The same defect characterised our steam sloops and gun vessels which were distributed in all parts of the world. They were constructed to keep the sea under sail, relying only upon steam when the wind failed. The result was that as sea boats they were admirable, but steaming against a moderate sea they were easily passed by the 8-knot steam collier plodding steadily on for the Suez Canal. But still in a tentative sort of way we recognised the necessity for swift vessels, and while providing those above enumerated, built two vessels, between 1870 and 1880, with a steam speed of 18 knots. These were the 'Iris' and 'Mercury,' remarkable also for being the first vessels built wholly of steel. The hull was not cased in wood and coppered, but simply coated with a composition to prevent fouling and preserve the steel from the action of sea water. Having twin screws, only a light rig was provided, and the armament consisted of ten 64-pounders. The 'Mercury,' attaining on trial a speed of 18½ knots, was the fastest of the two. These ships were called despatch vessels, and for such service are admirably suited. Speed had been made the first consideration, and beyond subdivision of the hull no attempt was made to afford protection, which involved carrying additional weight. The success of the 'Iris' and 'Mercury' led to the construction of others, in which some speed was sacrificed to give a steel deck over the machinery and vital parts, to exclude fragments of shell which might explode within the ship. The displacement was the same, 3750 tons, as also the length, 300 ft., but the armament was made heavier, and hence they
