length of the rod, then by observation it can be held in the position which will keep the point as clear of the fuse or wires as possible.

Tamping shot-holes charged with the nitro-compounds by means of water has many advocates and is extensively made use of, but in the author's experience such a method of tamping has not been attended with satisfactory. results, and is certainly not to be compared in any way with the system of tamping described in the preceding paragraphs.

204. Nature of an Explosion of Nitro-Compounds. It is generally believed, even by those of some experience, that the direction of impact of the nitrocompounds, when exploded, acts in a downward direction. This is not the case, however, as may be inferred by considering the following facts, which do not justify the general use of water as a means for tamping shot-holes.

rod.

If a cartridge of dynamite is suspended in mid-air, where the resistance around it is equal on all sides, it will, provided the explosive is of a uniform composition, and is detonated throughout its body simultaneously, cause the gases generated to expand equally in all directions. The only difference between the explo- FIG. 81.-Tampingsion of a charge of blasting powder and that of a dynamite cartridge is one of degree, the latter being much more quickly converted into a gaseous state; hence its action is rapid. If a cartridge of blasting gelatine or dynamite be placed on a block of stone and detonated, it is possible that under most circumstances the stone will be cracked in several directions. This does not prove, however, that the impact is entirely downwards, as only a limited amount of the explosive effect created may have acted in this direction by reason of its rapid action on the surface of the stone. A cartridge of the same kind and strength placed in the bore-hole of a block of stone of equal size and solidity and effectively tamped-in other words, similar to the one already operated upon-having equal resistance all round, would cause it to be shattered and the fragments would probably be thrown in all directions. It will thus be seen that, with an equal resistance, the gases generated by the explosion of dynamite will expand equally in every direction. Should the resistance be unequal, however, the explosive effect will be deflected in the direction of the line of least resistance. Hence in shot-holes tamped with water it sometimes happens that the deflected energy of the explosive follows the bore-hole, as it easily overcomes the resistance of the water, and expends its useful effect in the air. This explanation of the nature of an explosion of the nitro-compounds is amply borne out in practice. In carrying out some quarrying operations the

author had occasion to make use of water tamping, not as a matter of expediency, but from necessity. Of twelve shot-holes which were fired simultaneously three simply discharged the explosive into the air and on the quarry face opposite, the result being devoid of any useful effect. Two of these bore-holes were in a horizontal position and one vertical; from the last mentioned a small piece of loose rock surrounding the mouth of the hole was projected to a considerable height, and subsequently fell within a few feet of its previous position in the quarry. In bore-holes generally (especially in those drilled in a vertical direction) when there is a continuous flow of water (as was the condition of things in the case mentioned), it is advisable to place a few pellets of clay over the charge and to fill the remaining portion of the hole with sand. This assists in confining the gases when expanding, and therefore increases the effective results. Similarly, if the full force of an explosion is deflected when an accidental explosion of the nitro-compounds takes place, distant objects may be damaged, while objects comparatively near in different directions may not be affected.

205. Connecting the Shot-hole Wires.-When two or more shot-holes are to be exploded simultaneously, the wires leading from the detonators require to be joined to those on either side and also to the cables leading to the blasting machine, so as to form a complete metallic circuit. When the wires from the shot-holes are not long enough to join those on either side, it is necessary to make use of connecting wire, which should be of the same quality as the detonator wires and of a similar diameter. These latter are made from th to nd of an inch in diameter, and are insulated with paraffined cotton wound double, or with gutta-percha or fibre composition. The mode of joining the wires is to cut off the insulated covering for about 3 inches from the end of each wire, and then twist them together firmly in a somewhat reverse fashion, as shown in fig. 82.

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The bare ends should be scraped with a knife, as the contact surfaces must be clean, bright, and free from dirt or grease. These bare con

FIG. 82.-Method of connecting wires.

nections when the ground is wet should be protected by covering them with insulating tape; this is especially necessary when the explosion of the charge is effected by a blasting machine generating a high tension.

.current.

206. Circuits: different methods of connecting-up.- In effecting the metallic circuit between the shot-holes and the end of the cable wires, three different systems may be adopted in coupling-up the detonators, viz., (1) in series, (2) in parallel, or (3) in a manner combining both systems.

(1.) Series Circuit. -The method known as the series circuit, or conmecting the wires in series, and shown in fig. 83, is the one most frequently adopted when low tension or quantity detonators are used, which arrangement can be tested previous to firing. It affords a simple and effective means of forming the connections. Its only disadvantage is that a single defective

-detonator or faulty shot-hole wire will prevent all the others being fired, and cause what is termed a misfire; but by testing the exploders previous to inserting them in the shot-hole the risk of the defect mentioned may be reduced to a minimum.

(2.) Parallel Circuit.-The mode of connecting the wires in parallel or direct circuit is shown in fig. 84; the wires of each shot-hole are in direct communication with the cables, and as they are independent of each other, they are free from the disadvantage incidental to the series method of forming the connections. The great amount of wire necessary to form the connection by this system, especially when there are a considerable number of shot-holes to be fired, increases the cost of the operations considerably.

