As steel is much stronger in proportion to its weight than iron, it followed that the adoption of steel for building warships meant a great saving in the weight of the hull. The weight thus gained could be utilised in three ways: by increasing the extent of the armour carried, by increasing the weight of the guns carried, or by a combination of the two. As steel was still further improved it became possible to increase the size of the vessels, the power and weight of the engines and boilers--in which the power increased to a far greater proportion than the weight--the speed of the ships, the strength and extent of the armour carried, and the effectiveness of the guns. It permitted also of a destructive secondary armament. We have seen how from the old broadside ships of the _Northumberland_ type came the central battery ships like the _Hercules_, the last of these being the _Superb_. Their armament also underwent a modernising process as time went on, and many of these old ships, from the _Warrior_ onwards, were equipped with both quick-firing and anti-torpedo-boat guns, and were retained long after their fighting capacity had become a very doubtful quality, and their surrender to the tender mercies of the shipbreaker became imperative. Meanwhile from the converted _Royal Sovereign_ there descended a series of turret ships, some, like the _Cerberus_, _Devastation_ and _Dreadnought_, having two turrets on the centre line of the ship; others, like the _Rupert_ and _Conqueror_, having one turret only; others, like the _Monarch_, having two turrets in the centre, and yet others having their turrets _en echelon_ or placed diagonally, as in the _Inflexible_. The _Colossus_ was an improved _Inflexible_, but of steel, and practically marked the end of the heavily armoured vessels of this type. From the double turrets and the central battery ships we have the combination of the two in the _Temeraire_. The _Colossus_ and the _Edinburgh_, which were begun in 1882 and completed in 1886, may be said to have inaugurated a new era in the building of the world’s battleships. They were the first battleships to be built wholly of steel for the British Navy, and were asserted to be more powerful as fighting ships than any other ships in existence. This was due not only to the material of which they were constructed, but also to the fact that they were given breech-loading guns, Great Britain being the last of the great Powers to dispense with the old-fashioned muzzle-loader. These ships were of much the same type as the _Ajax_ and _Inflexible_, but their citadels were of greater length; they were of fourteen knots speed.