it home with a wooden ramrod, so that the paper wrapper would burst open, and the soft, putty-like stuff would be mashed out to fill the hole completely. Other sticks of dynamite were then put in, each being rammed up against the preceding one. In one of the sticks he jabbed a wooden, to make a hole for the detonating cap, After sticking the cap in place and fastening it with a half-hitch of the electric wires around the cartridge, he rammed it up against the rest of the dynamite, then put in a few more sticks, and finally closed the hole with a cartridge filled with sand. Extra heavy