A Treatise On The Principles And Practice Of Harbour Engineering

STONE: NATURAL AND ARTIFICIAL. 93 an extent admitting of the use of plates and wedges. These last are driven simultaneously, and as the fissure produced widens out, it is kept weil packed with hand rubble. When the block is detached and ready to corne away, an iron dog is attached to the stone, from which a chain passes to a crane, the tension of which, aided by men with crowbars and levers acting directly upon the block, causes it to become completely dislodged. It can then be converted into convenient sizes by plugging as before, and be dressed to requirements. When the height of the quarry face is considerable, it is desirable to place planks and pièces of old timber upon the ledges of rock and upon the floor of the quarry, to avoid undue breakage of blocks falling from the upper layers. Biasting* Agents.—The number of explosives available for working purposes in a quarry is legion. From a practical point of view, despite the dissimilarity of their ingredients and methods of production, they fall into two classes, viz., (1) those in which a high local intensity is produced, causing much shattering and splintering into small fragments, and (2) those in which the expansive power is more widely and less violently exerted, resulting in disruption and dislocation rather than shattering. The first class is repre- sented by dynamite, the second by gunpowder. The basis of dynamite is nitro-glycerine, which forms a number of Com- pounds possessed of similar attributes, but varying in power. Nitro-glycerine is a fluid combination of glycerine and of nitric and sulphuric acids. The majority of its various combinations, therefore, have a plastic, gelatinous nature, but the first to be noticed below has not this characteristic. Dynamite consists of nitro-glycerine with the addition of a granular absorbent, which may either be an inert substance or, in itself, an explosive. The material more specially employed is a silicious infusorial earth occurring in Hanover, and called “ kieselguhr.” Commonly, the proportions are 75 parts, by weight, of nitro-glycerine to 25 parts of earth. If cartridges remain immersed in water for any length of time, the nitro-glycerine exudes and the charge deteriorates. Moreover, the substance is affected by changes in temperature and freezes at a higher temperature than the freezing point of water, so that some trouble is incurred in winter-time in thawing cartridges, the operation requiring much care and circumspection. The effects of firing are, as stated above, an intense rapidity of explosion producing extreme local shattering. Other combinations of the same character are :— Blasting Gelatine, containing 93 per cent, of nitro-glycerine and 7 per cent, of nitro-cotton. This is probably the most powerful blasting agent known at the present day. It is also very little, if at all, affected by immersion in water. Gelatine Dynamite, somewhat inferior in strength to the foregoing, is a compound of nitro-glycerine, nitro-cellulose, and nitrate of potash. Gelignite contains nitro-glycerine, nitro-cotton, nitrate of potash, and wood meal. It is rather more powerful than ordinary dynamite. Forcite is a mixture of nitro-glycerine with cellulose, the latter being