Mixing and proportions.
The ingredients are now weighed out very accurately, in the proportion of 75 nitre, 15 charcoal, and 10 sulphur, in 42lb. charges, viz., nitre, 31lbs. 8oz., charcoal, 6lbs. 4oz. 13drs., sulphur, 4lbs. 3oz. 3drs., and thoroughly mixed in a machine, which consists of a cylindrical gun-metal or copper drum, about two feet in diameter, with an axle passing through its centre, on which there are metal flyers, like forks. The machinery is so arranged that the flyers and drum revolve in opposite directions when in motion, at a rate of about one hundred revolutions per minute. Five minutes is sufficient for a thorough mixture. The composition is then drawn off by a slip into canvas bags the proper size to hold the 42lb. charges, which are tightly tied, and taken to small magazines. Green charges.These are called green charges, and are now ready for the next process, incorporation.
THE INCORPORATING MILL.
Incorporation.
The Incorporating Mill consists of an iron or stone circular flat bed, about seven feet in diameter, fixed very firmly in the floor of the building which covers it, whereon two iron or stone cylindrical runners, from five to seven feet in diameter, fourteen to eighteen inches wide, and each weighing from 3 to 41⁄2 tons, revolve. They have a common axle, and a vertical shaft passing through the centre of the bed is connected with this axle, and to machinery above or below, which communicates the motion. These runners are not equidistant from the centre, by which arrangement in their revolution every part of the composition on the bed is subjected to their action, which is threefold, viz., crushing, grinding, and mixing; crushing, from the weight of the cylinders; grinding, from the twisting motion which they are forced into from so large a diameter revolving in so small a circle; and mixing, from a combination of the two former motions. To prevent the powder from falling over the side of the bed, a wooden rim, about two feet in height, is placed at an angle of forty-five degrees with it, like the side of a funnel, and fitted closely all round its circumference. This is called the “curb;” and in the centre of the bed a gun-metal ring, or “cheese,” as it is termed, about two feet in diameter, and five inches high, concentric with the bed, prevents the powder working beyond in that direction. Moreover, two scrapers, or “ploughs,” connected by stays with the horizontal axle, revolve with the runners, one rubbing against the inner, and the other the outer circle. These ploughs are made of hard wood, shod with leather and felt, and their use is continually to disturb and rout about the composition, and keep it under the path of the runners, so that every part should get its share of incorporation. The houses or sheds which cover these buildings have hitherto been constructed of wood, with either corrugated iron or wooden roofing. The new incorporating mills in this factory, which are just completed, are built with three sides of strong three-foot brickwork, and the fourth side and roof of corrugated iron and glass. They are also placed in a line contiguous to each other, the alternate ones only facing the same way, so that an explosion from one would probably communicate no further, and the lighter parts of the building would blow away, leaving the rest entire. Most of the machinery in the factory is driven by water-wheels; the motive power of these mills is steam. A horizontal shaft, worked by the engine, passes underneath the entire length of the building in a cast-iron tank, and a bevel wheel on this shaft is geared into another one on the vertical shaft under the centre of each bed, which, communicating with the runners, gives the necessary motion.
Water-tanks to prevent explosions.
In order, as much as possible, to guard against any explosion spreading, above each bed, placed so as just to clear the runners, is suspended or balanced a copper tank, holding about forty gallons of water. On one side of the tank is fixed a small shaft, which communicates with similar cisterns over the beds of the mills on either side. The other end of the tank rests on a flat board, which is subjected to a great part of the force of an explosion. This consequently lifts, disengaging the support of the tank, the contents of which drench the bed which has just exploded, thereby putting out all fire, and cooling the machinery, besides having a similar effect on the mills right and left, preventing, by this means, any extension of fire.