The first of these processes is the boring out of the interior. The boring is performed in certain machines called boring banks. They consist of square and very solid frames of iron, in which, as in a bed, the barrel is fixed, and there is bored out by a succession of operations performed by means of certain tools which are called augers, though they bear very little resemblance to the carpenter's instrument so named. These augers are short square bars of steel, highly polished, and sharp at the edges--and placed at the ends of long iron rods, so that they may pass entirely through the barrel to be bored by them, from end to end. The boring parts of these instruments, though they are in appearance only plain bars of steel with straight and parallel sides, are really somewhat smaller at the outer than at the inner end, so that, speaking mathematically, they are truncated pyramids, of four sides, though differing very slightly in the diameters of the lower and upper sections. The barrels being fixed in the boring bank, as above described, the end of the shank of the auger is inserted into the centre of a wheel placed at one end of the bank, where, by means of machinery, a slow rotary motion is given to the auger, and a still slower progressive motion at the same time. By this means the auger gradually enters the hollow of the barrel, boring its way, or rather enlarging its way by its boring, as it advances. After it has passed through it is withdrawn, and another auger, a very little larger than the first is substituted in its place; and thus the calibre of the barrel is gradually enlarged, _almost_ to the required dimensions. Almost, but not quite; for in the course of the various operations which are subsequent to the boring, the form of the interior of the work is liable to be slightly disturbed, and this makes it necessary to reserve a portion of the surplus metal within, for a final operation. In fact the borings to which the barrel are subject, alternate in more instances than one with other operations, the whole forming a system far too nice and complicated to be described fully within the limits to which we are necessarily confined in such an article as this. It is a general principle however that the inside work is kept always in advance of the outside, as it is the custom with all machinists and turners to adopt the rule that is so indispensable and excellent in morals, namely, to make all right first within, and then to attend to the exterior. Thus in the case of the musket barrel the bore is first made correct. Then the outer surface of the work is turned and ground down to a correspondence with it. The reverse of this process, that is first shaping the outside of it, and then boring it out within, so as to make the inner and outer surfaces to correspond, and the metal every where to be of equal thickness, would be all but impossible. TURNING.