apt to cause accident. The face of the runners should be nearly flat, with a slight bevel towards the edge. The runners are connected together by a powerful spindle of wrought iron, which rests in brass bushes placed in the cross-head, so as to allow the spindle and runners to rise and fall, according to tbo thickness or thinness of the layer of material on the bed. The spindle is placed in the crosshead, in order to bring one runner nearer to it than the other, and therefore to cause them to describe different paths when in Lotion. The spindle shonld have a large leverage on the runners, as their weight is so great. They must therefore be cast with as long a socket for the spindle as possible, otherwise they will very soon work loose on it. The cross-head r is fixed on a vertical shaft s, turned by a wheel p, driven from a pinion q on the driving shaft, which passes underneath the whole group of mills. By this arrangement the gearing is kept out of reach of damage from explosion. The cross-head r is fitted with a bracket on each side, to carry a plough of wood, shod with felt and leather, which travels round on the bed immediately in front of the runners, and thus keeps the composition from working away from them. The bed has a curb or edge round the outside and inside of the circular path described by the runners, that on the outside being formed by a sloping rim fixed all round the bed, that on the inside- by the circular base of the cheese or conical socket down which the vertical shaft of the cross-head passes. Both the inside and outside curbs have gun-metal rings round them for the ploughs to work against. Every fitting and bolt is arranged with the greatest care, so as neither to break nor become loose from the jolting of the mill, and thus drop into the charge.
Various modifications of the incorporating mill have been proposed from time to time. The most important was a plan for keeping the runners and cross-head stationary, and causing the bed to rovolvo underneath them. This plan has the merit of greater safety to recommend it, but the mechanical difficulties in working it are very great. Another proposal was to heat the beds of all incorporating mills by steam, as heat is supposed to assist and hasten the process of incorporation, for some unknown reason.
The ()barges, Dither green from the mixing house, or dust from the dusting or granulating houses, come to the mills in wooden tubs, and are stored in strong expense magazines or charge houses. In all eases they are sifted before going to the mills, as this is the only certain plan of preventing foreign matter's getting into the mill bed. The millman takes the charge and throws ono half on one side of the bed and the other on the other, and then with his rake and brush distri butes it evenly over the mill bed. The runners are then moved round a quarter revolution, and the piece of mill-cake left under the runners from the former eharge is broken up and distributed over the fresh charge. This portion of mill-cake is of course finished powder, and if the runners
are left standing for any length of time with their whole weight on it, it becomes extremely hard, It should therefore always be brokeu up and distributed over the fresh charge, for if otherwise, it may adhere to the bed, and being very hard may be the cause of so much friction as to cause explosion. It is a mueh better plan to leave the runners on a portion of powder on which they stop than to attempt to run them off on to a leather placed on the mill bed, as must be done before the mill is washed out for repairs. Before starting the mill, about two pints of distilled water are sprinkled over the charge, the runners are then started at a speed of about eight revolutions a minute ; the millman does not remain in the mill, but only goes in from time to time to push up the charge from the bed, and to add a little more water according to the state of the charge, from two to three pints are generally found to be necessary in very damp weather, and as many as eight or ten in very bright days. The watering, or liquoring, as it is called, of the charge is always left to the millman's judgment ; but it would be preferable to lay down a regular scale of liquor for all mill charges, according to the indications of the hygrometer.
Large-grained powder requires to be incorporated for three hours, all small-grained powder made with dogwood for five. The times of ineorporation vary with the power of the mills. Thus large-grained powder requires 31 hours working under strong runners weighing 31 tons, and making 7i rev. a minute, but only 21 under iron runners of 4 tons, making 8 rev. a minute. Small-grained dogwood powders require 51 hours in the former mills, and I in the latter. All dust charges are worked hour in all mills.
As the process of incorporation approaches completion, the charge requires to be carefully watched, in order to ensure each finished charge leaving the mill in as nearly as possible the same state as regards moisture. The appearance of the powder when finished depends mainly on the state in which the charges leave the mill. If more than from two to three per cent. of moisture be present as tho incorporation draws to a close, the charge must be repeatedly pushed up with the shover ; if too little, some more must be added from the watering pot. The colour of the charge gives a very good indication of the amount of moisture present.
When the process is finished, the charge now known as mill-cake, being partly in the state of soft cake and partly of dust, is scraped and swept up from the mill bed, placed in wooden tubs, and transferred to the charge house to await inspection. If the charges are found to be of a proper colour and consistency, samples from each are taken, which, after being roughly granulated by hand and dried, are flashed on a glass plate, to ascertain the thoroughness of the incorporation which they have undergone.