860 SHIP tened to the sides within at intervals from the deck to the keelson, which ia also of iron. The beams are also made of iron, shaped like thoSe used in house architecture, and in various other parts this metal is substituted for wood, the advantage being greater strength with less weight and the occupation of less room. Iron frames are now used exclusively in the English navy. Ships constructed wholly of iron are lighter than those of the same tonnage made of wood, and consequently can carry larger freights. Their size moreover being capable of enlargement beyond the dimensions to which wooden vessels mast be limited, they admit more than the latter of profiting by the prin- ciple, that the larger the capacity the less pro- portional part of it need be devoted to the transportation of the fuel required, and the more may be devoted to the cargo. Iron ships are built upon a frame of ribs and longitudinal pieces, upon which the outer plates are secured by bolts and rivets passing through their over- lapping edges. Lloyd's rules for iron ships will be found in " Ship Building in Iron and Steel, " by E. J. Reed, p. 49 1 . In 1 858 a steamer called the Rainbow, of 170 tons and 130 ft. length by 16 ft. beam, intended for the Niger expedition, was built with plates of steel. These were rolled from lumps of crude steel which were exposed four hours in a close furnace to a temperature a little below the melting point; by this process the steel was made to assume a more homogeneous texture and uniform strength. Its advantage over ordinary iron plates is that equal strength to that of the lat- ter is obtained with only half the weight. The boilers of the steamer were also made of it. The recent important changes in ships intended for naval service are : 1, the introduction of light and swift vessels propelled by steam, car- rying a few heavy guns, and able by their light draught to run into rivers and shoal waters ; and 2, that of floating batteries, some account of which has been given in the article IRON- CLAD SHIPS. Since 1858 the French and Eng- lish governments have vied with each other in the construction of fighting ships in which the maximum powers of offence are afforded the utmost security from hostile shot consistent with buoyancy. The contest between offence and defence is in reality coeval with the histo- ry of ship building. In the earliest sea fights protection from the missiles of the enemy was sought by placing shields, interlaced, on what now would be called the "gunwale" of war galleys. During the middle ages the same expedient was resorted to. At the siege of Tunis in 1535 the Santa Anna, one of the fleet of the renowned Andrea Doria, was plated with lead, and successfully resisted the artil- lery of the enemy. The light armaments of the last century often failed to penetrate the stout oak or teak sides of well built ships. Even so late as 70 years ago the ships of Nelson and Colling wood, so long under a concentrated fire as they bore down on the enemy's line at Trafalgar, would have been completely demol- ished had not the powers of attack and defence been so nearly equal. (See IRON-CLAD SHIPS.) Composite ships are designed to combine the advantages of an iron frame or hull with those of a wooden bottom sheathed with cop- per or zinc. As the bottoms of iron sea-going ships get so foul by the adhesion of shell fish and sea weed as to materially reduce the speed, the protection of the iron becomes a very im- portant consideration. The bottoms of wood- en vessels are protected by a sheathing of copper, which by exfoliation sheds or sloughs off such adhesions. The chlorine contained in sea water has a strong affinity for copper, form- ing a green chloride of copper, which is dis- solved by the water, and thus the copper is wasted away. This waste, which constitutes one chief value of copper as a sheathing, can readily be prevented. Chlorine is electro-neg- ative. If the copper sheathing were rendered electro-negative also, the chlorine would be repelled instead of attracted, and the metal would be protected from corrosion. Sir H. Davy proposed to do this by driving zinc nails into the copper. The zinc at once becomes elec- tro-positive, attracts the chlorine to itself, and generates an electrical current which is trans- ferred to the copper ; it thus becomes the gen- erating plate of a battery, while the copper be- comes the conducting plate. But while chlo- rine is repelled, lime and magnesia, electro- positives, are attracted to the copper, forming an earthy coating to which shell fish and sea weed readily and firmly adhere. This expla- nation shows why all the patent applications for the bottoms of iron ships fail to prevent fouling. No artificial coating possessed of the essential property of exfoliation has yet been devised. As any communication, through the medium of salt water, between copper sheath- ing and an iron hull would generate galvanic action highly destructive to the latter, it be- comes necessary to insulate the iron by apply- ing planking to the bottom and then sheathing that ; hence we have what are now common- ly known as composite ships. With copper sheathing the iron must be perfectly insulated ; with zinc this is not necessary, as it decom- poses instead of the iron when they are in galvanic communication with each other. The English frigates Shah and Inconstant are com- posite. They have a double thickness of wood sheathing outside the iron skin, with copper over all, and brass stems and stern posts. A sheathing of three-inch teak is first laid fore and aft and bolted to the shell of the ship ; next comes a layer of planking of the same thickness, but of lighter wood, secured with shifting butts and seams to the first by brass wood screws. The wood sheathing is calked, paid with pitch, and then coppered. The com- posite system will be adopted in the new mon- itors now (1875) in course of construction in this country. Ship Building. Few if any mechanical operations demand such a variety
