Owing to the many advantages attending the construction of vessels made entirely of wrought iron, excepting the decks, a decided preference is now given to them, and they are fast becoming almost universal. Amongst the numerous advantages may be mentioned,-

1. An iron vessel does not weigh half so much as a wooden one of similar dimensions; consequently draws much less water.

2. An iron vessel affords much more room for stowage, owing to the decreased thickness of its sides; those of the latter not exceeding four inches, including the angle iron ribs to which the plates arc fastened; while those of a wooden vessel of the same dimensions are not less than twelve inches thick. Thus a wooden vessel, of 24 feet beam outside, would be only 22 feet inside; whilst an iron one would be 23 feet 4 in.; making a difference of 16 inches all round; which difference, in a vessel measuring 150 feet long, 24 feet beam, and 16 feet deep, affords room for the stowage of 80 tons more of cargo than the wooden vessel.

3. Owing to the diminished draught, or the necessity of displacing a less weight of water, an engine of equal power to that on board a wooden one will propel it much faster, or one of less power equally fast. In the latter case, there is effected a proportionate saving of fuel; in the former, a saving of both time and fuel.

4. Iron vessels are much cooler, owing to the metal being a rapid conductor of heat.

5. The air in the holds of iron vessels is much purer, not being contaminated with foul bilge-water, as is the case with wooden ones.

6. They are less liable to leak; and the leak, when made, more easily stopped, the fracture or hole being generally much smaller, and capable, usually, of being beaten back into its former position.

7. Iron vessels do not, like wood, become heavier by saturation.

8. Iron vessels are not subject to be affected by lightning, that fluid being conducted into the water by the metal.

9. Owing to the greater tenacity of the metal, iron vessels may be constructed of greater dimensionsthan wood.

The first iron steam vessel built for sea was the Alburkah, constructed by Mr. Laird to take part in the expedition to the Niger, conducted by Mr. Laird. Her dimensions were, - length, 70 feet; breadth, 13 feet; depth, 6 feet 6 in. The plates in her bottom were only 1/4 inch thick, and those in the sides only an eighth. When launched, she drew but 9 inches of water; with her engines, boiler, and various fittings, about 3 feet 4 inches. Her weight, including the decks, was 15 tons. The engine, of 15 horse power, and boiler, added about 15 tons more.

The foregoing exhibits an example of one of the smallest iron vessels, and the following affords a magnificent specimen of the largest iron vessels hitherto constructed.

The Great Britain, having a tonnage of 3143, by the old measurement, was built at Bristol by the same Company which brought out the Great Western, the largest wooden steam vessel previously produced. The Great Britain was chiefly constructed under the direction and management of Mr. Thomas Guppy, with whom was connected Mr. Isamberd Brunei, as consulting engineer. The length of the keel of this noble vessel is 289 feet; total length, 322 feet. Her beam or breadth is 51 feet; depth, 32 feet 6 inches. Her draught of water when loaded, 16 feet. Displacement, 2984 tons. The ribs are of angle iron, 6 inches by 31/2, 1/2 and 7/16 thick. Distance of ribs from centre to centre amidships, 14 inches, increasing to 21 inches at the ends.

Ten iron sleepers run from the engine-room, (gradually diminishing in number to the fore end of the ship,) and under the boilers, the platform of which they support. In midships they are 3 feet 3 inches in depth, supported by angle irons in the form of inverted arches, a short distance from each other.

She has five water-tight partitions; stows 1200 tons of coal; has a space for 1000 tons of measurement goods. The engines weigh 340 tons, the boilers 200 tons, and they hold 200 tons of water.

The main shaft is 28 inches in diameter in the centre, and 24 inches in the bearings; in the rough, before it was turned, it weighed 16 tons: it was lightened by a hole of 10 inches diameter, bored throughout. A stream of cold water is caused to flow through this hole and the cranks while the engines are at work.

The screw shaft consists of one long, and two short, or coupling parts. The part next the engine is solid, 28 feet long, by 16 inches in diameter. The hollow intermediate shaft is 65 feet long, and 32 inches in diameter. The screw part is 25 feet 6 inches long, and 16 inches in diameter. The total length of this shaft is 130 feet, and weighs 38 tons. The screw has six arms, 15 feet 6 inches in diameter, and weighs 4 tons.

The main drum is 18 feet diameter, and drives 4 chains, weighing 7 tons The screw shaft drum is 6 feet diameter, and the weight with the pull when working is equal to 85 tons on the bearings of the main shaft.

The steam cylinders are four in number, 88 inches in diameter, and with a stroke of 6 feet. The condensers are of wrought iron, 12 feet by 8, and 5 feet deep. Under the whole space occupied by the engines up to the top, the angle irons are doubled.

The upper main and saloon decks are of wood, the two cargo decks are of iron. The officers and seamen are all accommodated on two decks under the forecastle. She has six masts fitted with iron rigging. Five of the masts are hinged for lowering during contrary gales. She is provided with seven life-boats, capable of carrying 400 people. She is built with lapped joints in preference to flush, the former having been proved, to the satisfaction of the Company, to be one-fifth stronger than the latter mode.

In each of the five compartments, the engine pumps, by means of pipes and cocks, can be applied. The water-tight divisions of each compartment add greatly to the strength of the ship, either as struts or ties.