When gun-cotton is the explosive agent employed, the wet material which constitutes the charge is put into the shot-hole in cartridges, one after another, until a sufficient quantity has been introduced. Each cartridge must be rammed down tightly with a wooden rammer to rupture the case and to make the cotton fill the hole completely. A length of safety fuse is then cut off, and one end of it is inserted into a detonator cap. This cap is fixed to the fuse by pressing the open end into firm contact with the latter by means of a pair of nippers constructed for the purpose. The cap, with the fuse attached, is then placed into the central hole of a dry “primer,” which should be well protected from moisture. When an electric fuse is used, the cap of the fuse is inserted in the same way into the primer. The primer is put into the shot-hole and pushed gently down upon the charge. As both the dry gun-cotton and the detonator may be exploded by a blow, this operation must be performed with caution.

Cotton-powder or tonite requires a somewhat different mode of handling. It is made up in a highly compressed state into cartridges, having a small central hole for the reception of the detonator cap. This cap, with the safety fuse attached in the way described, or the cap of the electric fuse, is inserted into the hole, and fixed there by tying up the neck of the cartridge with a piece of copper wire placed round the neck for that purpose. The cartridge is then pushed gently down the shot-hole, or, if a heavier charge is required, a cartridge without a detonator is first pushed down, and the “primed” cartridge put in upon it. No ramming may be resorted to, as the substance is in the dry state.

When dynamite is the explosive agent used, a sufficient number of cartridges is inserted into the shot-hole to make up the charge required. Each cartridge should be rammed home with a moderate degree of force to make it fill the hole completely. Provided a wooden rammer be employed, there is no danger to be feared from explosion. A detonator cap is fixed to the end of a piece of safety fuse, and, if water tamping is to be used, grease, or white-lead, is applied to the junction of the cap with the fuse. A “primer,” that is, a small cartridge designed to explode the charge, is then opened at one end, and the detonator cap, or the cap of the electric fuse, is pushed into the dynamite to a depth equal to about two-thirds of its length, and the paper covering of the primer is firmly tied to the cap with a string. If the cap be pushed too far into the dynamite, the latter may be fired by the safety fuse, in which case the substance is only burned, not detonated. With an electric fuse this cannot occur. The same result ensues if the cap be not in contact with the dynamite. The object of tying in the cap is to prevent its being pulled out. The primer thus attached to the fuse is then pushed gently down upon the charge in the shot-hole. It should be constantly borne in mind that no ramming may take place after the detonator is inserted.

Gun-cotton and tonite require a light tamping. This should consist of plastic clay; or sand may be used in downward holes. The tamping should be merely pushed in, blows being dangerous. A better effect is obtained from dynamite when tamped in this way than when no tamping is used. In downward holes, water is commonly employed as tamping for a dynamite charge, especially in shaft sinking, when the holes usually tamp themselves. But in other cases, it is a common practice to omit the tamping altogether to save time.

Firing the Charges.

—When all the holes bored have been charged, or as many of them as it is desirable to fire at one time, preparation is made for firing them. The charge-men retire, taking with them the tools they have used, and leaving only him of their number who is to fire the shots, in the case of squibs or safety fuse being employed. When this man has clearly ascertained that all are under shelter, he assures himself that his own way of retreat is open. If, for example, he is at the bottom of a shaft, he calls to those above, in order to learn whether they be ready to raise him, and waits till he receives a reply. When this reply has been given, he lights the matches of the squibs or the ends of the safety fuse, and shouts to be hauled up; or if in any other situation than a shaft, he retires to a place of safety. Here he awaits the explosion, and carefully counts the reports as they occur. After all the shots have exploded, a short time is allowed for the fumes and the smoke to clear away, and then the workmen return to remove the dislodged rock. If one of the shots has failed to explode, fifteen or twenty minutes must be allowed to elapse before returning to the place. Nine out of ten of the accidents that occur are due to these delayed shots. Some defect in the fuse, or some injury done to it, may cause it to smoulder for a long time, and the blaster, thinking the shot has missed, approaches the fuse to see the effects produced by the shots that have fired. The defective portion of the fuse having burned through, the train again starts, and the explosion takes place, probably with fatal consequences. Thus missed shots are not only a cause of long delays, but are sources of great danger. Accidents may occur also from premature explosion. In this case, the fuse is said to “run,” that is, burn so rapidly that there is not sufficient time for retreat.

Fig. 50.

When the firing is to take place by means of electricity, the man to whom the duty is entrusted connects the wires of the fuses in the manner described in a former chapter, and as shown in [Fig. 50]. He then connects the two outer wires to the cables, and retires from the place. Premature explosion is, in this case, impossible. When he has ascertained that all are under shelter, he goes to the firing machine, and, having attached the cables to the terminals, excites and sends off the electric current. The shots explode simultaneously, so that only one report is heard. But there is no danger to be feared from a misfire, since there can be no smouldering in an electric fuse. The face may, therefore, be approached immediately, so that no delay occurs, and there is no risk of accident. Moreover, as all the holes can be fired at the moment when all is in readiness, a considerable saving of time is effected. It is essential to the success of a blast fired by this means that a sufficient charge of electricity be generated to allow for a considerable loss by leakage. If Siemens’ large dynamo-machine be used, the handle should be turned slowly till a click is heard inside, and then, not before, the cable wires should be attached to the terminals. To fire, the handle must be turned as rapidly as possible, a jerky motion being avoided. As considerable force is required, the machine must be firmly fixed. If a frictional machine be used, care must be had to give a sufficient number of turns. As this kind of machine varies greatly, according to the state of the rubbing surfaces and the degree of moisture in the atmosphere, it should always be tested for a spark before firing a blast. In this way only, can the number of turns required be ascertained. It is important that the discharging knob should be pushed in, or, as the case may be, the handle turned backward, suddenly. A slow motion may be fatal to the success of a blast. In testing Bornhardt’s machine, the handle should always be turned forwards; but in firing, half the number of turns should be given in one direction and half in the other. The following table shows the number of turns required for a given number of André’s fuses with Bornhardt’s machine. The first column, containing the least number of turns, may be taken also for Julian Smith’s machine as manufactured by the Silvertown Company with the modifications suggested by W. B. Brain.

Firing Table for Frictional Machine.
When the Machine sparks with 10 Turns.When the Machine sparks with 12 Turns.When the Machine sparks with 14 Turns.
Fuses in Circuit.Number of Turns.Number of Turns.Number of Turns.
 4121517
 5121517
 6141720
 7161922
 8182225
 9202428
10222631
11242834
12253035
13263136
14273338
15283439
Note.—If the machine does notspark with 14 turns, the rubber should be taken out and brushed.