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410 on her first voyage struck a rock on the Irish coast, and went down instantaneously in deep water with every soul on board. Again, the possibility of making them sufficiently strong was demonstrated in the case of the Great Britain, which lay for four months on the rocks at Dimdrum Bay, exposed to the whole thrash of the Atlantic, and was got off a serviceable vessel at last.

Wooden vessels are constructed of a framing of timbers, which in war-ships are put together solid, and covered with a skin of planking inside and out. It was long before economy of cost would permit the idea that an inner framing was essential to give form and strength to an outer iron skin, imitating thus the ancient coracle formed of a leathern skin on a wicker basket, not without form, but with a very bad form, and void. Precisely of such structure is the Esquimaux canoe, built, like our Thames’ wherries, in the best form for skimming the water.

Even when it had been decided that ribs were essential to iron skins, it was still considered necessary to retain the old practice of making the deck beams of wood, and the decks of fir planks caulked, decks which in dry weather required to be treated like the sides and bottom — kept wet to prevent them from leaking. The phrase “wash- ing decks” meant, in truth, wetting decks to keep them from chinking open. The cleanliness grew incidentally from the necessity of the after “swabbing,” and, gradually, fastidious masters’ mates took to spending their own money in lemons to whiten them, till the phrase “as clean as a ship’s decks” became a sea proverb.

At length a move was made in the Great Eastern. A ribbed framework of iron was covered with an iron outer skin and an iron inner skin, and the whole was divided into water-tight compartments, fore and aft and athwartships, so that if one or more compartments filled, the others would float her. The practice of water- tight compartments obtained early in iron vessels. The Nemesis steamer, sent out to India by the East India Company, after beating the Admiralty in a long paper warfare — Steam versus Wind — was built of iron, in compartments; and, prac- tically, she decided the first Chinese war in our favour, though, one day, a rock or a Chinese shot — random, of course — struck her exactly at the partition of the two hinder compartments, and she would have sunk, but that the forward partition kept her floating. Our Scotch friends in the Clyde built some peculiar vessels, very flat-bottomed, with the intention of getting direct from Glasgow to Liverpool over the river shallows; these were divided by water-tight compartments, but they had great alacrity in going to leeward, not being provided with lee boards like our coasters and the Dutch craft. One of them took to the rocks on the Isle of Man, lying in their lee- way, and great exultation was exhibited by the rival owners of wooden craft at the failure of the new-fangled device. But in truth it was only a proof in favour of the scheme; for the canny owners of the iron craft, finding she would stow more cargo with a clear run in the hold, took out the water-tight compartments, and so ensured her sinking.

In addition to her double sides and bottom, the Great Eastern has another improvement. For the first time the decks also are of iron, and therefore for the first time an incombustible ship is attained. Shylock’s phrase, “ships are but boards,” no longer holds good. It is true that the compartments may be filled with combustibles and set on fire, or they may be blown up with gunpowder, but any one compartment may be drowned without risk, to put out a fire; and it is not an impossible thing so to stow the gunpowder that the minimum of damage may be inflicted by explosion, and the vessel remain a seaworthy craft notwithstanding.

The Admiralty built a number of iron steamers, imitating, as is always done, the experience of the merchant service. It then occurred to try their shot-proof capacity. Cast-iron shot splintered in going through their sides, and some of the projectiles drove in two square yards of plating at a blow, precisely as the Irish rock stove in the sides of the Tayleur. The plates broke away entire along the lines of rivets.

This leads us to the question of riveting — a very imperfect process. In round numbers, the strength of a vessel at the rivets is only two-thirds that of the solid portion, even when the riveting is carefully and honestly performed. But a large portion of the riveting is simply hand-labour, and the labour of many hands. The operation of punching holes does not always bring them opposite to each other, and a second operation, called “drifting,” takes place, skewing the rivet from one hole to another. In some cases the bad workman who has caused the defect covers up his bad workmanship with a leaden rivet. The most careful supervision cannot always guard against this.

Let us begin at the beginning, and find out what we want step by step. First, the ship should be such as cannot be sunk by collision. The bottom should be of the strongest possible structure. The fire-box of a locomotive engine furnishes an example of this. Two skins, some four inches apart, are connected by stay-bolts four inches from each other, and the pressure of steam, twenty tons to the square foot, between them, has no power to rend them asunder. If the pressure, instead of being internal were external, and the hollow space were filled up with solid matter between, the strength of resistance would be increased many fold.

Again, if a surface of iron plates, riveted together and extended on an open frame, be struck with a shot, a large mass will be driven in. But if the whole volume of the plates be lined with a solid mass of timber, of sufficient thickness, the shot will simply punch a hole in the iron plates.

If, therefore, we construct the bottom of the vessel, and part of the rising sides, of two skins, say four feet apart, with parallel surfaces, stayed with strong stays four feet apart, and then fill the inclosed space with hot bitumen mingled with blocks of wood or with stone ballast, the bitumen when cold would be a tough substance,