of two atoms of carbon and one of nitrogen), which is made from a variety of animal matters. It is a new application without any change of principle; the time occupied in this steelifying process is sometimes only minutes instead of hours and days, as for example when iron is heated in the open fire to a dull red, and the prussiate is either sprinkled upon it or rubbed on in the lump, it is returned to the fire for a few minutes and immersed in water; but the process is then exceedingly superficial, and it may if needful be limited to any particular part upon which alone the prussiate is applied. The effect by many is thought to be partial or in spots, as if the salt refused to act uniformly; in the same manner that water only moistens a greasy surface in places. The prussiate of potash has been used for case-hardening the bearings of wrought-iron shafts, but this seems scarcely worth the doing. In the general way, the conversion of the iron into steel, by case-hardening, is quite superficial, and does not exceed the sixteenth of an inch; if made to extend to one-quarter or three-eighths of an inch in depth, to say the least it would be generally useless, as the object is to obtain durability of surface, with strength of interior, and this would disproportionately encroach on the strong iron within. The steel obtained in this adventitious manner is not equal in strength to that converted and hammered in the usual. way, and if sent in so deeply, the provision for wear would far exceed that which is required. Let us compare the case-hardening process with the usual conversion of steel. The latter requires a period of about seven days, and a very pure carbon, namely, wood charcoal, of which a minute portion only is absorbed; and it being a simple body, when the access of air is prevented by the proper security of the troughs, the bulk of the charcoal remains unconsumed, and is reserved for future use, as it has undergone no change. The hasty and partial process of cementation is produced in a period commonly less than as many hours with the animal charcoal, or than as many minutes with the prussiate of potash; but all these are compound bodies (which contain cyanogen, a body consisting of carbon and nitrogen), and are never used a second time, but on the contrary the process is often repeated with another dose. It would be, therefore, an interesting inquiry for the chemist, as to whether the cyanogen is absorbed after the same manner as carbon in ordinary steel, or whether the nitrogen assists in any way in hastening the admission of the carbon, by some as yet untraced affinity or decomposition. It may happen that the carbon is not essential, as the Indian steel or wootz is stated to contain alumina, silex, and manganese. This hasty supposition will apply less easily to cast-iron, which contains from three to seven times as much carbon as steel, and although not always hardened by simple immersion, is constantly under the influence of the case-hardening process; unless we adopt the supposition, that the carbon in cast-iron which is mixed with the metal in the shape of cinder in the blast furnace, when all is in a fluid state, is in a less refined union than that instilled in a more aeriform condition in the acts of cementation and case hardening. CHAPTER XI. ON THE APPLICATION OF IRON TO SHIP-BUILDING. THERE is probably no branch of industry in which the use of iron is more important than that of ship-building. The strength, ductility, and comparative lightness of this material are all in its favor; and, although much has been done in the application of iron to this important purpose, a great deal more remains to be accomplished. Vessels composed of iron plates have been employed for more than fifty years in the navigation of canals; but it is not more than twenty-five or twenty-six since they were first introduced as sea-going vessels. It is true that the late Mr. Aaron Manby projected an iron vessel in 1820, which was built in the ensuing year, and early in 1822 was navigated by Captain (since Admiral Sir Charles) Napier from London to Havre, and on to Paris; this, however, was not a sea-going vessel, but an iron steamer constructed for the Seine, and which for many years navigated that river between Paris and Rouen. From this period little appears to have been done in furtherance of the application of iron to the construction of ships till 1829-30, when the introduction of a new system of traction at high velocities on canals led to new developments; and from this time to the present, iron, as a material for ship-building, has been extensively used, and is increasingly in demand. From 1829 to 1832, iron ship-building may be considered to have been experimental; and the trials conducted by Mr. Fairbairn on the Forth and Clyde Canal,* simultaneously with those of Mr. John and Mr. McGregor Laird at Liverpool, led to a new era in the history of ship-building. Among the first iron vessels for sea-going purposes was one of small tonnage, built at Manchester for the Forth and Clyde Canal Company. She was built with paddle-wheels on the quarter near the stern, and propelled by two high-pressure engines of, collectively, 30 horse-power. This vessel attained great speed, considering the date at which she was built; and for many years traded between Grangemouth and the coast of Fife, round to Dundee. Previously to the building of the "Manchester," another small vessel, called the "Lord Dundas," was constructed for the same * Vide "Remarks on Canal Navigation," by W. Fairbairn. Longman, 1831. company. She was strictly experimental, and was propelled by a locomotive engine of 16 horse-power, with 8-inch cylinders. Such was the lightness of her construction, that the plates were only 1-14th of an inch thick, riveted to light T iron, which formed the ribs of the hull. This vessel had stern paddles, and was of the following dimensions: Length, 68 feet. Breadth on beam, 11 feet 6 inches. Diameter of paddle-wheel, 9 feet. Whole weight, including engine, paddle-wheel, etc., 7 tons 16 cwt. Draught of water with cargo on board, 16 inches. The "Lord Dundas" was built in 1830, conveyed through the streets of Manchester on trucks, and launched into the Irwell, where numerous trials took place in regard to her speed in narrow channels, such as canals; including such other direct experiments as were likely to result from vessels of this kind propelled by steam. Subsequently to these trials