724 THE POPULAR SCIENCE MONTHLY. 



This part of the process is more dangerous than any other, and 

 explosions in the incorporating-mills are very frequent. The houses 

 are built of light planking, nailed on a strong framework, so as to 

 diminish the force of the explosion by yielding easily before it. The 

 men are forbidden to remain in them while the mill is in motion, and 

 a very simple arrangement has been devised for preventing an explo- 

 sion from extending from one mill to another. A shaft runs horizon- 

 tally through the upper part of the walls of each row of mill-houses. 

 A shutter, balanced by a weight on the other side of the shaft, pro- 

 jects from it over each mill, and this shutter supports one side of a 

 water-tank, the other resting on a pivot. Now, if an explosion takes 

 place in any of the mills, the shutter above it will be blown up, turn- 

 ing the horizontal shaft, and raising all the shutters attached to it ; so 

 that the tanks, being left unsupported, turn over, and drench the con- 

 tents of the mill-beds below. 



On leaving the mill, the gunpowder is in the form of a soft cake, 

 which easily breaks up into meal and dust. The old plan for making 

 gunpowder, still followed in some places, was to moisten this mill-cake 

 and force it through fine sieves, so as to break it into grains ; but the 

 moisture partly dissolved the saltpetre, and thus, to some extent, de- 

 stroyed the previous incorporation, and the result was an inferior gun- 

 powder, which, on account of the softness of the grains, often broke up 

 into dust in transport. In the modern process, the mill-cake is first 

 presssd in layers between plates of copper or gun-metal, to increase its 

 hardness and density, and then made into grains of the required form 

 by machinery. As a preparation for the press, the mill-cake is rough- 

 ly broken down into meal and dust by being passed between grooved 

 gun-metal rollers. It is then ready to be poured into the press-box. 



This is a large box of gun-metal, lined with oak, and capable of 

 holding about 800 pounds of powder. The sides are hinged to facilitate 

 unloading, and by means of a small crane it can be swung into or out 

 of the hydraulic press. To be loaded it is turned on one side, a wood- 

 en cover placed on the top, and the uppermost side is turned back on 

 its hinges. Then, by means of gun-metal racks, the plates are 

 arranged in the box, with the proper intervals between them to pro- 

 duce a thick cake for cannon-powder, or a thin one if rifle powder is 

 to be made. The powder-meal is then poured in between the plates, 

 the racks withdrawn, the side closed and bolted down. It is then 

 swung by the crane on to the table of the press, and the cover taken 

 off. The press is an ordinary hydraulic one ; the table which supports 

 the box is placed on the head of the ram, and as it rises a block of oak 

 fixed overhead enters the box, and pi-esses the powder, the amount of 

 the pressure being measured by the extent to which the block enters 

 the box. The pumps which supply the press with water are fixed in 

 an adjoining room, and worked by a water-wheel; and, in order that 

 the men may know when the pressing is complete without having to 



