The Incorporating Mills, twelve in number, extended along the canal beyond the Refinery building and further back from its bank, having the Laboratory between the two; they were two hundred and ninety-six feet long. This separation was for safety, as they worked explosive material. The walls were massive, being four to ten feet thick, the horizontal section of each being that of a huge mortar of seventeen feet wide by twenty-four feet long; the height of the walls was twenty-eight feet; they faced alternately in opposite directions, so that an explosion of one would not be communicated to those adjoining. The fronts were constructed of light wood and glass, and the roofs of sheet zinc, so that but slight resistance would be offered, upwards and outwards, to the explosive force. A wing wall, nearly as high as the main walls, and three feet thick, extended outwards from the centre of the exterior back wall of each mill twenty feet, to guard still further against the effects of an explosion. Behind these the powder-makers stood, for safety, while starting or stopping the motion of the ponderous rollers. This was done by means of a long lever, which threw in or out of gear the friction arrangement, which worked each set beneath the floor, in the thick archway which extended from end to end beneath the mills. It has already been stated that this archway contained the great iron shaft which imparted motion to all the mills, and which derived its own from the large steam engine, which was located above, in the centre apartments separating the mills into two divisions. In addition to the above precautions to prevent the explosion of a mill from extending to the others, above each set of rollers was balanced a vessel containing about thirty gallons of water. This was connected by means of a small iron shaft with a similar vessel to each mill of the division. Thus, on an explosion in one mill, its bed-plate was instantly drenched with water, and this caused the same to take place at the same moment with all the others. These precautions were rendered the more necessary by the carelessness of the powder-makers, who might not remove the broke up powder cake from the mill enclosure before placing a new charge under the rollers, thus having one hundred and twenty pounds of material to take fire at the same time--as once happened--producing a powerful explosion. There occurred only three explosions at these mills--all before the steaming process was adopted--and in the first only was any one injured. In that one no material harm was done, as the two powder-makers--exposed by their own carelessness--were at work again in a few days. This explosion completely destroyed the slight roof, as well as the wood and glass front, but did scarcely any other damage to the mill, and had no action on the other mills further than drenching their beds with water. The other two explosions were insignificant.