evolved from the powder can escape freely through the damaged covering.

Blasting by Electricity. The practice of igniting shots by the aid of the electric current has been gaining ground for a considerable number of years; with it no question can arise as to whether shots have missed fire or not. Ignition with ordinary fuse sometimes hangs for a considerable time, even up to twenty-four hours; sparks from the fuse are got rid of by numerous devices, but as no sparks are produced by electricity it must be better. Then again, there is no chance of premature explosion. Every one can be in a place of safety before the shots are fired; indeed, in some collieries where blasting might produce an explosion, all the shots are fired from the surface when the pit is free from men.

Two systems are in use; in one, electricity of high tension and small quantity is employed, while in the other the electricity is of low tension and of large quantity. The former are called "tension," or "machine," fuses, and the latter, "quantity," or "battery," fuses.

Tension Fuses.-These consist of two copper wires, with the ends separated from each other by a small interval, in which is placed a priming composition, and the whole inserted into a detonator. The current, in leaping across the interruption, meets with great resistance from the low conductivity of the material passed through, heat is generated, and the priming and detonator fired. The priming composition generally employed is known as Abel's, and consists of a mixture of 10 parts of sub-phosphide of copper, 45 parts of subsulphide of copper, and 15 parts of chlorate of potash, well rubbed together in a mortar, with sufficient alcohol to moisten the mass, and afterwards carefully dried.

Quantity Fuses.-Here the two copper wires are joined together by a very thin, short length of platinum wire, and surrounded by a substance inflammable at a low temperature. The current passing down the copper wires meets with great resistance in passing across the small section platinum wire, and generates sufficient heat to fire the priming. As the circuit is uninterrupted, and quantity only is required to heat the wire to redness, an ordinary battery may be used.

Comparison. The advantages of high tension lie chiefly in the convenient form and ready action of the machines employed to excite electricity. These are of small dimensions, light weight, simple in construction, and do not readily get out of order. In addition the means of discharging the machine may be removed until the required moment. For this reason such system is useful in mines where the operations are carried out by men of no scientific knowledge. A great advantage, however, is the fact that a large number of shots can be fired simultaneously with more certainty than with a battery, and that line resistance has small effect on the current, so that cables of small diameter can be used.

The disadvantages are, that the fuses are more or less affected by moisture and heat, and that the wires carrying the current have to be well insulated. Low tension fuses are more trustworthy than high. The insulation of the line wires need not be very perfect. Certainty of action is almost always possible, as each fuse can be tested before use by coupling it up to a galvanometer and passing a weak current through it. It does not, however, necessarily follow that miss-fires

cannot take place after low tension fuses have been tested in the galvanometer, because two conditions may happen. Either the two copper wires may be in contact, and allow the current to pass, or even the small testing current may break the delicate bridge of connecting wire, and the fuses thus appear all right at the test, but all wrong afterwards. When low tension fuses had to be fired by batteries they were not so convenient as high tension, as only a limited number of shots could be fired simultaneously, unless a large battery power was available. Batteries soon get cumbersome, and always require a considerable amount of attention. Low tension fuses can, however, be fired from an electric light circuit, while high tension ones cannot, and as dynamos are common at collieries, low tension fuses are becoming more and more applied. Specially constructed magneto machines generating a current of low intensity are now made for firing low tension fuses, and are recommended for use in preference to batteries. Low tension fuses cost a fraction of a penny more than high tension ones, but year by year are becoming more used in Europe, while in Canada and the United States high tension exploders and fuses are practically unknown.

For firing tension fuses, two types of machines are employed-(a) the frictional, (b) the magneto type.

Frictional Machines.-The machine most in favour is that of Bornhardt, which, from its simplicity, compactness, and portability, possesses many advantages. Electricity is generated by the friction of two revolving discs of ebonite against two small cushions covered with cat-skin, and is received by two cones, and transmitted by a metallic conductor into the interior of a Leyden jar, from which it is discharged by pressing a button. The apparatus is, however, very delicate; both glass and ebonite being so hygroscopic, that a machine can rarely be depended upon to work many hours consecutively. Unless the places in which it is used, and the rubbers, are dry and warm, the machine will not furnish any current, as the electricity is conducted away by the condensed moisture as fast as it is generated.

