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, which rotate with great rapidity,—­usually, about four hundred times in a minute.  These stones are covered with large, movable wooden cases, to keep the water from flying about the room, or over the workmen.

An iron rod is inserted into the bore of the barrel, and is fitted very closely.  The rod is furnished with a handle, which is used by the workman for holding the barrel against the stone, and for turning it continually while he is grinding it, and thus bringing the action of the stone upon every part, and so finishing the work in a true cylindrical form.  In the act of grinding, the workman inserts the barrel into a small hole in the case in front of the stone, and then presses it hard against the surface of the stone by means of an iron lever which is behind him, and which he moves by the pressure of his back.  The work is very rapidly and smoothly done.

There are twelve sets of stones in the grinding-room in constant operation day and night.  These stones, when set up, are about eight feet in diameter, and are used to within twelve inches of the centre.  They last about ten days.