268 LECTURE VI. ON THE CONSTRUCTION OF IRON VESSELS EXCEEDING THREE HUNDRED FEET IN LENGTH. In the previous Lecture I endeavoured to inculcate principles on which iron ships ought to be built in order to secure perfect safety, and to give to the public increased confidence in the stability of these constructions. In pointing out how these desiderata may be obtained, I confined my attention to vessels varying from 500 to 1500 tons burden, and not exceeding 300 feet in length and 41 feet 6 inches beam. In these constructions I attempted to prove that the present system was defective, and that in certain positions a vessel built upon this principle must of necessity break up and go to pieces. These views were not founded upon theoretical speculations, but upon experimental facts, and to which I considered it my duty to direct public attention. It cannot be denied that the most disastrous effects have followed from these defects; and it appears imperative, for the sake of life and property, that a new and more perfect system of construction should be adopted, founded on definite laws by which the resisting powers of materials in different forms and conditions are governed. As respects iron nearly the whole of these laws are known, and we are at no loss to discover its ultimate powers of resistance in whatever positions it is placed, or to proportion its dimensions to meet with safety the forces to which it is subjected. Possessing this knowledge, and having it in our power to apply it, why should we neglect its application in structures of such vast importance as those in which our lives and fortunes are so often embarked? The surveyors of Lloyds, most excellent, well-meaning, gentlemanly men as they are, may say what they please, but I have no hesitation in stating that their regulations are very defective and require immediate revision, and such a revision in my opinion should be based upon principles of exact science, and calculated to secure a maximum strength in the iron ship. I do not wish to find fault, nor do I assert that the alterations I have to propose are in every sense the best calculated to produce a maximum effect; on the contrary, they may require correction in practical details; but this I believe, that the present build of ships is decidedly imperfect, and admits of great improvement both as regards security and economy in the use of the material of which they are composed. The cellular system has been objected to on the ground of the inconvenience of longitudinal stringers along the deck on each side of the hatchways, and their liability to oxidation. Now, so far as regards the deck these objections have in reality no weight, for the proposed cellular stringers need not exceed fifteen inches square, or eighteen inches wide by fifteen deep; and these, with the cells which form part of the bulwarks, will afford all that is wanted to give the required stability to that part, forming, if properly put together, perfectly rigid horizontal columns to resist the force of compression on the one hand, and tension on the other. Again, as regards oxidation, none can occur to any injurious extent so long as these cell: stringers are below deck and are riveted water which may be done with perfect safety and witho nution of their strength. From these remarks, at previous statements it will be seen that the excess of material is not required in the vicinity of the neutral axis, where the strain is least, but at the extreme top and bottom, where the strains are most severe when the vessel is pitching in a heavy sea. It is a universal law of construction that the resistance provided for should be proportional to the assailant force in each part, and in order to effect security it should always be greatly in excess. In building a ship, as in other similar structures, the first thing is to ascertain the points of greatest strain, and to provide at those parts the greatest power of resistance; but to build a ship with equal thicknesses of plates throughout, or any other vessel liable to be ruptured by forces that act with double the intensity in some parts that they do in others, is not only a great waste of valuable material, but is absolutely injurious, in so far as it adds by increased weight to the destructive element that tends to break up the vessel. This being the case, how essentially necessary is it that the strengths should be carefully proportioned to the strains, and the material arranged in such a form as to offer a harmonious resistance to the forces thus acting upon it. To effect this distribution, the object of the previous investigation, and keeping in view the same principles I there ventured to advocate, I now come to a larger class of vessels, which involve at the present time considerations of vast importance to the owners and builders and others interested in the extension of commerce. To these vessels I would now venture to apply the same principles, so as, in my opinion, to secure the necessary strength under the varied forms and circumstances to which they are subjected. We do not know what changes are in store for us as a result of the performance of the Great Eastern; that vessel has not yet been fully tried, and it would be premature to anticipate results; as it is, we can only assume that she will prove commercially successful, and although probably not to the extent expected by her more sanguine advocates, yet that she may possess qualities favourable to a considerable increase in the dimensions of our vessels both in relation to their capacity for cargo, speed, and other good properties. If we assume this as the result of the forthcoming performances of the Great Eastern, we may take as the basis of our inquiry a vessel of 500 feet length between the perpendiculars, and 68 feet beam. The question for consideration then is, on what principle should she be built for the purpose of attaining the greatest security with the least material? To answer this inquiry we may consider ; 1st. The general principle of construction. 2nd. The frames and ribs, and their distribution as affecting the transverse strength. 3rd. The plating or sheathing, including stringers, cells, &c., as affecting the longitudinal resistance to fracture. 4th. The decks, bulkheads, and internal fittings. 5th. The bows and stern in their resistance to concussion. 6th. The resisting powers, durability, and economy of the ship taken en masse. In our attempts to apply sound principles in construction, we have two things to determine:-first, the properties of the material we have to deal with, and second, the forms and conditions in which it should be applied. In regard to the former, it is essential to sound construction. that we should have good material, and on this point it will be requisite to offer a few suggestions. To those acquainted with the iron trade, it is well known that we have five or six different sorts of plate and bar iron, na cinder plates, common plates, best plates, double plates, and the superlatively good best-best plat same varieties may be had in bars, and it rec small degree of skill and penetration to determine from appearance what is good and what is bad. One thing is however evident, that no description of plates or angle iron should be employed in shipbuilding that would not stand a test of 20 to 24 tons tensile strain per square inch. That these plates should be made from good puddled bars, piled and rolled at the proper heat, is also essential to durability and security in naval construction; and the additional cost of 20s. per ton should not be an object when compared with the superior quality of the iron employed. In fact, it is a mistaken economy to suppose that a reduced rate per ton in the first cost of an iron ship is an advantage. On the contrary, it involves in reality a serious loss; the inferior material can never be depended upon, and the risk incurred in consequence is too great to lend to its employment any countenance or support. On the other hand, when a better quality of iron is used, less weight is |