Compressed Air Work And Diving 1909

i oo COMPRESSRD AIR WORK. this class of work it is necessary to shatter the rock in orcler to get it into the buckets and waggons. The more powerful explosives will also be found more economical in the end, although higher in first cost, because of the saving in the number and diameter of the holes to be drilled, Explosives formed of nitro - glycerine cannot be exploclecl by being set fire to except indirectly by the heat generated. They require to be “ cletonatecl.” In this respect they are unlike gunpowder, which is con- verted into a gas by combustion. The process is then a gradual one, because the grains of powcler are set fire to in succession. Detonation is almost instantaneous. For this reason, until cordite was invented, nitro-glycerine coulcl not be used for guns or small arms, the tendency being almost as much to burst the breach as to expel the shell or hullet. Cordite is a gelatinised form of gun- cotton and nitro-glycerine made up into sticks of different thickness, and by this means the rate of combustion is delayed. By altering the thickness of the sticks any required rate of combustion can be obtained. Detonators for use with the safety fuse are small copper tubes containing a compound of 80 per cent, of fulminate of mercury and 20 per cent, of chlorate of potash. The tube is left open at one end, and when getting ready for firing, the safety fuse is inserted and fixed by squeezing the copper tube round it with a pair of copper nippers. A hole is then made in the cartridge at one end, and the detonator inserted in such a way that about one-third of the copper tube shows outside the explosive. This is to prevent the fuse from setting fire to the explosive before the detonator goes off. The paper wrapping of the cartridge shoulcl finally be tiecl round the fuse above the detonator to prevent its pulling