Fig. 2.—Safety Lamp Testing Gallery.
Fig. 3.—Mine Gallery No. 2.
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, in inches | Number 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.