GUNPOWDER 331 acted charcoal. The article is usually judged by the velocity it gives to a projectile, and the amount of fouling. In both respects erroneous judgments are likely to be formed, since the mode of charging is more frequently the cause of a poor performance than any defect in the quality of the powder. If a given brand is found to give a lower velocity than desired, it is better to increase the charge than to resort to a more violent kind ; for the smaller charge is more apt to strain and erode the gun than the larger charge of milder powder. There is seldom any sufficient reason for excessive foul- ing, since this may generally be corrected by the use of a patch and lubricant. The force of gunpowder is measured by an instrument called a pressure gauge. Three forms have been used, two of which were invented by Gen. Rodman of the United States ordnance depart- ment. One of these is applied to the exterior FIG. 2. Rodman's Internal Gunpowder Gauge, cylindrical steel box ; b b, cover ; c, steel piston rod ; d, steel disk, carrying e, the indenting knife , /, disk of soft copper ; g, copper cup for gas check ; h h, copper washer. The smaller figures show the parts e, d, e, and /, enlarged, and viewed from the side, and the indentation made on the copper disk. of a gun, and communicates with the chamber by a narrow passage. The other is inserted in the cartridge bag at the base of the charge, and remains in the gun after the discharge. The internal gauge consists of a cylindrical box of steel, with a cover screwed on. Through the axis of the cover is a cylindrical hole, in which a steel rod is fitted. Within the box is a thick disk of steel, having a knife edge pro- truding from its lower face. This knife has a double shear, the edges of the two shears meet- ing at the centre in a very obtuse angle. At the bottom of the box the apex of the knife rests upon a disk of soft, annealed copper. The inner end of the steel rod is stepped into the steel knife disk, and its outer end is a little below the top of the cover. A copper cup rests upon the top of the rod, to serve as a gas check. The pressure of the explosion is re- ceived by the rod, which communicates it to the knife, the apex of which sinks into the copper, giving a cut, the length of which serves as the measure of the pressure! The working parts of the external gauge are quite similar to the foregoing, but the manner of housing them is different. Another form of gauge, invented by Capt. Noble of the English artillery, substi- tutes for the copper disk a short cylinder of copper, which is crushed by the pressure, the amount of crushing being employed to measure the pressure. This gauge is screwed into the wall of the gun in such a manner that the end of the rod receiving the pressure is very near the surface of the bore. Both forms of gauge are liable to grave objections, since the measure obtained is essentially dynamical, while the quantity to be measured is statical. The Eng- lish gauge is much inferior to the American, and cannot be relied upon to give even approx- imate indications of pressure when violent powder is used. In the composition of blast- ing powder, nitrate of soda (Chili saltpetre.) has of late years been chiefly used in the United States, instead of nitrate of potassa ; the latter being subject to a high duty, while the former is duty-free. So far as explosive properties are concerned, the difference between the two is not very great ; but the former absorbs mois- ture from the air and deliquesces, while the lat- ter does not, unless the air be very damp and the exposure long continued. Hence nitrate of soda rapidly deteriorates, a matter of compara- tively little importance in blasting powder, which is commonly used soon after manufac- ture. (For the constitution of other blasting powders, see EXPLOSIVES.) But as war mate- rial is frequently accumulated and stored, or transported long distances and more or less exposed, military powder must be made with nitrate of potassa. Powder for blasting differs essentially from military or sporting powder in the formation- of CO instead of C0 2 , yielding the same volume of gaseous product for the same temperature, but less heat; hence a low- er temperature of products, and consequently a lower immediate explosive energy. The com- bustion is also slower. The theory of its use has been to loosen the surrounding rocks to as great a distance as possible from the bore hole, and to waste as little force as possible in the hurling of fragments or the production of very small pieces, or dust. Usually a good deal of work with the pick and bar, and of subsequent breaking or blasting of large fragments, has ac- companied the main blasting operations. The introduction of nitro-glycerine compounds has led to a change of practice ; and many engi- neers now find a gain in the more complete shattering of the rocks by quick explosion, which permits an easier handling and a more rapid progress, besides, what is economically most important, the substitution of single-hand drills and small holes.- Even rifle powder is now used in this way, it is said, with good re- sults. The most judicious authorities appear to agree, however, that the nature of the rock