Magneto-Machines.-These consist of an electro-magnet, between whose poles an armature, wound to a very high resistance, is caused to rapidly revolve by means of crank motion and gearing. An electric current of high potential is generated, and at the moment of maximum intensity is sent out to the outside circuit, in which are the fuses, the explosion of these being instantly accomplished. To prevent the risk of miss-fires the machine should have a considerable reserve of power, and care should be taken to see that the connecting ends of all wires are clean and are firmly twisted together.

Simultaneous Blasting. The advantages of firing a number of shots simultaneously, especially in shafts or headings, are self-evident, particularly where machine drills are employed. In the first place, as soon as the machines have been removed and the holes charged, the rock should be shot down as quickly as possible. Then all the shots going off at once assist each other, their force is applied collectively, and the whole of the rock is brought clean away, while, if fired separately, each individual blast has to tear out the mass of rock allotted to it, the result being that in the former case less explosive is required and, in addition, a minimum amount of time is taken up in the operation. Another advantage of simultaneous firing is that all the smoke produced by the explosion is generated at one time and the men only

have to wait for this to clear away, while if shots are fired independently they have to wait after each blast.

For firing a large number of shots at once electricity is particularly useful, the reduced quantity of explosive used balancing the cost of the electric fuse, the saving in time already referred to remaining as an advantage. Another point of importance is the question of missed shots. When firing with fuse one can never be sure whether the shot has really missed or only hung fire, and, unless explosion takes place, the working has to be fenced off for a considerable time, thus entailing a loss; but with electricity nothing of the sort occurs. After the current has been passed through the wires the place can be approached at once without danger. In order to avoid miss-shots it has been proposed to use a hollow tube during stemming, one end of which is inserted into the charge and the other projects out of the hole. After ramming is complete, the tube is withdrawn, and the detonator and connecting wires pushed down the passage into the charge. Should a

Figs 85, 86, and 87.

miss-fire occur the detonator can be withdrawn and another inserted. The objections are the danger in pushing the detonator into, or withdrawing it from, the hole, and the reduced effect of the blast due to the fact that a vent hole of at least inch diameter is left extending from the explosive to the atmosphere.

For firing by electricity two main systems of connecting the wires to the machines are in use. In the first, the fuses are connected in series--that is to say, one wire of the first hole is connected to one wire of the second hole, and the remaining wire of the second hole to one wire of the third hole, and so on until all are joined, when there will be one wire of the last hole and one wire of the first hole left unconnected. These are now joined by means of conducting wires to the machine a considerable distance away in a place of safety (Fig. 85).

The second system is known as the parallel one. In this, one wire of each shot is connected to one cable, and the other wire to the second (Fig. 86). Modifications of both these systems are possible, as the holes may be connected in multiple series (Fig. 87).

The disadvantage of the series system is that the power of the machine has to be equal to that required to fire each fuse, multiplied by the number of fuses, and that unless the fuses have all the same resistance, or vary only within narrow limits, only the most resistant will be fired.

Bickford's Volley Fuse.-To render the operation more simple than with electricity, Messrs. Bickford have designed a method in which ordinary and special fuses are employed for simultaneous blasting. A length of safety fuse is connected to one side of an explosive disc in a tin tube. The required number of special fuses are snugly tied together, their ends cut clean and level, and inserted into the tin tube, touching the other side of the explosive disc. The mouth of the tube

is protected by a waterproof substance, such as pitch. To fire, the length of the safety fuse is lighted, this ignites the explosive disc, which starts all the special fuse burning at the same time. The particular point, however, consists of the special fuse, which is manufac tured to burn at the rate of 9000 feet per minute, the speed of ordinary fuse being only 2 feet per minute. To enable operators to adapt the instantaneous fuse to

Fig. 88.