FIG. 84. Connecting the shot-holes in parallel.

The connecting wires, instead of being placed so as to take the circuitous direction indicated in fig. 84, may be arranged in straight line between the shot-holes and the ends of the cable wire. They must, however, be kept -entirely separate from each other in order to avoid complications, and the bare parts must be properly protected to ensure perfect insulation.

(3.) Series and Parallel Circuit.-The other method which may be adopted is a combination of the series and parallel circuits; this arrange

ment is shown at fig. 85. It will be observed that should a defective detonator exist in the front row of shot-holes a b, none of the others in that row can be fired; consequently, the back row of shot-holes, if they are

FIG. 85.-Connecting the shot-holes in series and parallel.

in order, will be exploded. The force exerted, however, will not be sufficient to dislodge the rock in front, which, had the charges in all the shot-holes been exploded simultaneously, would have been effected.

207. Connecting to Cables and Terminals of the Blasting Machine.A wire from each of the detonators in the shot-holes nearest to the blasting machine is connected to the ends of the cable; and the other wire, as already explained, is connected to the next shot-hole wire when in series, and to the other cable when the parallel circuit is adopted.

The double cable is preferable for open quarry blasting; when hightension detonators are employed the material of which the wires are composed is of little importance; it may be of iron. For low-tension detonators the leading wires or cables are made of copper, and of a comparatively large section, so as to admit of as small a line resistance to the current as possible. They are protected by gutta-percha or other suitable insulating composition, and for convenience in using and storing should be wound on a reel. This is a matter of some importance, as, when a cable is used without such an accessory, the time lost and the trouble occasioned by the wires becoming a tangled mass is very great, and result in the insulation being destroyed, and possible damage to the wires. As the blasting depends on a thoroughly efficient metallic circuit it is necessary, before using the cables in actual work, to test this by attaching them to the blasting machine and holding the free ends together, when, if they are in good order, a spark will appear on the current being sent through the cable wires.

208. Firing or Blasting Machines. Machines for exploding the detonators, and consequently detonating the charge in the bore-hole, are, as may be inferred, of two kinds. The blasting machine for firing tension. detonators must be capable of producing a high tension or voltage, this being necessary to decompose the priming compound contained in the cap or detonator. The chief advantage of this class of blasting machine is the small effect of line resistance upon the current, in consequence of which the charges in the shot-holes may be situated at a relatively great distance from

the blasting machine. As there is no possibility of testing high tension detonators except in actual blasting, it is usual to connect or couple up the shot-holes in parallel, so that each may be independent of the other; this minimises the trouble which may be experienced from a misfire should a faulty detonator happen to be in the circuit.

All the wires connecting the blasting machine and the detonators in the shot-holes must be perfectly insulated, however, as the smallest leakage of the electric current will greatly reduce the useful effect in blasting, and possibly lead to total failure. The high tension type of blasting machine is much employed in mines, and in single-shot firing; those principally used in this country are known as the 'Rackbar Exploder' (Smith's), and the 'Siemens' Exploder.' The Rackbar blasting machine, weighing 42 lbs., is capable of exploding simultaneously twenty-six high tension detonators in parallel. The Siemens' Exploder can only be used for firing high tension detonators, and is made of different sizes. The smallest size, 100 volts, weighs 81 lbs., and can fire sixteen detonators, while the largest size, having four times that voltage, and weighing 21 lbs., can fire twenty-five high tension detonators.

For general quarry operations, however, the low tension system of blasting is in most favour, and possesses many points of great superiority compared with that of high tension 'exploders.' Low tension detonators possess the advantage that they can be tested, previous to being inserted in the shothole, by means of an electric galvanometer or by an electric fuse and blast tester. There is also the additional advantage that the whole charge can be tested by a weak current of electricity before connecting the leading wires to the blasting machine.

This testing machine is of great convenience, for, although detonators are now manufactured with great care, and seldom cause a misfire when proper care is exercised in handling them, still a detonator-testing machine is a valuable accessory to a blasting outfit, and its use may save considerable trouble and expense.

It is not of great importance that the wires for blasting on the low tension system should be thoroughly insulated, as bare wires are often used; but in damp places it is advisable to employ insulated wire, and to cover the joints or places where the connecting wires are coupled-up to avoid any possible escape of the electricity. The line resistance in connection with blasting machines generating a low tensiou current necessitates the employment of copper wire (as already mentioned) for the cables, which need not be of great length for ordinary quarry work. The length of the leading and connecting wires has a considerable influence on the total resistance in the circuit, consequently as this becomes greater the number of shot-holes which can be fired effectively will become less.

It is therefore usual to connect the shot-hole wires in series, because, as the detonators can be tested prior to the blasting taking place, there is but a

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