In number 22, Vol. XX., of the SCIENTIFIC AMERICAN, we discussed the method, hitherto in vogue, of forging shafts for sea-going steamers, from mixed scrap iron. We most decidedly disapproved the method, maintaining that a perfectly homogeneous shaft of such materials, even if its achievement were possible, must necessarily be a highly improbable result, of a plan opposed, not only to scientific principles, but to common sense. Since writing the article alluded to, we have seen no reason to alter the opinions we then entertained and expressed with reference to this subject; and we now have the pleasure to state that those opinions are not only winning adherents, but that their truth is actually being tested, practically, by the Pacific Mail Steamship Company. This company have recently had a shaft forged for them at the Franklin Forge, corner of Twenty-fifth street and Third avenue, New York, of the Collins Iron Companys Lake Superior charcoal pig iron. This shaft is intended for the steamer Japan, San Francisco and China, and is, in the rough, 39 feet 7 inches long, weighing 80,000 lbs. The body of the shaft will be, when finished, 26 inches in Hiametcr, and the diameter of collars 31 inches. The forging of this shaft required a working force of 38 men, and consumed 15 days of ten hours each. The iron. from which this shaft was forged, was puddled by Tugnot, Thompson & Co., of the above works, expressly for the purpose, and twice hammered before the shaft was forged. None of the iron used has had less than three heats after the billets were prepared. We were present on two occasions during the forging, and our opinion as to the great superiority of shafts made of such iron over those of scrap-iron, has been greatly strengthened by our observations. Mr. Tugnot, of the above works, under whose supervision the whole work has been performed, is one of the most experienced iron masters and forgers in this country, and the work throughout is of the most perfect character. The steam hammer used weighs nine tuns, and under its ponderous strokes, the heated billets seemed as plastic and cohesive as wax. Such a shaft must, necessarily, cost more than one made of scrap-iron, but its greater strength and consequent security, will more than compensate for its increased cost. The iron from which it is made isoa very superior quality, a larger quantity of charcoal being used in its manufacture than is ordinarily employed. It is made of half hematite and half specular ore, a mixture of which gives an iron of remarkable tensile strength. A chain link of this iron, made of 1-i-inch bar, was once tested by D. B. Martin, formerly Engineer-in-Chief to the Secretary of the United States Navy, and broke only at the enormous tension of 169,120 lbs. We have also seen a specimen of this iron which had only been subjected to two heats, and which was tested by Paulding and Kendall, of the West Point foundery, which, after breaking at a tension of 63,376 lbs. per square inch, was found, upon examination, to be defective. These facts speak sufficiently for the excellence of this iron, and we are glad that the importance of using shafts made of the very best material is beginning to be appreciated by capitalists. A steamer with a broken shaft is almost as helpless as a ham-strung horse; it may, if it has good luck, finally crawl into port, after a delay which has cost more than two shafts would, or it may encounter bad weather and go to the bottom. Where so much is depending, considerations of first cost should weigh little in the scale against security, and it does weigh little to the engineer who knows his business. Unfortunately, however, these facts are too often overlooked by the men who invest their money, and who, not acquainted with the nature and quality of different kinds of iron, are too apt to consider them pretty much on a par. Nothing could be more unwise than such a conclusion, and the difference between a shaft made of inferior iron and one of the best quality, is so great in its contingent results that, within reasonable limits, cost should not be considered. We hope the precedent established by the Pacific Mail Steamship Company, will prove the beginning of a wiser practice than has hitherto prevailed in reference to this subject.