“Threepenny skelp iron” is made from an inferior quality of scrap to that from which “charcoal iron” is made; but unlike it, there is no pretension of quality. Its inferiority is not denied; it is poor in quality, and suits parties who cannot buy better. The method of preparing is by an air-furnace, forge, tilt and rolling mill, as before described. The greatest strength of a bar 11-16ths broad by 3-16ths thick, containing 1·5468 solid inches, is 7,840 lbs.; or equal to an internal pressure of 3,841 lbs. to the inch of tube. One particular fact attaches to all kinds of inferior iron--the greater the mass acted upon by the rollers the greater the variation of strength. This arises entirely from the increased sponginess of the metal, and its greater expansibility. For instance, a rod 1-16th thicker, is 15 per cent. weaker in proportion; and so on to the greatest extent. But on the other hand, it is capable of recovering a great increase of strength by cold hammering; greater than better iron. A considerable quantity of this iron is sold to engineers, and used in the construction of locomotive and other engines; the price and uniformity of texture in grain fitting it for that purpose. “Twopenny” or “Wednesbury skelp” is almost too bad to be used in making an article which may endanger the limbs of our fellow creatures, and is now little used, fortunately. It is made of an inferior scrap to the former, in precisely the same manner; and in point of strength is still lower. The bar is generally 1 and 1-16th inches in breadth, by 3-16ths thick, the solid contents 2 inches and 25-64ths, and will bear a weight of 7,840 pounds; consequently the strength will be 2,840 pounds to the inch of tube. This is a great falling-off in strength; and I would ask any one who values the safety of his hand, if he would like to risk it, by using a gun made of iron possessing so low a degree of strength, as compared to the force of the charge it has to bear? Let him recollect that the force of the charge may be increased by a variety of circumstances. The pressure of a certain quantity of powder, on which a certain weight of shot is placed, is so many pounds to the inch; and if you double that weight of shot, you nearly double the pressure. In estimating the force of pressure, the opposing friction is also to be taken into account. If the gun be allowed to get very foul, then friction is increased, and of course a still greater pressure is thrown on the tube of the barrel. All these circumstances being taken into consideration, I repeat, that _no barrel is safe, whose power of resistance is not more than double the strength of a charge of sufficient force for general shooting_. Every bad gun should be thrown aside as unsafe, or used with the greatest caution. Bad and inferior guns are made from the foregoing material; and not many years have elapsed since it was thought good enough for military arms.