This is a comparatively new branch of American iron manufactures, and is at present retained under the control of a very few persons, who endeavor to keep the processes as profoundly secret as possible. The advent of railroads called into existence extensive man-nfactures of copper and brass tubing for the flues of locomotive boilers. Previous to that era, gun barrels and gas pipes of small oali-lier had been manufactured of wrought iron,' but lap-welded iron tubes, such as are now lused, were unknown. Owing to the high, price of copper and brass in comparison with iron, many persons in England especially were incited to invent machinery to manufacture unriveted wrought iron tubes of various sizes, to supersede those of the more expensive metals. Their efforts were at last successful, and although copper and brass tubes are still preferred by many, the lap-welded iron tubes are now extensively used for all kinds of multi-tubular boilers and various other purposes. Lap-welded iron tubes were first introduced into our country, from England, about the year 1845, by Thomas Prosser & Son, of this city, and were manufactured under a European patent previously secured. Soon after this, various attempts were made to make this a branch of our home manufactures, and for this purpose one firm in Philadelphia employed several mechanics who had been engaged in the business in England ; but after a considerable expenditure of money, their efforts failed of success, and it was not until 1852 that American lap-welded iron tubes became a " fixed fact." This was accomplished by machinery designed, constructed and operated by Joseph McCully, f or the well-known firm of Morris, Tasker & Morris, of Philadelphia. Mr. M. has as yet received but little benefit from his discoveries. By hisinvention lap-welded tabes are finished in four continuous operations. The sheet of metalf or a tube is first drawn through a scarfing machine, which has two cutter heads for matching the edges of the sheet to form the lap, much in the same manner as the stationery cutters for matching boards, only the form of the chisels are different. After the scarfing operation is performed, the sheet or skelp is heated in a reverberatory furnace to prepare it for the bending operation. This is executed by a machine, through which the heated skelp is drawn, and it is upset or bent by dies, and the lap laid ready for welding. It is now heated a second time, in a similar furnace, to the welding heat, after which it is passed over a bill pointed ball placed bekween rolls fluted in such a manner that, as they roll and press upon the formed tube, they weld the lap without leaving any fin upon it, and at the same time they feed the tube back as fast as it is welded, over a long movable rod or bar attached to the ball on which the tube is squeezed. After the lap-welding is completed from end to end of the tube, the iron rod is withdrawn and the tube—still hot—is taken out and passed between friction rollers which straighten, smooth and finish it ready for market. In this manner, by four operations and two heats, American lap-welded iron tubes, are manufactured from sheets or skelpsof wrought iron. The machinery employed is simple and well arranged for performing the successive operations, and it is, in many respects different from the machinery and processes employed and practiced in England in the same business. Hitherto, tubes of this character have only been made from one to eight inches in diameter, but the machinery of Mr. McCully, we understand, is capable of making twelve inch pipes, which, when manufactured, will undoubtedly take the place of riveted flues in long cylindrical boilers, also the riveted pipes employed f or the chimneys of some steamers and locomotives. Such flues can be made cheaper than the riveted kind, and they are much stronger and handsomer. There are only two firms in our country which manufacture lap-welded iron tubes and flues at present, namely, Messrs. Morris, Tas-ker & Co., of Philadelphia, and Messrs. Seyfert, McManus & Co., of Reading, Pa. The English lap-welded tubing of good quality still meets ours in the market, but the home manufacturing business, we believe, is very profitable and prosperous in the select hands to which it is confined. The price of this kind of tubing varies according to the size. Flues of two inches outside diameter, cost thirty cents per foot of two pounds, or fifteen cents per pound—not half the price of copper tubes of the same weight. Since 1852, when American lap-welded tubes were first made, the business of manufacturing them has largely extended, and when it is taken into consideration that, in such a steamer as the Baltic, there are 5624 two inch lap-welded iron tubes in the four boilers, amounting to 29,526 feet, we can form some idea of the extent to which this business will yet be carried as our steamers increase, and when these tubes will take the place of all others in locomotives. We can also obtain a more extended idea of the amount of such tubing which will yet be required for such a purpose as artesian wells, when we reflect that very large tracts of California and the Great West—in the Mississippi valley—can only be supplied with water through such agencies. Judging, therefore, from its utilitarian character, the manufacture of American lap-welded iron tubes must ultimately attain to gigantic proportions.