Their average speed was about six and a half knots. Although Admiral Popoff is usually given the credit of the invention of this type of vessel, Mr. John Elder, the Glasgow shipbuilder, designed and patented a circular floating battery in 1867. He proposed that the circular ship should carry twenty-six guns in a lower battery and ten in a central one, and that the sharp edge of the circumference should be used as a ram. According to his design his vessel would have had a diameter of 144 feet, a freeboard of 6 feet, and a draught of 9 feet. The great ironclads described and their armament represent what may be regarded as the apotheosis of the iron turret ship and the heavy iron gun. Before passing on to the great change introduced by the adoption of steel in shipbuilding and gun manufacture it may be as well to note something of what has been accomplished in the production of warships of other types and modifications of types, and how some of them acquitted themselves in actual conflict. One drawback to all the heavy British ironclads of this period was that so much weight and space were taken up by the armour and its backing, that comparatively little space was left for bunker accommodation. Obviously the very heavily armoured ships could not travel for long at high speed under steam without exhausting their coal supply. In order to obtain speed and allow space for the engines of the necessary dimensions, together with adequate coal supplies, the amount of armour carried had to be reduced, and in July in the summer of 1869 the first of a new class of armoured frigates, the _Inconstant_, was launched for the Navy. She was constructed of iron sheathed with three thicknesses of wood and coppered. She was the first vessel which had a stern post and rudder frame made of brass. She carried sixteen guns, viz. ten 9-inch muzzle-loading guns on the main deck, and six 7-inch muzzle-loading rifles on the upper deck; her engines, of 1,000 h.p. nominal, gave her a speed of about sixteen knots, at which she was faster than any other warship in the world. She was unusually narrow for her length, in order to add to her speed, her length being 337 feet 4 inches, and her beam 50 feet 3½ inches. At one time on her trials she made nearly eighteen and a half knots.