Make a clear fire in the forge, using charcoal if it can be obtained, heat the barrel very carefully until small pieces of solder will be melted when placed on the inside of the thimble. Have the soldering copper heated, and by using it and applying the acid an even amount of the solder can be applied to the joint outside the thimble where it joins the barrel. When all are soldered let the barrel cool, remove the binding wires and wash with warm water to remove the acid flux, which would rust the work. A stiff brush is best to wash with. Scrape off the superfluous solder, rub the thimbles bright with emery-cloth, or let them remain the black color, as may be desired. CHAPTER XXI. ON RIFLING OF GUNS. _Importance of Rifling._—In a rifle the grooving is of the utmost importance; for velocity without accuracy is useless. To determine the best kind of groove has been, accordingly, the object of the most laborious investigations. The ball requires an initial rotary motion sufficient to keep it “spinning” up to its range, and is found to gain in accuracy by increasing this rotary speed; but if the pitch of the grooves be too great, the ball will refuse to follow them; but being driven across them, “strips”—that is, the lead in the grooves is torn off, and the ball goes on without rotation. The English gun-smiths avoided the dilemma by giving the requisite pitch and making the grooves very deep, and even by having wings or lugs cast on the ball to keep it in the grooves—expedients which increase the friction in the barrel and the resistance of the air enormously. The American gun-makers solved the problem by adopting the “gaining twist,” in which the grooves start from the breech nearly parallel to the axis of the barrel, and gradually increase the spiral, until, at the muzzle, it has the pitch of one revolution in three to four; the pitch being greater as the bore is less. This gives, as a result, safety from stripping, and a rapid revolution at the exit, with comparatively little friction and shallow groove-marks on the ball, accomplishing what is demanded of a rifled barrel, to a degree that no other combination of groove and form of missile ever has. There is no way of rifling so secure as that in which the walls of the grooves are parts of radii of the bore. They should be numerous, that the hold of the lands, or the projection left between the grooves, may divide the friction and resistance as much as possible, and so permit the grooves to be as shallow as may be. Fig. 41 represents grooves cut in this way, but exaggerated to show more clearly their character. In the Kentucky rifle this law is followed, except that for convenience in rifling, the grooves are made of the same width at the bottom and top, as shown in Fig. 42, which is, for the grooves of the depth of which they are generally made, practically the same, the depth in the cut being two or three times that generally used. [Illustration: FIGURE 41.]