The great and ultimate object to be attained in the manufacture of Gunpowder is, to produce that which shall give equal results with equal charges; the greatest regularity should therefore be observed in this stage. The millman should have great experience; the runners and beds should be, as nearly as possible, the same size and weight, and driven at the same speed throughout the factory; at any rate, each charge should be worked to the same number of revolutions; the motion of the runners should also be as uniform as possible, which is very satisfactorily accomplished by each water-wheel being regulated by a governor. BREAKING DOWN THE MILL CAKE. ~Breaking down the mill cake.~ ~Object of mealing.~ The mill cake, after it comes off the bed of the incorporating mill, is placed in wooden tubs, and taken to small-expense magazines, and from there, in about twelve hours, to the breaking-down house; the object of the machine from which this takes its name, is to reduce the cake to a convenient size for the hydraulic-press box, and also that, by being crushed again to meal, it may get a more even pressure. It consists of a strong gun-metal framework, in which are fixed two pairs of fine-toothed or plain rollers, which revolve towards each other, working in spring collars, so that on any hard substance getting in by mistake, they would open, and allow it to pass through, thereby preventing the dangerous friction which would otherwise result. A hopper, or upright wooden funnel, capable of holding about 500 lbs. is fixed at one end of the machine, and an endless canvass band 2ft. 6in. wide, having strips of leather sewn across at intervals of four inches, passes over one roller at the bottom of the hopper, and one at the top of the machine. When set in motion, this conveys the cake from the hopper to the highest point of the band; it then falls through the first pair of rollers, and from thence through the second, passing in the form of meal into small wooden carriages underneath, which, as they are filled, move forward by a self-acting motion, making room for others. The mill cake thus broken down, is fit for the press. PRESSING THE MEAL BY THE HYDRAULIC PRESS. ~Hydraulic Pressure.~ The meal is now subjected to very powerful pressure; and, in order to explain the way in which this is effected, a short description of the apparatus must be given. The principle of the hydraulic press is so familiar to most, that it will be unnecessary to do more than show how the power is applied (vide plate 2). ~Description of box.~ A very strong oak box, 2 feet 6 inches square, and 2 feet 9 inches deep, is constructed so that two of the sides of the lid will fall back on hinges, or form a compact solid box when screwed firmly together. Forty-six copper plates, 2 feet 5¹⁄₂ inches square, slide vertically into this box, and are kept five-eighths of an inch apart by two metal slips with corresponding grooves, which can be removed when necessary.