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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small chance of a misfire. When the means of testing the detonators are not at hand the wires may be coupled-up in parallel, but of course a less number of shot-holes can then be fired simultaneously.

209. The principal blasting machines for firing low tension detonators used in this country are 'Siemens' Exploder,' Rackbar Exploder' (Smith's Blasting Machine), 'Crescent,' and 'Victor.'

The Rackbar exploder or Smith's blasting machine, already mentioned, is extensively used in this country. It is a very serviceable machine, the largest size being capable of firing twenty-six high tension or thirty-five low tension detonators; it is of simple construction, and very effective when properly manipulated. The Crescent blasting machine is of strong construction, and although of considerable weight it is a very effective machine, being capable of firing thirty-five shot-holes simultaneously. This is the only electric blasting machine, so far as the author is aware, which discharges a uniform current of electricity on every occasion, no matter who may operate it. To obtain the best possible results with the other machines mentioned the operator must have had some previous experience, as it is possible to give a weak or a strong current at will, or by improper manipulation. Even those experienced have difficulty at times in operating these machines so as to discharge exactly the same quantity of current at each operation.

210. Figs. 86, 87 illustrate the interior construction of a Smith's or Rackbar blasting machine, which, for effective working, compares favourably with any of the others, while the small size and moderate weight are greatly in its favour.

They all depend on the manner in which the electric current is generated, which is practically the same as in a dynamo.

The iron magnet A is of the horse-shoe type, wound about with coils of insulated copper wire, and between the poles of the magnet there is a revolving cylindrical armature B, on the body of which insulated wire is coiled longitudinally. The rapid revolution of the armature generates and sustains in the machine an accumulative current of electricity of great power, which at the moment of its maximum intensity is switched off to the outside circuit, in which are the detonators. C is a loose pinion, its teeth engaging with the rackbar G, and, by a clutch, engaging with the spindle of the armature in the downward stroke of the rackbar. D is a spring which, when struck by the foot of the descending rackbar, breaks the contact between two small platinum bearings E, and causes the whole current of electricity to pass through the outside circuit. F is the commutator, while at H are shown the binding-posts or screws, to which the cables are attached. These machines are sometimes made with three wire connections, namely, two out-current wires and one in-current or return cable. It is claimed that more effective work is performed by machines possessing this arrangement, but as yet few of them are in actual use.

211. Final Arrangements for Blasting.-The final arrangement necessary to be made in carrying out the blasting is to place the machine at a safe distance from the quarry face, so that the operator and the machine itself will be out of reach of any pieces of rock which may accidentally be projected from the shot-holes when the charges are exploded. In most cases 100 to 150 yards is a safe distance, and provided the bore-holes are deep and the tamping is properly executed, there is little chance of any mishap occurring at this distance. In blasting, the rock operated on falls or is thrown in the direction of the least resistance, that is, from the rock mass

FIGS. 86, 87.-Sections of Smith's blasting machine.

towards the open portion of the quarry. The blasting machine, therefore, should be placed in a position opposite in direction to that in which a rock mass is expected to fall, so as to ensure its being placed in the least exposed situation. It is always necessary to bear in mind that from some cause or other a few detached pieces of rock may be projected upwards in an inclined direction; therefore, when consistent with the preceding observations, it is advisable to locate the position of the blasting machine so that the operator's view of the quarry will not be interrupted by the sun's rays.

The cables are then connected to the binding-posts or thumb-screws of the blasting machine, the workmen are ordered to retire to a place of safety,

everything is made ready, and the customary signals are exchanged. The handle or rack bar is then put in operation, and the requisite electrical current generated, which is switched off into the circuit, when, if everything is in proper order, a simultaneous explosion will be the result, accompanied by a low rumbling noise which indicates effective work.

212. As a substitute for electrical firing, friction fuses have been employed at times, and also instantaneous fuses; the latter are still much used in some of the large quarries in England and Wales.

213. The instantaneous fuses or volley-firers made by the firm of Beckford, Smith & Co., of Tuckingmill, which are claimed to act simultaneously, differ from this firm's ordinary safety fuse in having a core of cotton wick which has been drawn through a paste of meal powder, instead of a powder train. When it is desired to fire several shots by means of this fuse, the required number are inserted into a tin socket or volley-firer, at one end of which is a plug of wood perforated to receive the time-fuse for igniting the instantaneous fuses leading to the shot-holes, and all the shot-hole fuses are concentrated in the receptacle and brought into contact with the disc. A gunpowder disc is placed against the plug, the whole being insulated with gutta-percha. The instantaneous fuse burns at the rate of 50 yards per second, and for open quarry work the T-pattern of volley-firer is generally made use of when the fuse is of any length.

These fuses have to be specially made when the number of the shots and the distance apart is great, which particulars must be ascertained some time previous to the blast taking place. As a matter of convenience, they are generally made up and connected by the makers at their works.

This method of firing may be approximately simultaneous for a few shotholes, but when a large number are intended to be ignited at once, the varying lengths of the patent fuse must necessarily mean a difference of time, and although to all appearance the explosion is simultaneous, it that a few seconds must elapse between the actual explosion of several of the charges. With electrical firing this is not so, as the current must pass from the blasting machine through the outer circuit and return to it again before all the detonators can be exploded. For quarrying in mines and tunnels Beckford's patent fuse, when a limited number of shot-holes are to be fired, may be adopted with great advantage.

214. Misfires. Should a misfire occur, which unfortunately happens sometimes in practice, it may be traced to various causes, such as (1) a defective detonator, (2) faulty shot-hole wires, brought about by careless handling, (3) the weakness of the current generated by the blasting machine. The first two causes should not occur in low tension blasting, as they are discoverable by testing previous to firing. The best way of guarding against the third cause is not to have more shot-holes than the machine will fire with certainty.