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.

These incorporating mills consisted, each, of an iron circular flat bed
of seven feet diameter, fixed in a mass of masonry built up above the
brick archway, through the center of the floor, to a convenient height.
On this bed two massive iron rollers, six feet in diameter and fifteen
inches face, revolved. Each wei