ghed five tons. They had a common axle of
wrought iron, of five inches diameter, and a vertical shaft of cast iron
passing through the centre of the bed, having a rectangular cross-head
through which the axle worked. This shaft connected below with the
machinery which gave it motion from the main shaft.

These rollers were not equi-distant from the centre of revolution, by
which arrangement every part of the charge of materials on the bed was
subjected to their action--which was crushing, grinding, mixing and
compressing; grinding and mixing from the twisting motion which followed
from so large a diameter revolving in so small a circle, and crushing
and compressing from the weight of the rollers.

To keep the powder on the bed, a wooden curb, funnel-shaped, two feet
high was placed around the circumference, fitting closely, extending
outwards at an angle of forty-five degrees. In the centre of the bed was
a short cylinder of metal, two feet in diameter and six inches high,
through the top of which the vertical shaft passed. This prevented the
powder working inwards. It also acted as a steam-chamber to keep the
bed-plate warm; but this was not used for the purpose, since the
steaming process rendered it unnecessary. A scraper, or plow, followed
each roller, which continually broke up the powder-cake, mixed its
fragments, and kept them in the path of the rollers.

At the commencement of the operation the charge of sixty pounds of
steamed materials was uniformly distributed over the bed; the rollers
were then set into motion, revolving about ten times each minute, which
continued for an hour; the broken up powder, or mill cake, which was
about five-eighths of an inch thick, was then removed from the bed,
having a blackish grey color and taken to the cooling magazines. These
were excavated in the clay and rock on the other side of the canal,
about one hundred yards distant; were four in number and separated from
each other; here the mill cake became cold and hard, and was ready for
the next operation, that of granulation.

The permanent building in which this was done was about fifteen hundred
feet distant from the Powder Mills, on the same side, further up the
canal; this, as well as each of the other permanent structures, was made
of brick, having thin walls and light roofs. Wood in the damp atmosphere
of the canal speedily decayed.

A natural growth of trees and brush-wood intervened between the
buildings along the