With the large grained, the ignition is more rapid, but the combustion slower; with small grains, the contrary is the case. 16. Why should the grains be angular? Because they present a greater surface to the action of the flame, and therefore burn quicker. 17. Why should powder be free from dust? Because the dust fills up the intervals between the grains, and forming a compact mass, retards combustion. [Sidenote: 86]] 18. To what special purpose are large and small grained powders applied? The large for cannon, and the small for small arms. 19. How is the size of the grain for each kind of powder tested? By means of sieves or gauges. 20. How many grains of powder are in 10 grs. Troy weight? _Cannon_, 150; _Musket_, 2,000 to 2,500; and _Rifle_, 12,000 to 15,000. 21. What is the object in glazing powder? Glazed powder does not absorb moisture, or break up in transportation, so much as unglazed. 22. What is the established mode of proving the strength of powder in the U. S.? A sample is taken from each barrel, and the strength determined by the eprouvette mortar. 23. What is the least range allowed? The general _mean-range_ of new powder must not be less than 250 yards; but no powder ranging below 225 yards is received. 24. When is powder in magazines considered unserviceable. When it does not range over 180 yards. 25. What is the range of good powder? Cannon from 280 to 300 yards. Small grained from 300 to 320 yards. 26. What other means is there for determining the strength of powder? The GUN AND BALLISTIC PENDULUM, and NAVEZ’ ELECTRO-BALLISTIC MACHINE. The latter is considered the best for determining the initial velocity. [Sidenote: 87]] 27. What is the hygrometric proof of powder? Samples are placed in shallow tin pans, set in a tub, the bottom of which is covered with water; the pans should be about an inch above the water, and the tub covered. Good powder will not absorb more than 2½ per cent. in 24 hours. 28. How can the relative quickness of two kinds of powder be determined? By burning a train laid in a circular or other groove, which returns into itself, made in a piece of hard wood: one-half of the groove being filled with each kind of powder, and fire communicated at the junction of the two trains, the relative quickness is readily deduced from observation of the point at which the flames meet. 29. What are the qualities of good powder? It should be perfectly free from dust, uniform in strength and size of grains, angular and irregular in form; in color, brownish black, or slate color; so hard as not to be easily crushed by pressure with the finger; and should leave no beads or foulness when flashed in quantities of 10 grs. on a copper plate. 30. What is the expansive velocity, and pressure of ignited powder? The expansive velocity is about 5,000 feet per second, and pressure about 2,000 atmospheres. 31. What is the weight of a cubic inch of powder?