Volume 4, Slice 1 Part 13 (2/2)
Bla.s.s is chiefly known for his works in connexion with the study of Greek oratory: _Die griechische Beredsamkeit von Alexander bis auf Augustus_ (1865); _Die attische Beredsamkeit_ (1868-1880; 2nd ed., 1887-1898), his greatest work; editions for the Teubner series of Andocides (1880), Antiphon (1881), Hypereides (1881, 1894), Demosthenes (Dindorf's ed., 1885), Isocrates (1886), Dinarchus (1888), Demosthenes (Rehdantz' ed., 1893), Aeschines (1896), Lycurgus, _Leocrates_ (1902); _Die Rhythmen der attischen Kunstprosa_ (1901); _Die Rhythmen der asianischen und romischen Kunstprosa_ (1905). Among his other works are editions of Eudoxus of Cnidus (1887), the [Greek: Athaenaion politeia]
(4th ed., 1903), a work of great importance, and Bacchylides (3rd. ed., 1904); _Grammatik des neutestamentlichen Griechisch_ (1902; Eng. trans, by H. St John Thackeray, 1905); _Hermeneutik und Kritik and Palaographie, Buchwesen, und Handschriftenkunde_ (vol. i. of Muller's _Handbuch der kla.s.sischen Altertumswissenschaft_, 1891); _uber die Aussprache des Griechischen_ (1888; Eng. trans, by W.J. Purton, 1890); _Die Interpolationen in der Odyssee_ (1904); contributions to Collitz's _Sammlung der griechischen Dialektinschriften_; editions of the texts of certain portions of the New Testament (Gospels and _Acts_). His last work was an edition of the _Choephori_ (1906).
See notices in the _Academy_, March 16, 1907 (J.P. Mahaffy); _Cla.s.sical Review_, May 1907 (J.E. Sandys), which contains also a review of _Die Rhythmen der asianischen und romischen Kunstprosa_.
BLASTING, the process of rending or breaking apart a solid body, such as rock, by exploding within it or in contact with it some explosive substance. The explosion is accompanied by the sudden development of gas at a high temperature and under a tension sufficiently great to overcome the resistance of the enclosing body, which is thus shattered and disintegrated. Before the introduction of explosives, rock was laboriously excavated by hammer and chisel, or by the ancient process of ”fire-setting,” i.e. building a fire against the rock, which, on cooling, splits and flakes off. To hasten disintegration, water was often applied to the heated rock, the loosened portion being afterwards removed by pick or hammer and wedge. In modern times blasting has become a necessity for the excavation of rock and other hard material, as in open surface cuts, quarrying, tunnelling, shaft-sinking and mining operations in general.
For blasting, a hole is generally drilled to receive the charge of explosive. The depth and diameter of the hole and the quant.i.ty of explosive used are all variable, depending on the character of the rock and of the explosive, the shape of the ma.s.s to be blasted, the presence or absence of cracks or fissures, and the position of the hole with respect to the free surface of the rock. The shock of a blast produces impulsive waves acting radially in all directions, the force being greatest at the centre of explosion and varying inversely as the square of the distance from the charge. This is evidenced by the observed facts. Immediately surrounding the explosive, the rock is often finely splintered and crushed. Beyond this is a zone in which it is completely broken and displaced or projected, leaving an enveloping ma.s.s of more or less ragged fractured rock only partially loosened. Lastly, the diminis.h.i.+ng waves produce vibrations which are transmitted to considerable distances. Theoretically, if a charge of explosive be fired in a solid material of perfectly h.o.m.ogeneous texture and at a proper distance from the free surface, a conical ma.s.s will be blown out to the full depth of the drill hole, leaving a funnel-shaped cavity. No rock, however, is of uniform mineralogical and physical character, so that in practice there is only a rough approximation to the conical crater, even under the most favourable conditions. Generally, the shape of the ma.s.s blasted out is extremely irregular, because of the variable texture of the rock and the presence of cracks, fissures and cleavage planes. The ultimate or resultant useful effect of the explosion of a confined charge is in the direction where the least resistance is presented. In the actual work of rock excavation it is only by trial, or by deductions based on experience, that the behaviour of a given rock can be determined and the quant.i.ty of explosive required properly proportioned.
Blasting, as usually carried on, comprises several operations: (1) drilling holes in the rock to be blasted; (2) placing in the hole the charge of explosive, with its fuze; (3) tamping the charge, i.e.
compacting it and filling the remainder of the hole with some suitable material for preventing the charge from blowing out without breaking the ground; (4) igniting or detonating the charge; (5) clearing away the broken material. The holes for blasting are made either by hand, with hammer and drill or jumper, or by machine drill, the latter being driven by steam, compressed air, or electricity, or, in rare cases, by hydraulic power. Drill holes ordinarily vary in diameter from 1 to 3 in., and in depth from a few inches up to 15 or 20 ft. or more. The deeper holes are made only in surface excavation of rock, the shallower, to a maximum depth of say 12 ft., being suitable for tunnelling and mining operations.
