along against each side of the vessel, the edge of which is fitted up against the ribs and riveted on to the flat angle iron beams. This continuous plate is made of the ordinary boiler plates, united at the end with a jointing fillet "single riveted" to each, and over it are laid the deck planks, to which they are bolted; it being therefore firmly secured between the beams and planking, cannot fail to aid very materially in resisting any sudden and partial resistance externally, and to maintain the original form.

The upper, or main deck, is planked longitudinally 3 inches thick in the middle, 6 inches near the sides, from which there is a mass of timber forming the "water ways" increasing from 6 inches to about 2 feet in depth against the outside plating, forming a curve surface against the ship's side above and below, to admit of which, the iron beams are bent down at the ends. The planking of the first saloon deck consists also of longitudinally laid planks 6 inches wide, 4 inches thick, with "water ways" 10 inches thick at the sides; and, as it lies on the before-mentioned horizontal plates, the projection is all above the surface of the deck. The planking of the third deck runs across the ship, with 6 × 4 inch water ways, as in that immediately above.

Mr. Grantham thinks that cabins, decks, masts, &c., will ultimately be made of iron. To decks made of iron plates I think the sailor would have a very standing objection, and iron fittings for cabins would not be very consistent with the requirements of refined society. Mr. G. has entitled himself to great credit for having given the subject of iron ship building a most interesting and masterly investigation.

Mr. Fairburn says, that plate iron will bear a strain of 221 tons, in the direction of its fibre, per square inch, and still more across the fibre; while the Franklin Institute make it rather less, as any one would from common experience expect, but this does not materially affect the present inquiry. He also says, that double riveting is stronger than single, in the proportion of 70 to 56; or, the strength of plate being 100 single riveting, will be 56 and double 70 respectively. A plate 2 feet wide x thick will bear a strain in the direction of its length of 337 tons, and at the joints, if single riveted, 189 tons, both vertically and hori

zontally (the plate being nearly of equal strength in both directions), whilst it will require, in a timber-built ship, fir planks of about 4 inches thick, to be of equal sectional strength, and which will be making no allowance or deduction for the butt joints. In this comparison of strength, I have entirely disregarded the frame timbers in timber-built ships, and ribs in those of iron, and shall only consider each as forming the frame work. A horizontal seam of rivets as above, if 100 feet in length, will bear a strain of 9450 tons, whilst the strength of a plank joint, independent of the trenails (which places both under the same condition), is nothing.

This absence of union and strength in the planking joints of timber ships is the primary cause of their weakness, and but for the friction and compression caused by the caulking, in horizontal planking, would be much more so. To this circumstance, chiefly, may be attributed the "hogging" or dropping of the extremities of a ship by the 'unsupported overhanging ends, on its being launched and taking its immersed support on the water, which I have understood has occasionally amounted in ships of war to from 8 to 10 inches.

The diagonal planking of Seppings is admirably calculated to make a much better use of materials than the former plan, for by the planks crossing each other, and forming an infinite number of triangles with the apex downwards, and the upper parts being united by strong tension stringers-probably the strongest wooden fabric possible is obtained. This observation may raise the question (much easier asked than answered), why it is that the diagonal plan was not in use much earlier, since every boy who rides on a five-barred gate knows that without the diagonal his horse's head would drop? The very best combination of wood, or of wood and iron, that can be devised, appears, however, very far below the strength which may be obtained by plates of iron in forming the hull of a ship.

The greater capacity of an iron-built ship will also be a matter of considerable importance; as, from the absence of floor and frame timbers, kelsons, linings, deck-beams, and knees, the stowage will be materially increased over a timber ship, though the external dimensions be

the same, and which, in the Great Britain, I imagine would not be less than 1000 cubic yards.

