Page:The American Cyclopædia (1879) Volume II.djvu/719

 BLASTING 699 pound depends upon the nature of the work to be performed. In quarrying, gunpowder of slow igniting power is preferred, because it is desired to avoid pulverization ; but in simply clearing away material, a more instantaneous explosive is found to be more effectual. Gun- cotton was used in Europe to some extent soon after its discovery, but has never been employed in any important work in this country, except as an experiment. Nitro-glycerine, or some preparation of it, as giant powder, is the com- pound now relied upon when rapidity and an approach to accuracy of result are desired ; and it is generally preferred when the disengaging of surface portions of rock is the immediate object. It often happens in some situations, especially in excavating chambers under water, where it is of the greatest importance to keep the water bed as firm and intact as possible, that a seamy structure of rock requires the use of an explosive which will expend its force as much as possible in detaching only a certain superficial mass, upon the same principle that a small hammer, propelled with a sharp quick stroke, is better adapted to drive a nail in an unstable and slight body than a heavy one. When gunpowder is used, the holes are usually drilled deeper than for mtro-glycerine, and when practicable the powder is poured into the cavity instead of being introduced in a cartridge. Therefore the holes are drilled in a downward direction, as nearly perpendicular as the course of lamination and other circumstances will admit. The small hand drill is held and driven by one person, and after each stroke it is turned sufficiently to allow of a chip being cut from a section of the bottom. The degree to which this turning is done at each stroke is a matter of consequence, as upon it depends much of the rapidity and economy of the operation. When the bottom of the cavity becomes obstructed, instruments called scrapers or dippers are used to clear it out. Some of these tools are merely wires bent at right angles at one end, which is flattened so as to form a shelf upon which the rubbish may bo taken ; but the flattened end should be slightly depressed on one side, so that by a twisting motion the shelf or pan may he made to pass under. A worm is often formed at the other end for carrying a piece of sponge or other material to the bottom of the cavity to absorb water. It is generally advan- tageous to pour water into the cavity while drilling for the purpose of softening the rock, and keeping the bit from heating. It often happens that water percolates into the cavity, and in either case some contrivance is required to occasionally remove it. When the hole has reached a sufficient depth it is to be thoroughly cleaned and dried with the scraper and a piece of sponge or cloth attached to a stick or to the worm at one end of the scraper. Then the proper charge of powder is poured in and covered with a tamping, which may consist of dry sand, brick dust, or moist clay. When dry sand is used, it is not tamped down, but brick dust or clay is, the material being intro- duced in small quantities at a time, and suc- cessively compacted with a tamping rod, which is simply a straight bar of copper, brass, or wood. The end of a fuse, which is made of gutta-percha cylinder, impervious to moisture, filled with a mixture of gunpowder, charcoal, and nitre, is passed into the hole and inserted in the body of the charge before the tamping material is introduced, the other end remaining outside and being of a sufficient length to burn the desired time before producing the explo- sion. When a fuse is not employed, a priming needle made of copper is passed down one side of the hole, with the point extending into the powder. It has a tapering form, so that its withdrawal will not disturb the tamping, which in this case must be more or less damp. When the needle is withdrawn the canal is filled with fine powder, and its ignition effected with a slow match. When the cavity, in consequence of percolation from surrounding rock, cannot be dried, the powder must be used in the form of a cartridge, the case of which is made of tin or pitched paper. When nitro-glycerine is used, it is placed in cartridges and exploded by means of some kind of fulminate, as fulminate of mercury or chlorate of potash, or both to- gether. The fulminate may be ignited either by a fuse or by a galvanic battery. The use of nitro-glycerine in its raw state being consid- ered very dangerous, preparations of it have been made, which with careful handling are no more hazardous than gunpowder. Of these, giant powder or dynamite, which is composed of 75 per cent, of nitro-glycerine with 25 per cent, of a certain silicious infusorial earth, holds the first rank. When an explosive compound is fired, the great and almost instantaneous expansion of liberated gases, which in the case of gunpowder is many hundred times its vol- ume, produces an equal pressure in all direc- tions. Those surfaces which offer the least re- sistance of course give way to the greatest ex- tent; and the slower the explosion and conse- quent expansion, the more will these surfaces be displaced, receiving by direct action and re- action most of the explosive force, while the firmer material will be left undisturbed. When, however, nitro-glycerine is used, the expansion of gases is so nearly instantaneous, that the tampings, even when they are quite unstable, offer an amount of resistance which is consid- erable. Even when it is fired upon the surface of a rock under a depth of only a few feet of water, so great is the reaction produced by the inertia of the water that a sufficient force is exerted against the rock to rend it in some in- stances to a large extent. Under similar cir- cumstances even gunpowder will explode with considerable effect. Mr. Maillefert in the years 1851 and 1852 succeeded, by the use of gun- powder in surface blasting under water, in re- moving large portions of several of the obstruc- tions to the navigation of the East river at Hell Gate. Kocks known as Pot rock, the Fry-