And now to the armour itself, which is sometimes affixed to an iron and sometimes to a wooden hull, and in a few cases has wood _outside_ it. These facts, by no means generally known, must be studied, for it can hardly yet be said to be determined which is the better form. It may be said, in general terms, that the “adoption of armour-plating was accompanied in this country by the introduction of iron for the construction of the hulls of ships of war, and our ironclad fleet is for the most part _iron-built_. We have, it is true, a number of wood-built ironclads, but most of these are converted vessels.”(44) Several were built of wood (and then armoured) for the purpose of utilising the large stocks of timber accumulated in the dockyards. In the future it is probable that nearly all will be of iron, with wood backing. The armour of the _Warrior_ is only 4½ inches thick, with, however, a “backing” of 18 inches of timber. This type includes the _Black Prince_, _Achilles_, _Defence_, _Hector_, _Valiant_, and _Prince Albert_. Then we come to another series, of which the _Bellerophon_, _Penelope_, _Invincible_, _Audacious_, _Swiftsure_, _Triumph_, _Iron Duke_, and unfortunate _Vanguard_ furnish examples. They average 6 inches of iron-plating to 10 inches of wood backing. The lost _Captain_ was somewhat heavier in both plating and backing. Then again we advance to a still heavier type—12 inches of iron to 18 inches of wood: the _Glatton_, _Thunderer_, and _Devastation_ furnish examples. Then there is the _wood-built_ class, the thickness of their (wooden) sides ranging from 19½ to as high as 36 inches, with 4½ to 6 inches of armour. The _Royal Sovereign_ (a turret ship) is a leading example of this class; she has 5½ inches of armour, covering 36 inches of wood.