Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/346

Rh 326 where it falls between the first pair of rollers ; after passing through the second pair, which are directly below, the meil is received in wooden boxes, placed upon a carriage, and is raady for the press. The press-box is usually of oak, with a strong gun-metal frame, and so constructed that three of the sides can turn back on hinges, or be screwed firmly together. Being laid sideways, the top temporarily closed by a board, and the uppermost side alone open, a number of copper plates are placed vertically in the box, and kept apart (at a distance depending upon the description of gunpow der required) by two racks, which are afterwards re moved ; the box is then loaded with some 800 E&amp;gt; of meal, which is rammed evenly down between the plates with wooden laths, anl the racks withdrawn, so that the plates are only separated by the meal be tween them. The present upper side of the box being firmly screwed down, the box is turned over, and placed on the table of the ~ hydraulic ram, under the fixed press-block, the plates being now horizontal (see fig. 5). The pumps which w&amp;gt;rk the press (in a sepa rate house) are then set in motion, and the press-block allowed to enter the box a certain distance, when the FlQ - 5. Press. edge of the latter releases a P ress - Wock &amp;gt; spring catch and rings a bell as a signal to stop the pumps; the powder is kept under pressure a few minutes, after which the ram is lowered, and the box removed and un loaded. For all granulated powders the press-cake is broken up into pieces and put into tubs, but for the cubical cannon powders the slabs are pressed to the exact thickness required, and are carefully kept whole. The above mode of regulating the pressure is found to give more reliable rjsults than trusting to the indicator gauge of the hydraulic press, for the reason that the elas ticity, or resistance to pressure, of the meal varies with the amount of moisture present in it, and the state of the at mosphere. To get uniform density, equal quantities of rneal, containing equal amounts of moisture, must be pressed at the same rate into the same space. In practice, however, the moisture in the meal will slightly vary, whatever care be taken with the mill-cake, owing to the hygrometric state of the air causing a difference by the time it comes to the press. It is therefore nece.ssary to alter the exact distance the press-block is allowed to enter the box, not only with the niture of the powder, but with the season of the year, On the Continent, the operation of pressing is sometimes altogether omitted, and the requisite density given merely by the weight of the runners revolving very slowly, the charges being worked with a considerable amount of mois ture in them, and the less dense edges of the cake rejected ; by this plan it is, however, almost impossible to ensure uniformity in the powder produced. The meal is also sometimes pressed by passing it, on an endless band, between large rollers revolving at a slow speed, the less dense edges of the cake being cut off by fixed knives. A, press-box; B, hdraulic ram. For some centuries, gunpowder remained in the form of Granu. dust or &quot;meal,&quot; being, in fact, simply the ingredients lating. ground up together. Granulating or &quot; corning&quot; the powder was a great step in advance, but it is doubtful whether this operation was intended to increase its strength, or merely to rendei it more convenient for charging small-arms, for which alone corned powder was used for many years, whilst meal powder was still employed for heavy guns ; the latter was called &quot; serpentine &quot; powder in the time of Edward VI., probably in allusion to the name of one of the pieces of ordnance then in use. However, during the reign of Elizabeth, the experience of the great additional strength imparted by the corning process, for the reasons already explained, led to the universal introduction of corned powder, except for priming, both in cannon and small- arms, for which purpose meal powder remained in use as late as the reign of Charles I. The old method of granulating was to place the press- cake in sieves, provided with two bottoms of thick parch ment prepared from bullock s hides, and perforated with holes, those in the lower bottom being much the smaller ; a large number of those sieves were attached to a wooden frame, hung by ropes from the ceiling, which received a violent shaking motion by means of a crank underneath. Into each sieve was put a disk of lignum vitae, to break up the cake, and force it through the larger apertures ; the grain produced was retained between the bottoms of the sieves, the dust passing through the fine holes in the lower ones, and falling on the floor of the house. The grain was afterwards separated into sizes by being passed through wire sieves. These machines were clumsy and dangerous; and the accidents which happened with them have caused them to be generally supplanted by better apparatus, although some of the old frames are still in use. The granulating machines used in the royal factories in England and in India, as well as in the best private works, are constructed upon a principle introduced by Sir William Congreve, but since improved upon. Three or four pairs of gun-metal rollers with pyramidal-shaped teeth, are fixed obliquely one above the other in a strong framework (see fig. 6) ; the sizes of the teeth vary according to the kind of grain required, but decrease from the top pair, and for fine-grain powders, the lowest pair would be smooth ; one roller of each pair Avorks in a sliding bearing, having a counterweight attached to prevent undue friction. Each pair of rollers is connected with that next below, by a short rectangular screen of copper wire, while, underneath all tlic rollers, are placed, at the same slope, two or more long wire- screens fixed in a frame having a wooden bottom ; both tlic frame and the short connecting screens are hung to the machine by strips of lance wood, and, when at work, a quick vibratory motion is given to all the screens by means of a polygonal wheel upon the main frame working against a loose smooth wheel attached to the screen-frame. A large hopper, which rises by the action of the machine, feeds the press-cake upon an endless canvas band, as in the breaking- down machine, and carries it to the top pair of rollers, whence it falls upon the first short screen ; all that is fine enough to pass through is sifted out by the shaking action of the long screens below, and travels down upon whichever screen has meshes fine enough to retain it; the pieces too large to pass through the short top screen are carried to the next pair of rollers, and so on. At the lower end of the long screens are placed boxes to receive the different sizes of grain ; the &quot;chucks,&quot; or pieces too large for any grain, are again passed through the machine, while the dust which falls upon the wooden bottom, and is received in the inner most box, goes to the mills to be worked up for forty minutes as a dust charge. Formerly at Waltham Abbey both musket (F.G.) and cannon powder (L.G.) were granu-
 * iiid even according to the prevailing state of the weather.