When gunpowder is used, means must be adopted to protect it from the water, since a small proportion of moisture is sufficient to lessen, in a very important degree, the force developed, while a large proportion of moisture will destroy altogether its explosive properties. It is no easy matter, under the most favourable circumstances, to keep the water from the charge; but when the depth of water is considerable, it becomes very difficult to attain that object. The pressure of a considerable “head” will force the water through substances that, without a pressure, are sufficiently impervious. At ordinary depths, metal canisters are usually employed to contain gunpowder. Old oil-cans are as good as anything for this purpose. The fuse, whether safety or electric, is passed through the cork, and the latter is luted with some waterproofing composition. The best consists of:
Tallow 1 part. Rosin 3 parts. Guttapercha 4 parts. Swedish pitch 12 parts.
Instead of metal canisters, indiarubber bags are sometimes used. These are, however, more expensive than the oil-cans, and, in many cases, they are scarcely more efficient or suitable. Small charges of gunpowder may be put into short lengths of indiarubber tubing, so as to form a kind of cartridge. But care must be taken to close the ends securely. The best way is to insert a cork, or if that cannot be obtained, a cylindrical piece of wood, and to tie the tubing to this very tightly with twine. The ends should then be dipped into the luting composition described above. Tubing suitable for this purpose is sold under the designation of “blasting tubes.” For large blasts, wooden casks are the most suitable receptacle for the charge. The casks should be well tarred, or, if the depth of water be great, laid over with pitch applied very hot. Great care must be taken to protect the aperture through which the safety fuse, or the wire of the electric fuse, passes.
In blasting under water with gunpowder, only the best and strongest qualities of that compound should be used. The extra strong mining powder of the Messrs. Curtis’s and Harvey’s, commercially known as the E.S.M. powder, is, of all, the most suitable. It is also highly conducive to success to detonate the charge. If the charge be not detonated, the enclosing vessel is ruptured when only a small proportion of the number of grains have been ignited, and, consequently, a large proportion of the charge is blown away into the water unburned. Were gunpowder in blasting charges always fired by a detonation, it would compare in its effects far more favourably with the nitro-cotton and the nitro-glycerine compounds than it does under the circumstances attending the common method of firing it.