l line drawn across a pane of
glass in a window. The workman then placed the barrel of the musket upon
a rest in such a position that the reflected line in the mirror could be
again reflected, through the bore of the barrel, to his eye,--the inner
surface of the barrel being in a brilliantly polished condition from the
boring. When the barrel is placed at the proper angle, which practice
enables the person performing this duty to accomplish at once, there are
two parallel shadows thrown upon opposite sides of the inner surface,
which by another deflection can be made to come to a point at the lower
end. The appearance which these shadows assume determines the question
whether the barrel is straight or not, and if not, where it requires
straightening. Although this method is so easy and plain to the
experienced workman, to the uninitiated it is perfectly
incomprehensible, the bore of the barrel presenting to his eye only a
succession of concentric rings, forming a spectacle of dazzling
brilliancy, and leaving the reflected line in as profound a mystery
after the observation as before.

At present, the mirror is discarded, and the workman holds the barrel up
directly to the pane of glass, which is furnished with a transparent
slate, having two parallel lines drawn across it. The only purpose
subserved by the mirror was that of rendering the operation of holding
the barrel less tiresome, it being easier to keep the end of the musket
presented to the line pointing downwards than upwards. Formerly, this
means of detecting the faults, or want of straightness in the barrel,
was, like the working of the rolling-mill, the secret of one man, and he
would impart it to no one for love or money. He was watched with the
most intense interest, but no clue could be obtained to his secret. They
gazed into the barrel for hours, but what he saw they could not see.
Finally, some fortunate individual stumbled upon the wonderful
secret,--discovered the marvellous lines,--and ever since it has been
common property in the shop. Each workman is obliged to correct his own
work, and afterwards it is passed into the hands of the inspector, who
returns it to the workman, if faulty, or stamps his approval, if
correct. The next process is that of grinding, for the purpose of
removing the marks left upon the surface by the tool in turning, and of
still further perfecting its form. For this operation immense
grindstones, carried by machinery, are used,