QUARRY.

169

Fig. 1.

should be avoided as much as possible, in order to save powder, as the full effectdue to the powder can only then be produced by making such an increased allowancefor the charge as will counterbalance the loss consequent on the diminished resistance.

Boring for Blasting .—These apertures are made by boring with iron rods calledborers or jumpers, according as they are struck on their heads with a hammeror merely jumped up and down and allowed to penetrate the rock by their ownweight. These are of various sizes, and have wedge-shaped pieces of steel weldedto their end, called bits, which are brought to an edge, so as to cut into therock. The holes may be made in almost any direction, but thatwhich is most advantageously worked is vertical, in which casethe weight of the cutting tool is brought into useful co-operationwith the force employed to drive it. With the jumper it is theweight alone which produces the effect.

The speed with which holes are sunk into rock depends on thenature of the rock, and on the size and weight of the boring tools;it having been ascertained, from long experience, that three menare able to sink, in granite of good quality, at the following rates:

With a 3-inch jumper, 4 feet in a day.

,, 2^ ,, 5 ,,

tt 2J- „ 6 ,,

,, 2 ,, 8 „

„ n „ i2

In working the two last classes, where the holes are not verydeep, a strong boy will answer to turn the jumper. With a 1-inchjumper, a strong man bored 8 feet in a day.

In using borers, it is necessary to pay some attention to theweight of hammer used for striking them. If too heavy, byfatiguing the men and the reduced number of blows given, a lesseffect is produced than if lighter hammers are used; and if toolight, the strength of the miner is not kept fully employed inraising the hammer. The usual weight is from 5 to 7 lbs.

It therefore is evident that it is desirable to sink the holesof as small bore as possible, considering the size of the charge,which should in all cases be determined by weight, and oughtnot to occupy too great a length in the bore-hole; the objectbeing to get the centre of the charge as near as possible to thecentre of explosion. In order to effect this object of placing alarge charge at the bottom of a small hole, which is also ad-vantageous as presenting greater facilities for tamping, various planshave been adopted. At the bottom of the bore, small charges havebeen placed and fired, so proportioned as not to produce fractureto the rock; they then have the effect of enlarging the space(called chamber) for another charge, which can he inserted by boringthrough the tamping. This operation may be repeated severaltimes when it is desired to place a large charge at the bottom.

Another plan, applicable principally to calcareous stones, hasbeen tried with good effect. The rock having been pierced inthe usual way, a copper pipe, the size of the bore, is introduced(seefig. 1), the end a reaching to the bottom of the hole, whichis closed up tight at b with clay, so that no air can escape. Thispipe has a bent neck c, for a purpose hereafter to be explained.