A Treatise On The Principles And Practice Of Harbour Engineering

92 HARBOUR ENGINEERING. sending an electric current through a thin platinum wire contained in the detonator, whereby it is made red-hot and so ignites the inflammable material in which it is incased. lu high tension fuses (fig. 81), the wire terminals are not connected, but a spark is passed between them, effecting the same result. Fig. 81. —High-tension Fuse. Seam Firing'. —It has already been remarked that the efficacy of a blast depends to a very large extent upon the judicious selection of the site and direction of the bore-holes. Tlie natural faults to be found in rock manifestly lend themselves to an economical disposition of the disruptive forces, and it is often a wise plan to try the effect of a small preliminary charge by way of ascertaining to what extent any latent lines of weakness are developed. Thus, in a quarry at Penmaenmawr, in North Wales, where the rock is a hard, compact trap, it is the practice to bore a hole, say 20 to 23 feet deep, and charge it with only 5 Ibs. or so of powder. On explosion, certain cracks are produced, the traces of which are followed and enlarged by additional holes until a final charge can be suitably placed for bringing down the whole mass. Similarly, at Goodwick, Pembrokeshire, a hole 20 feet deep and 2| inches in diameter at its extremity would be sprung several times by means of single gelignite cartridges of 1{ inches diameter, producing a pear-shaped cavity capable of receiving the larger quantity—50 or 60 Ibs.— required for complété dislocation. Another method adopted in a granite quarry at Kirkmabreck, Kirkcud- brightshire, where the seams are fairly regular, extending along the face-line for some distance, is to open out a seam by means of plugs and wedges until it is wide enough to admit of the lodgment of a charge of powder. A chamber for the charge is prepared by shaping a couple of boards to fit the seam and setting them temporarily a couple of feet apart. Tamping is tightly rammed against the outsides of the boards, which are then withdrawn. The method of charging is to insert the fuse after the deposition of a third of the powder, the remainder being added on top and covered with a layer of lightly pressed hay followed by tamping above. The first foot of tamping is lightly rammed, the rest compactly. Wedging.—-Where the rock is of good quality and is required in the form of large, sound blocks for ashlar work in copiugs, facings, and string courses, a system of obtainment by wedging is adopted in preference to that of blasting, which may produce unsuspected planes of weakness as well as undesirable cracking and cleavage. In this case, a series of plug-holes, about 1J inches in diameter and 9 inches deep, are driven along the line of some natural joint. Plugs and feathers are inserted into these holes, and driven by a succession of blows from a 26-lb. hammer until the seam has been sprung to