The grains of black blasting powder are graded by a separator, similar to those used in powder mills, but of reduced size. It consists of an inclined wooden box, with slots on the sides to carry a series of screens, and a vertical conduit at the end for carrying off the grains as they are screened into separate small bins (Fig. 1, Plate X). At the upper end of the screens is a small 12 by 16-in. hopper, with a sliding brass apron to regulate the feed. The screens are shaken laterally by an eccentric rod operated by hand. The top of the hopper is about 6½ ft. above the floor. The box is 6 ft. 10 in. long, from tip to tip, and inclines at an angle of 9 degrees. After separation the grains fall through a vertical conduit, and thence to the bins through zinc chutes, 1 by 2 in. in section. Care is taken to have no steel or iron exposed to the powder. The screens are held by light wooden frames which slip into the inclined box from the upper end. In this way, any or all of the screens may be used at once, thus separating all grades, or making only such separations as are desired. The screens with the largest meshes are diagonally-perforated zinc plates. Table 2 gives the number of holes per square foot in zinc plates perforated with circular holes of the diameters stated. TABLE 2.--Number of Holes per Square Foot in Zinc Plates with Circular Perforations. -------------+------------ Diameter, | Number in inches | of holes. -------------+------------ 1/2 | 353 4/10 | 518 1/3 | 782 1/4 | 1,392 1/6 | 1,680 1/8 | 3,456 1/10 | 6,636 1/16 | 12,800 -------------+------------ The finer meshes are obtained by using linen screens with holes of two sizes, namely, 1/20 in. square and 1/28 in. square. Until a few years ago, black blasting powder was manufactured in the sizes given in Table 3. TABLE 3.--Gradation of Black Blasting Powder. ---------+----------- Grade. | Mesh. ---------+----------- CC | 2 - 2½ C | 2½ - 3 F | 3 - 5 FF | 5 - 8 FFF | 8 - 16 FFFF | 16 - 28 ---------+-----------