W

any available length to suit their particular requirements, the inventors supply on demand the ignitors with fuse looped as in Fig. 88, so that if the whole length of fuse so looped is, say, 10 feet, the miner can cut it into single lengths of 3 feet and 7 feet, or any proportion of 10 feet (taking care not to detach it from the ignitor). This does not affect the simultaneousness of the explosion, as, owing to the rapidity of burning, small differences in the lengths of the special fuse are not of any moment.

Position of Holes.-The situation and inclination of holes in rock drifts depend on the nature of the rock, and on the system of drilling employed. With hand drilling and single blasts, everything depends on the skill of the miner, who carefully examines the faces and decides on the position, direction, and depth of the hole; the conditions that have to be fulfilled being that the rock should be as free as possible on one side, and that neither too much nor too little rock should be attempted to be dislodged. In the former case, if there is too much resistance the hole will act like a

cannon, and the tamping will be forced out, producing what is known as a "blownout shot," while, in the latter case, the explosives will be wasted.

Longitudinal Section through Centre Cut Holes.

Face of Heading

Figs. 89 and 90.

Plan of Heading

Longitudinal Section through Side Foles.

Figs. 91 and 92.

With machine drills and simultaneous blasting, there is not so much necessity to consider the lines of least resistance, although such is generally done. Many different arrangements can be employed. The following may be considered a typical example.* A wedge, or core, is first blasted out of the centre of the heading, this being known as a centre-cut, the sides being blasted out afterwards. A centre-cut needs about eight holes, divided into two sets, four each, arranged in nearly vertical lines, at equal distances from the centre line of the heading. Each hole of one set of the centre-cut is drilled * The Vosberg Tunnel, Leo v. Rosenberg, p. 24.

in a direction intended to meet the corresponding one of the other set at the centre line of the heading, so as to form a wedge. These are drilled 10 feet deep. Where the character of the material only requires one set of holes in the sides, these are three in number, and from 7 to 9 feet deep. The inclination of the holes in the different sets are shown in Figs. 89 to 92. The holes inclining upwards are drilled dry, those horizontal, or inclined downwards, wet. Sometimes second side rounds are required; these consist of two holes each. At the Newhouse Tunnel, Colorado, U.S.A., the work was said to be much hastened by the addition of a so-called plunger hole at the upper centre of the cut, which inclined slightly downwards.*

Blown-out Shots.-The combustion of powder produces large quantities of gaseous products, which, in the case of blown-out shots, are driven violently into the roadways, and at the point of discharge act like a piston, driving back the air flowing past the hole in both directions, and producing a partial vacuum, into which the gas contained in the coal is exhausted, and diluted with the air current until the firing point is reached. Clouds of dust may be raised at the same time, and if this mixture comes into contact with flame, a serious explosion is readily produced. It is also suggested that the

sound wave produced by a blown-out shot may cause sufficient pulsation in the atmosphere to force flame through the gauze of a safety lamp. It is, therefore, desirable that blown-out shots should be prevented, care being taken that all the holes are placed in such position that they do the work allotted to them, and bring down the coal. It is most important that the stemming should be unfissured, and

adhere closely to the sides of the hole, so that the gaseous products cannot escape before the complete ignition of the powder. To remove, however, any chance of such an occurrence, various tamping plugs have been designed, the majority of which consist of an arrangement of metal wedges tightly secured in the hole, generally by the aid of a screw. They are expensive in first cost, and easily lost. A later device is the employment of a cylinder, or rough octagon of pinewood, with a wedge-shaped piece cut out of it and a saw cut made as a continuation of the wedge-shaped cut. The wedge a (Fig. 93) is placed against the charge, the block b above it, and the explosion drives the wedge up into the body of the block, and binds it firmly against the sides of the hole. The use of tamping plugs does not seem to afford any greater security than ordinary tamping, if the latter is properly applied.

VARIOUS METHODS TO SUPERSEDE BLASTING.— Numerous methods have been proposed to do away with blasting, such as the application of compound wedges.

* Eng. and Min. Journ., 1902, lxxiii., 552.