Surface blasting is possible only with high explosives, such as nitroglycerin, dynamite, or guncotton, which explode by detonation so sud denly that the shock is transmitted with shatter ing effect to the rock with which it is in contact. Gunpowder, when fired in an unconfined space, explodes so slowly that its force is expended through the air without producing any effective local shock. Surface blasting has been exten sively used in removing reefs and similar obstruc tions to navigation in Long Island Sound. Small shot submarine blasting is frequently used for similar work, and the method is well illustrated by the following typical example. In remov ing a submarine ledge in Oswego (N. Y.) Har bor in 1893, a drill-scow 82 X 26 X feet was employed, along one side of which was a track carrying two movable drill-frames. Each drill frame carried a percussion drill operated by steam, the drill-rod of which descended down the side of the scow. The holes were drilled 5 feet apart in rows of 14 holes to a depth of from 4 feet tc 7 feet, and charged with 2 X 18-inch dynamite cartridges, weighing 2% pounds, and containing 75 per cent. dynamite. The entire 14 holes were fired by electricity in one blast.
In charging the drill-holes with explosives for blasting, the method now in use is to employ the explosive in the form of cylindrical cartridges.
Formerly, and more particularly in ease gun powder was used, the explosive was placed in the hole in powdered form. All high explosives are now tired by electricity, several holes being fired simultaneously. As each hole is charged, a de tonator or cap with two wires attached is in serted in the top of the cartridge. The first wire of the first hole runs to the battery, as does the last wire, the other wire being used to connect several holes with each other. When the first and last wires are secured to the poles of the fir ing-battery or frictional electric machine and the current passed through them, the detonators or caps in the cartridges are exploded by the cur rent, and in turn cause the explosion of the cart ridge. Where a time-fuse is used, the end of this fuse is inserted into the detonator, which is itself inserted into the top of the cartridge; the outer end of the fuse is then lighted and the fire fol lows the fuse until the detonator is reached and exploded, thus causing the explosion of the cart ridge. For a comprehensive discussion of blast ing operations, consult: Drinker, Tunneling, Explosive Compounds and Rock Drills (New York, 1S74) ; for a more concise description, in Prelini. Tunneling York, 1901). See Ex PLOSI YES.