_Hand Drilling_.--The work is either ”single-hand” or ”double-hand.”
In single-hand drilling, the miner wields the hammer with one hand, and with the other holds the drill or ”bit,” rotating it slightly after every blow in order to keep the hole round and prevent the drill from sticking fast; in double-hand work, one man strikes, while the other holds and rotates the drill. For large and deep holes, two hammermen are sometimes employed.
[Ill.u.s.tration: FIG. 1.]
[Ill.u.s.tration: FIG. 2.--Sledge-hammer.]
A miner's drill is a steel bar, occasionally round but generally of octagonal cross-section, one end of which is forged out to a cutting edge (fig. 1). The edge of the drill is made either straight, like that of a chisel, or with a convex curve, the latter shape being best for very hard rock. For hard rock the cutting edge should be rather thicker and blunter, and therefore stronger, than for soft rock.
Drills are made of high-grade steel, as they must be tempered accurately and uniformly. The diameter of drill steel for hand work is usually from to 1 in., and the length of cutting edge, or gauge, of the drill is always greater than the diameter of the shank, in the proportion of from 7.4 to 4.3. Holes over 10 or 12 in. deep generally require the use of a set of drills of different lengths and depending in number on the depth required. The shortest drill, for starting the hole, has the widest cutting edge, the edges of the others being successively narrower and graduated to follow each other properly, as drill after drill is dulled in deepening the hole. Thus the hole decreases in diameter as it is made deeper. The miner's hammer (fig.
2) ranges in weight from 3 to 4 lb. for single-hand drilling, up to 8 or 10 lb. for double-hand. If the hole is directed downward, a little water is poured into it at intervals, to keep the cutting edge of the drill cool and make a thin mud of the cuttings. From time to time the hole is cleaned out by the ”sc.r.a.per” or ”spoon,” a long slender iron bar, forged in the shape of a hollow semi-cylinder, with one end flattened and turned over at right angles. If the hole is directed steeply upward and the rock is dry, the cuttings will run out continuously during the drilling; otherwise the sc.r.a.per is necessary, or a small pipe with a plunger like a syringe is used for was.h.i.+ng out the cuttings. The ”jumper” is a long steel bar, with cutting edges on one or both ends, which is alternately raised and dropped in the hole by one or two men. In rock work the jumper is rarely used except for holes directed steeply downward, though for coal or soft shale or slate it may be employed for drilling holes horizontally or upward.
Other tools used in connexion with rock-drilling are the pick and gad.
Holes drilled by hand usually vary in depth from say 18 to 36 in., according to the nature of the rock and purpose of the work, though deeper holes are often made. For soft rock, single-hand drilling is from 20 to 30% cheaper than double-hand, but this difference does not hold good for the harder rocks. For these double-hand drilling is preferable, and may even be essential, to secure a reasonable speed of work.
[Ill.u.s.tration: Fig. 3.--Ingersoll-Sergeant Mining Drill.]
_Machine Drills._--The introduction of machine drills in the latter part of the 19th century exerted an important influence on the work of rock excavation in general, and specially on the art of mining. By their use many great tunnels and other works involving rock excavation under adverse conditions have been rapidly and successfully carried out. Before the invention of machine drills such work progressed slowly and with difficulty. Nearly all machine drills are of the reciprocating or percussive type, in which the drill bit is firmly clamped to the piston rod and delivers a rapid succession of strong blows on the bottom of the hole. The ordinary compressed air drill (which may, for surface work, be operated also by steam) may be taken as an ill.u.s.tration. The piston works in a cylinder, provided with a valve motion somewhat similar to that of a steam-engine, together with an automatic device for producing the necessary rotation of the piston and drill bit. While at work the machine is mounted on a heavy tripod (fig. 3); or, if underground, sometimes on an iron column or bar, firmly wedged in position between the roof and floor, or side walls, of the tunnel or mine working. As the hole is deepened, the entire drill head is gradually fed forward on its support by a screw feed, a succession of longer and longer drill bits being used as required.
Among the numerous types and makes of percussion drill may be named the following:--Adelaide, Climax, Darlington, Dubois-Francois, Ferroux, Froelich, Hirnant, Ingersoll, Jeffrey, Leyner, McKiernan, Rand, Schram, Sergeant, Sullivan and Wood.
[Ill.u.s.tration: Figs. 4 and 5.--Darlington's Rock Drill.]
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