The three ingredients are now ground separately to a very fine powder. The mills (vide plate 1) which effect this, and incorporate, are so similar, that a description will be given under the head of “Incorporation.” After being ground in this way, the saltpetre is passed through a slope cylindrical reel, covered with copper sieving wire of 60 meshes to the inch, which, as it revolves, sifts it to the required fineness, being then received in a box or bin underneath. The charcoal and sulphur are likewise passed through similar reels of 32 and 60-mesh wire respectively, and that which remains without passing through, is ground again under the runners. A very excellent machine has been invented by Mr. Hall, the engineer, of Dartford, for grinding charcoal, which makes a most useful addition to the Gunpowder factory. It consists of a conical drum, working in a conical box, on the same principle as a coffee-grinding machine, the axis being vertical. The mill is fed with charcoal by a hopper, and, as it passes through in fine powder, falls into a revolving reel, which sifts it in the same manner as before described, the whole being covered in, to prevent the great annoyance of dust, which was felt until lately, from the old charcoal mill. The three ingredients having been pulverized, are now fit for the mixing process. MIXING THE INGREDIENTS. ~Mixing and proportions.~ ~Green charges.~ 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. These are called green charges, and are now ready for the next process, incorporation. THE INCORPORATING MILL. ~Incorporation.~