A Treatise On The Principles And Practice Of Harbour Engineering

STONE: NATURAL AND ARTIFICIAL. Ql most cartridge is fitted with the cap, or detonator, required to produce explosion. Tamping’. — The next step after charging, prior to explosion, is tamping. This consists in packing the hole with granular or plastic material, so as to completely confine the charge and its gaseous products. For powder, dry, tough clay and powdered brick make good tamping material. By reason of its rapid action, dynamite does not require tamping to such an extent as is necessary for slower explosives. Even water will serve the purpose in deep vertical holes. Mud, sand, brickdust, and clay, are all used in connection with high explosives. Care must be taken in pressing home the first portion of a tamp, so as to avoid prematurely exploding the cap. Only a blunt wooden tamping-rod should be used. Firing’.—There is a difference of procedure in regard to the circumstances attendiug detonation. Low explosives, such as gunpowder, expand progres- sively by combustion, the gases accumulating until the resistance to expansion gives way. High explosives, on the other hand, act instantaneously, but they require a sharp initial explosion to develop their action fully, and this is provided by means of the detonator. The detonator in general use is a small, solid-drawn copper tube, closed at one end and partly filled with an explosive compound (fulminate of mercury and chlorate of potash in varying proportions) which is capable of producing intense local force and heat. The detonator itself may be exploded either by means of a combustible fuse or by the electric current. The ordinary (safety) combustible fuse burns at the rate of 2 to 3 feet per minute. There is also a “lightning” fuse for simultaneous firing, burning at the rate of 150 feet per second. Electrical discharge, of course, takes place instantaneously. There are two methods of firing. One, called the low tension (fig. 80), consists in