As the form may easily be maintained by internal framing, bracing, and trussing, what is there to limit the size of ships, except the draft of water in ports, mercantile convenience wharfs, quays, &c.? I do not see any reason connected with the construction, or in an engineering point of view, why the Great Britain may not hereafter be considered an ordinary sized vessel, instead of being "outrageously large" as she is now called,

There may be more advantages obtained by large steam vessels than at present occur to me, but I will mention what appear some of the most prominent. The greater the length of a vessel the greater will be the broad-side resistance against the water, and consequently less "lee way" will ensue, and less will be the disturbance by waves from a straight line, and consequently, the velocity will be less detracted by the water impinging against the rudder. Adverse storms and heavy seas will be of less consequence, and such as would founder moderate size ships may be regarded in a large one as insignificant. The displacement, and therefore capacity of the vessel increasing as the cube, whilst the section of resistance increases as the square of dimensions, it follows that a less proportion of power to tonnage will be sufficient. Large engines and large fires will work more advantageously than small ones. The friction of parts generally will increase as their diameters, whilst the areas under pressure of useful effect will increase as the square of the diameters, and twisting or torsion as the cube of the diameters. Radiation of heat also from the surface of boilers, steam-pipes and cylinders will follow the same law.

What the durability of iron plates in a sea vessel may be, does not appear to be very clearly established. Mr. Mallet says, that a half-inch plate will last 100 years exposed to sea water, but it must be considered that the inside of the bottom of a ship will be exposed to the bilge-water, and therefore the period will be reduced to 50 years. If in 50 years a half-inch plate be half worn away on the outside by corrosion, and the other half be reduced to an oxide on the inside, it is perfectly clear that it could not form part of a ship, even 25 years, when the

ths,

The first iron steamer, as appears by Mr. Grantham, was the Aaron Manby, constructed by the Horsley Company in 1821, and I am pleased to see that she is still in use, from the circumstance of having been present at Horsley when the parts were shipped into the canal boat. My recollection of the thickness of the plates is imperfect, but I have an impression that they were not more than if even so much, and that all the joints were flush. Mr. Grantham thinks that "plates may probably be yet required of much greater strength than ths of an inch thick." Surely no peaceful maritime occupation can require it, and as good citizens of the commonwealth we ought no longer to indulge in forming plans to defeat cannon balls, for which only, or "hammering on rocks" can such a thickness be required. If larger vessels than the Great Britain are built, and more strength be considered necessary, the most advantageous disposition of it would be in ribs and bones and not in the skin.

The Machinery.

The boiler, as will be seen by the accompanying sections, presents a great quantity of surface to the action of the fire and heated air, and appears amply strong for condensing engines.

In wooden-built steamers there is an absolute necessity to guard against the possibility of the timbers taking fire; and for this reason the boilers have been constructed with water spread all over the surface of the bottoms below the fires; but as the same necessity does not exist in iron vessels, it occurs to me that the entire quantity of boiler plates and water below the line of the fire-bars, but for the deposition of salt, is useless and unnecessary. It is not to be supposed that any portion of the current of flame will dip down at the back of the bridge and impart heat to the sides, even much less will it touch the bottom, and then creep

up against the back before it escapes to the upper flue; the cause of the air being drawn into the fire, being the inferior specific gravity of that contained in the chimney, caused by its expansion by heat; and as the drawing power of the chimney extends to the fire, the centre of the current of heat will take the shortest course (except, that some trifling deviation will take place in the curves of the currents at the bends of the flues), and all flue space to which the current does not extend, will be occupied by the heavy carbonaceous gases in a quiescent state, possessing a very small power of transmitting heat from the current to the

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flue plates. I believe in almost all cases of boilers with large flues, by far the greatest quantity of heat is imparted to the water by the roof of the fire-place, and flues, and my present impression is, that were all that mass of water, amounting to probably not less than 2,500 cubic feet, or about 70 tons weight, and nearly 40 tons of boiler work removed, and the bottoms of the flues made good in another way, the heating power and effect of the boilers would be still nearly equal.

If this view of the question be correct, there is then also a sacrifice of space under the boiler of about 20 feet long, by the width of the ship, and at least 6 feet