of explosive used was either too much or too little for the work to be done. Overcharging the shot-holes is a dangerous practice, and should be carefully guarded against in all quarrying operations.

197. The position and direction of the bore-holes in road-stone quarry No. 2, taken from actual practice, are shown in fig. 74, as also the quantity of explosive (dynamite) used and the depth of each shot-hole; for instance, in 10 the lower figure indicates the depth of the bore-hole, the upper one the amount of charge in pounds, while the holes are indicated by the numerals 1, 2, 3, etc.

The quantity of explosive used in each shot-hole was determined by the method described; in the top holes 1 lb. of dynamite was allowed for dislodging 13 tons of rock, while in the bottom holes 1 lb. was allowed to

every 10 tons of rock. The total quantity of material calculated as likely to be dislodged was 3400; but the actual quantity of rock quarried and passed over the steelyard was 3475 tons, so that on an average 121 tons of rock was thrown or dislodged and broken up for each pound of explosive used. The appearance of the quarry face during drilling operations, and previous to blasting, is shown at fig. 88, while the result of the blasting, taken immediately after the shot-holes were fired, is represented in fig. 89.

198. Another example of a road-stone quarry, No. 3, is shown in fig. 75; the position and direction of the bore-holes, and the quantity of explosive used in each, are represented in a manner similar to that described in the preceding paragraph; a cross-section A A and the quarry face are shown in fig. 76. In this case the quarry, although in use for several years, had never been blasted, the limited quantity of metalling required each season having been obtained from the loose material on the top by means of bars and wedges.

The rock consists of basalt of a very tough and stubborn nature; consequently it was considered advisable to place the bore-holes closer to each other than in the cases referred to.

The amount of rock required for the purpose of forming a new road in the vicinity was 2000 tons, and considering the nature of the rock it was deemed advisable to work within a safe limit, the amount of the charge allowed being 1 lb. of explosive to every 10 tons of rock. The appearance of the quarry face previous to blasting is shown in fig. 90, and the result obtained is represented in fig. 91.

The actual quantity of rock dislodged and broken up suitable for sledging turned out, after weighing, to be 2102 tons, or equal to 10 tons

of throw for each pound of explosive used. The actual cost of drilling, charging, and blasting in these two quarries is given in detail on pp. 171 and 172.

199. Charging the Shot-holes.-Before commencing the operation of charging the shot-holes, it is necessary to remove any sludge or water which may be in them by the means already described, in order that they may be rendered as dry and clean as possible. Moisture will cool the gases produced from the explosive when fired, and therefore a part of the energy is uselessly expended in evaporating it, while the action of some explosives is lessened or prevented by it. If blasting powder in a loose state is to be employed as the explosive agent, particular care should be exercised, for the reasons given. The boreholes ought to be thoroughly wiped dry by means of a piece of tow or other suitable material attached to the eye of the scraper, which is passed slowly up and down the bore-hole to absorb the moisture.

FIG. 76.-Cross-section at A A of road-stone quarry No. 3.

After determining the amount of the charge by the rule given, the requisite quantity is placed in the shot-hole; if the explosive is blasting powder in a loose state this is accomplished by means of a tin or copper tube, so as to prevent its coming in contact with the sides of the bore-hole. Blasting powder is also made up in cartridges having a waterproof covering, in which form it has many advantages. One pound of blasting powder in a loose state occupies about 30 cubic inches, and 27 cubic inches when pressed, so that the quantity to be placed in a bore-hole can be determined exactly by measurement. Dynamite and the gelatinous explosives are made up in cartridges with paraffined paper wrappers, and are generally 3 inches long and 1 inch in diameter for the former and ths of an inch for the latter. Boxes containing 50 lbs. are subdivided into ten cardboard boxes made up of 5 lbs. each. The number of sticks to a pound of the nitro-glycerine explosives can thus be easily ascertained, being generally 8 or 9, and this at once is a measure to guide the quarryman in charging the shot-holes.

The plastic nature of the nitro-compounds affords a great practical advantage in charging, as it admits of their being readily rammed home into the shot-hole, so as to fill all the seams and crevices. It is advisable, in order to save time when charging machine-drilled shot-holes, to place three sticks of explosive at a time into them instead of singly; this is done by tying them together with a piece of twine previous to commencing the charging operations, the combined thickness being sufficient to fill the lower part of a 14-foot bore-hole. They are then dropped into the vertical holes, and pushed into the horizontal ones, to the required number, and thoroughly rammed home, each lot of three separately, by means of a tamping rod made of wood, without using undue force; the more compact the cartridges are the greater will be the effect of the explosion.

