A Treatise On The Principles And Practice Of Harbour Engineering

94 HARBOUR ENGINEERING. gelatinised by heating in water under considérable pressure. Nitrated cellu- lose is also used in admixture with oxidising salts. Gun cotton, which is cotton dipped into a mixture of nitric and sulphuric acids and itself an explosive, gives rise to the following, amongst other, compounds :— Tonite is finely divided, or macerated, gun cotton, combined with an equal weight of nitrate of baryta. There are two varieties—the white and the black. The former is very shattering in its action, and is therefore chiefly applicable to the breaking up of extremely hard stone, such as quartz. Black tonite, containing a larger proportion of baryta and some charcoal, is more disruptive. Chlorate of potash forms the basis of two well-known explosives, viz., Rack-a-rock and Cheddite. The former consists of compressed cartridges of chlorate of potash, impregnated with dead oil, either alone, or in conjunction with bisulphide of carbon, or mixed with nitro-benzol. Cheddite, an admirable product of more recent date, contains chlorate of potash, naphthaline, and castor oil. It is needless to extend the list further. There are many other excellent explosives on the market, and fresh compositions are continually being evolved, each with its own special advantages. But while many of them are character- ised by extremely high power, resulting in the production of almost incredible downfalls of rock, yet in ordinary quarrying operations where, as has been pointed out, intense local effect is by no means sought after, it is probable that in the majority of cases biasting powder is every whit as serviceable, and certainly more economical. Gunpowder, the earliest of explosives, is a mixture of saltpetre, sulphur, and charcoal, in proportions ranging between 6:1:1 and 15:3:2. These are the proportions used for service powder for military purposes. Blasting powder is distinguished from gunpowder, properly so called, in that it contains rather less saltpetre and that it is not manufactured with the same particular selection of material and delicacy of treatment. The effective power is therefore lower. Quarrying1 for Goodwick Breakwater.—The following particulars relating to the quarrying of stone for the breakwater in Pembrokeshire, forraing a protection to the Fishguard terminus of the new Fishguard, Rosslare (Great Western) route to Ireland, have been compiled from information kindly supplied by Mr G. Lambert Gibson, the engineer in charge. When the works were begun in the year 1896, they were carried out tentatively with a small outfit of plant, but with a considerable body of men. The start was a difficult one, the men having to attack the face of precipitous cliffs of an intensely hard and vitreous texture, rising from the sea to heights of one and two hundred feet. The boring of the rock to receive explosives was done entirely by hand ; and, owing to the want of foothold, the men had often to be slung by ropes from the top of the cliff. After six years of some- what slow progress in this manner, more vigorous measures were decided