The drying-house is a large room with double doors, and fitted with racks from floor to ceiling. On these racks copper and wooden trays are placed, containing the powder, spread out in thin layers. Steam-pipes are introduced from a boiler in an adjoining building, and thus the air of the room is kept at a temperature of about 135°. The fire-men in charge of the drying are forbidden to enter the room for fear of carrying in a spark in their clothes, but they ascertain the temperature by a register thermometer placed inside a small window, and this thermometer also acts as a telltale, by showing if the temperature has at any time been allowed to become too high or too low. So perfect are all the arrangements at Waltham, that no explosion has ever occurred in the drying-house.
The dried pebbles are finished by being placed in a revolving barrel (called a glazing-barrel), with a certain amount of powdered black-lead. On being taken out, every pebble is found to have a perfectly smooth surface coated with black-lead, the effect of which is still further to diminish the rate of burning. The pebbles are then thrown into sieves to separate small fragments; all irregular pieces are picked out by hand, and the remainder is packed in ordinary powder-barrels, which would hold 100 pounds of rifle-powder, but contain 125 pounds of the pebble-powder, on account of its greater density.
The following results of experiments with the 8-inch gun will give the reader an idea of the effects of the different kinds of powder. We need only explain that R. L. G. means the old "Rifle Large Grain" powder, still in use for field-artillery, and draw attention to the fact that the pebble-powder gives at once the highest velocity and the lowest strain:
| POWDER. | Charge, Pounds. |
Initial Velocity, Feet per Second. |
Maximum Pressure, Tons on Square Inch. |
| R. L. G | 30 | 1324 | 29.8 |
| Russian Prismatic. | 32 | 1366 | 20.5 |
| Service Pellet | 30 | 1338 | 17.4 |
| Pebble | 35 | 1374 | 15.4 |
Visitors to the laboratory at Waltham can see there a number of experimental varieties of pebble-powder, the largest of which consists of cubes as hard as stone, each side of which is two inches square. A shower of this alone fired from a gun would be quite as effective as grape, and it is possible that 300 pounds of this tremendous powder will form the charge of the new 80-ton gun.
For rifle-powder the meal is pressed into thin cakes; these are broken up into irregular pieces by hand, and carried to the granulating machine. This machine consists of four pairs of toothed cylinders, between which the broken cake is passed. As it falls from them in grains, it is received upon a series of screens of net-work. There are
