The great advantage derived in this instance from air-furnace welding is a chemical one; for while the small pieces of steel are fusing on the bed of the air furnace, the oxygen is extracting the carbon, and leaves the resulting metal mild steel, or iron of the densest description; while the succeeding hammering and rolling and re-welding, produce the mechanical arrangement of making the whole of an extremely fibrous description. The polishing secures a clean metal; indeed, so free from specks are the generality of barrels thus made, that it is scarcely possible to imagine clearer metal. When contrasted with the best of ordinary iron, by a powerful microscope, the closeness and density of grain are strongly apparent.
To such an extent has this been carried, that I can produce specimens of a considerably increased specific gravity. The barrels made of this metal, in general, beat all tried against them; with this great advantage, that the finer the polish in the interior the better they shoot, and continue longer free from lead. The only difficulty is in the working; as the boring, filing, &c., are more difficult. Moreover, greater care is required to see that they are not annealed,[8] when in the hands of the borer or filer; for in such case they would be considerably injured, though not to the same extent as barrels of a softer nature. I tested a great variety of bars by drawing them asunder longitudinally by the testing machine, and the average strength of a rod of 6-16ths broad by 5-16ths thick and 12 inches long, containing 1·40625 solid inches of iron, was equal to a tension of 11,200 lbs. This furnished a barrel having a thickness of metal in all parts of the arch equal, or 3-16ths of an inch thick, capable of bearing an internal pressure of 6,022 lbs. to the inch of the tube.
[8] Dr. Ure falls into an error in describing the process of barrel boring: he says “the barrel is first properly annealed, and allowed to cool gradually,” &c. The barrel-maker that would take such a proceeding with a barrel of ours should never do so to another. The Doctor ought to have pointed out the evil tendency of this. We never saw it done, and we doubt much whether he did, though we have heard of the practice, which induces us to notice it, but the Doctor describes it as a _necessary_ proceeding.