Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/345

325 fixed in the floor of the building, whereon a pair of iron or. stone cylindrical edge-runners revolve ; formerly, both were made of a dark grey limestone which took a liigh polish, but the newer mills have cast-iron beds and runners, the edges of the latter being now usually surface-chilled. The average diameter of the runners is 6 feet, and their width about 15 inches; they have a common axle, which rests in gun-metal bouches in a solid cross-head attached to a vertical shaft; this shaft passes through a bearing in the centre of the bed, and is in gear with the driving ma chinery, which is placed above the runners in the old water mills, and beneath the bed in cast-iron tanks in the new steam mills. Each run ner weighs about 4 tons, and the speed averages 8 revolu tions per minute. The bed has a slop ing rim on the outside, called the "curb," and on the inside .in ed"e FIG. 3- Plan ol runners and bed. formed by the " cheese," or bearing, through which the vertical shaft passes. The runners are not equidistant from the centre of the bed, one working the part of the charge near the centre, and the other the outer portion, but their paths overlap. Two "ploughs" of wood, shod with leather, are attached to the cross-head by arms and brackets; one working next the vertical shaft, and the other close to the curb, the ploughs throw the composition under the runners as it works away from the latter. The charge, when spread evenly on the bed by means of a wooden rake, contains about 2 pints of water (the moisture of the saltpetre) and a further quantity of from 2 to 6 pints is added from time to time, according to the state of the atmosphere, for the threefold purpose of preventing powder dust from flying about, facilitating the incorpora tion, and reducing the effect of an explosion ; if too wet, the runners would lick up the composition from the bed. During the time of working, the millman enters the mill occasionally, takes a wooden " shover," and pushes the outside of the charge into the middle of the path of the runners, so that every portion may be equally incorporated. The action of the runners is a combination of rolling and twisting, and has, on a large scale, somewhat the effect of a pestle and mortar, crushing, rubbing, and mixing the ingredients, so as to effect an intimate union. The time of milling depends upon the nature of the powder. For good fine-grain powders it varies from four to eight hours, but the very best sporting gunpowder is said to be as long as twelve hours under the runners ; blasting and cannon powders are incorporated from two to four hours, the period being rather longer where the lighter stone runners are used. It is of the greatest importance that the mills should be carefully attended to by experienced men, as the whole goodness and uniformity of the powder depends upon this process, and no after treatment can remedy defective incorporation. When the composition, which has now become "mill- cake," is ready for removal from the bed, it should be homogeneous in appearance, without any visible specks of sulphur or saltpetre, and of a dark greyish or brownish colour, according to the charcoal used. The mill-cake is carefully tested to ascertain whether it contains the proper amount of moisture; this should be from 2 to 3 per cent, for fine-grain powders, and 3 to 5 per cent, for the larger de scriptions. Sometimes a small portion is roughly granu lated, and " flashed " on plates of glass or porcelain ; a good powder should flash off, leaving nothing but some smoke marks, but, if badly incorporated, the plate will be coated with specks or beads of solid residue. In former days the ingredients were incorporated in " stamp-mills," which were simply large mortars and pestles, the latter merely raised up by some cam arrangement and allowed to drop by their own weight, the charge being about 12 ft>, and the weight of the stamp 50 fi) or there about. The stamp or pilon mills are still used in France and Germany, as well as the moulins-cL-tonneaux, in which the composition is put, with about an equal weight of brass or lignum vitse balls, into large barrels, which are made to revolve for a certain time on their axes ; this method of incorporation is sometimes employed in conjunction with edge runners. There is more danger of an explosion during the milling than in any other process of manufacture ; but, owing to the limitation by law of the weight of charge permitted to be under the runners at one time to 50 ft>, as well as to the great precautions taken, there is seldom any fatal result. The millmen only enter the mill occasionally to "liquor" the charge or give it a shove over, and at Waltham Abbey they wear incombustible clothing with a cap fitting over the ears, and gauntlets of the same material. The roof, and front and rear sides of the mills are usually constructed of very light boards, or even of canvas on a wood frame, while the partitions between each pair of runners are of solid masonry. The force of the explosion of a mill charge materially depends upon the length of time the incor poration has been in progress. Directly over the bed of each mill is a flat lever-board or " shutter" (see fig. 1) in gear with a tank of water, so arranged that, when the shutter is raised on its pivot by an explosion, the water is upset into the bed; a horizontal shaft connects all the shutters in a group of mills, so that the ex plosion of one mill at once drowns all the remaining charges. The set of tanks can also be pulled over by hand. The process of breaking down, although a subsidiary one, Breal is strictly necessary in order to reduce the mill-cake to a fine in s meal, so that it may be conveniently loaded into the press box, and receive as uniform a pressure as possible. The breaking-down machine (see fig. 4) consists mainly of two pairs of gun-metal rollers, set in a strong frame of FI<J. 4. Breaking-down machine. H, hopper; B, endless band ; K, rollers ; M, boxes to receive meal. the same material ; one roller of each pnir works in sliding bearings connected with a weighted lever, so that any hard substance may pass through without dangerous friction. An endless canvas band, having strips of leather sewn across it, conveys the pieces of mill-cake from a hopper,

capable of holding 500 ft to the top of the machine,