If a charge of blasting powder is to be fired by means of a straw, a pointed metal rod of small diameter called a pricker is placed against the side of the bore-hole, with its lower or pointed end in the charge. The tamping is then proceeded with, which, considering the heaving tendency of blasting powder, requires not only that it should be firmly rammed, but also that the material forming the tamping itself should be of a dense nature, as, for instance, moist clay.

The pricker is then carefully withdrawn, leaving a circular passage through the tamping down to the charge. Into this passage the straw, properly filled with powder of fine grain, is inserted; the charge is then ready for firing.

This method of firing charges, although defective in many respects and attended with danger owing to the number of supposed misfires, was the method exclusively adopted years ago, but is now prohibited by the regula tions of the Quarries Act, 1894.

The firing of a powder charge is generally effected by means of a safety fuse, a piece of which, sufficiently long to reach from the top of the charge

and to project a few inches above the bore-hole, is placed in a similar position to that occupied by the pricker in the case referred to, and the tamping proceeded with in a similar manner as described in the preceding paragraph. Better results will, however, be obtained if the fuse is placed in the centre of the charge so as to ensure its igniting and exploding properly.

It is a well-established fact that an electrical current will produce a more powerful initial impulse in firing shot-holes charged with powder than by the ordinary safety fuse; this method of blasting will be described presently. In blasting with dynamite and the other nitro-compounds as the explosive, the firing cannot be effectively carried out by safety or any other kind of fuse alone. To bring about instantaneous decomposition in the nitroglycerine class of explosives it is necessary to produce in their midst an explosion of some other substance. This is effected by means of a detonator, or copper capsule containing fulminate of mercury.

A short piece of safety fuse with a clean-cut end is inserted into the open end of the detonator till it reaches the fulminate; the detonator is then pressed tightly round the fuse by the aid of a pair of pliers specially made for the purpose. The detonator is then pushed to a depth equal to two-thirds of its length into the primer, a hole being previously made in the primer by means of a small round piece of wood having a similar diameter to that of the cap or detonator, the paraffined paper wrapper is then laid over and secured to the fuse. To ensure proper detonation the safety fuse in the cap ought not to touch the dynamite primer, as, should this occur, the explosive may become ignited previous to the fulminate of mercury in the cap exploding, which, besides creating noxious fumes, would detract from its explosive effect. The method of fixing the fuse and the other details necessary are shown in fig. 77, a being the fuse, the detonator, c the primer or cartridge of dynamite, and d where the paraffined paper wrapper is secured to the fuse by a piece of twine..

When damp or wet bore-holes are encountered, it is advisable to smear the connection between the detonator and the fuse with tallow or some other kind of water-resisting composition.

a

d

It is a common practice to insert a paper wad on the top FIG. 77.of the primer and detonator before the tamping is commenced; this forms a cushion and protects the detonator.

Dynamite primer and ordinary

fuse.

When a number of shot-holes are to be blasted simultaneously, the employment of electricity to fire the charge presents numerous and great advantages; even in single-shot firing it is much superior to the safety fuse and detonator by its producing a greater useful effect.

200. Electrical Detonators.-These consist of a charge of explosive compound, usually fulminate of mercury, suitably placed in the circuit of an

electric current, and acted upon in a manner and degree sufficient to produce an explosion. The mode in which the current of electricity is made to act depends on the source or manner in which it is generated, being either of high intensity but small in quantity, or of low tension and of great quantity.

Two kinds of electrical exploders are therefore manufactured, consisting of three types to suit these two different conditions or sources from which the electric current is produced. These three types are the slot detonator, fig. 78, the bridge detonator, fig. 79, and the incandescent detonator, fig. 80, but are commonly called tension and quantity detonators.

Slot Detonator.-The slot detonators are thin copper tubes-a, about inch in diameter, closed at one end and partially filled with a detonating composition, b, consisting of fulminate of mercury in varying quantities up to 2 grammes or 31 grains. On the top of this composition there is placed a layer of priming or flashing mixture, c, which is generally made of antimony sulphide and potassium chlorate. The wire for conveying the electric current is bent at the lower end in the form of the letter U, around which a plug, e, of melted sulphur and powdered glass is moulded, a fine slot being cut through the wire as shown at d. The plug is then inserted into the copper casing, so that the two bent ends of the wire project into the priming mixture; it is then made water-tight with india-rubber solution.

Bridge Detonator.-The bridge detonator is similar in form to the previous one; the wire terminals are, however, filed off flush with the inside face of the plug, along which and between the ends of the two wires a line

is drawn with a soft black lead pencil. This forms a path for the electriccurrent to pass along, with many points at which sparking takes place.

Incandescent Detonators.-The incandescent detonators are made in a similar manner to the slot type, but the ends of the wires are placed in the priming or flashing mixture and connected by a very fine platinum wire, d.