Before this, Messrs. Ditchburn and Mare built at Blackwall, in 1842, and Messrs. J. and G. Rennie engined, a small iron steamer of 164 tons builders’ measurement, and 98 tons displacement, called the _Mermaid_. She was a screw steamer, and was fitted with George Rennie’s conoidal propeller; this was a three-bladed screw propeller with the blades arranged to resemble a cone with its widest part at the boss of the propeller shaft and tapering towards the tips of the blades. The engine, having two vertical cylinders of 40 inches diameter each, and 32 inches stroke, and with a pressure of 8 lb. in the boiler, indicated 216 h.p. A spur gearing transmitted the power to the screw shaft, giving it 153 revolutions per minute and driving the vessel at its trial, in May, 1843, at a little above 10½ knots, or over 12 miles; as the Admiralty had promised to take over the vessel if she attained a speed of 12 miles, she was accordingly purchased and under the name of H.M.S. _Dwarf_ has the honour of being the first iron screw steamer the British Government possessed. The _Dwarf_ was largely used afterwards for experimenting with various kinds of screw propellers. With the exception of the bombardment of Acre, in 1840, there was a long interval during which the world’s navies were not called upon for any serious engagements, and the development of warship building which took place during that period was the result rather of scientific research than of actual fighting experience, and there was consequently no need, while the nations were recovering from the wars of the eighteenth and early nineteenth centuries, for any remarkable advances to be made. For the first fifty years of the nineteenth century the warships were much as they were in Nelson’s time, except that some of them were fitted with mechanical means of propulsion.