she was navigated to Liverpool, and from thence to Glasgow via the Isle of Man. As this Voyage was rather a perilous one, when the slightness of the vessel's build and the thinness of her sheathing-plates are considered, and as it was among the first-if, indeed, it were not the very firstwhich indicated the necessity of adjusting the compass in order to neutralize the local attraction of the material by which it was surrounded, we may probably be permitted to give a brief narrative of the circumstances as they occurred during the voyage. The "Lord Dundas" sailed from Liverpool at four A.M. on a fine morning in June, 1831, and steered direct for the floating-light. She made the light in good time, notwithstanding a thick haze in the atmosphere, which, during the forenoon, thickened into a dense fog. Towards one o'clock land was descried upon the starboard bow, showing apparently that she had made considerable deviation in a westerly direction. A dispute arose as to what land it was- -one party contending that it was the western side of the Isle of Man; the other, better acquainted with that side of the island, that it was not. After a considerable contest and examination of the charts, it was at last discovered that the little vessel was on the north of Morecambe Bay, approaching the coast of Cumberland. On the discovery of this error, and in consequence of the frail bark show. ing symptoms of weakness, from the effects of the swell which was rolling in from the west, it was considered desirable to look out for shelter; and consequently her course was altered in the direc tion of the Island of Peel Foundry, where she was sheltered for the night. On the following morning she crossed to Ramsey, where the question of the variation of the compass was investigated, and rectified by the simple process of nailing a block of iron to the deck, in the immediate vicinity of the compass-by this means neutralizing the local attraction of the iron by which it was surrounded. After this, the remainder of the voyage from Ramsey to Greenock was effected in a direct course with perfect safety. We have noticed these circumstances as illustrating the imper fect state of our knowledge, as respects the influence of large masses of iron upon the ship's compass. It has been ascertained that the angle-iron and T iron ribs, when carried above the deck so as to form part of the bulwarks, had a remarkable effect upon the compass, each of them forming, as it were, a separate magnet, whose influence, unless neutralized by some greater magnetic power, caused a considerable deviation of the needle, so that it indicated a point wide of the magnetic north; and as this deviation altered with every change of the position of the vessel, no reliance could be placed upon it. Captain Johnson and Professor Airey, by an interesting series of experiments, ultimately settled this question, and provided a remedy in the adjustment and correction of the compass on board iron ships. The object contemplated by this light vessel and light machinery was, to ascertain how far quick speeds could be attained upon canals by steam-power. As much as fourteen miles an hour had been accomplished by horses, with a tractive power of 352 lbs. by dynamometer, and that without the least appearance of surge;* but the experiments made with the "Lord Dundas" steamer indicated a very different law, and, under the most favorable circumstances, never exceeded more than eight to eight and a half miles an hour, and that with an enormous swell washing over the banks of the canal in every direction. In fact, the object for which the boat was built was never attained, and it was found impossible to effect by steam what was done by horses. It nevertheless led to a more important and a greatly-enlarged branch of industry-namely, th construction of iron vessels upon a large scale for ocean traffic. These experimental vessels, the "Lord Dundas" and the "Manchester," already mentioned, in conjunction with the "Alburka," and some other vessels, by Messrs. J. Laird and Co., of Liverpool, may be considered as the first successful attempts in iron shipbuilding. Shortly after the completion of these vessels, several large establishments were founded for this branch of construction, amongst whom may be enumerated Messrs. W. Fairbairn and Co., Millwall, and Messrs. Ditchburn and Mare, Blackwall, London; Messrs. Laird and Co., Liverpool; Messrs. Tod and McGregor, Glasgow; and several others, all of whom were engaged for many years in the construction of iron ships. In this chapter we shall be unable to go much into detail, and must confine ourselves to a few general observations in connection with the more important application of iron as a material of construction for ocean steamers and sailing vessels, exposed to all the changes and vicissitudes of wind and tides in the open sea. "Remarks on Canal Navigation," page 57. A Fig. 108 exhibits a half cross section of one of Her Majesty's frigates of the second class, and will, to a certain extent, illustrate the principles of construction. It will be seen that the iron-ship is composed of a series of frames or ribs, placed at various distances apart; these are connected together in the interior of the vessel by transverse beams, mostly of iron, but sometimes of wood, which support the decks. Over the exterior of the ribs the iron sheathing-plates are riveted, so as to form a continuous water-tight covering over the entire exterior of the vessel. RIBS. One of the ribs is shown at a a a, Fig. 108; and its section will be seen in Fig. 109, which is a longitudinal section through the line bb; it consists of a vertical plate c, to which two angle-irons are riveted, one at the top and the other at the bottom. On the lower angle-iron the sheath ing plates d are riveted; and on the upper, interior plates, some Fig. 109. of which in large vessels are riveted diagonally, so as to form stringers and braces from the keelsons round the bilge to the upper decks. These ribs are placed at distances of about fifteen inches to eighteen inches apart, according to their position in the direc Fig. 110. tion of the length of the ship. Öther kinds of frames might be used with double angle-iron, as shown at e e, in the annexed sketch (Fig. 110), but they are more expensive; and from the increased complexity of construction, the extra strength obtained does not compensate for the difference of cost. Although the frames shown |