So long ago as the year 1825, the paddle was recognised as a source of danger for warships, and in that year a two-blade propeller, designed by Commander Samuel Brown, was accepted. In 1836, Ericsson (subsequently to be of _Monitor_ fame) patented some propellers in England, but as he met with very little sympathy from the authorities, he retired to America. The main objections to the propeller appears not to have been due to any lack of appreciation so much as opposition from those who had invested heavily in paddle-propulsion plant. [Illustration: _SALAMANDER_ PADDLE WARSHIP.] In 1842, however, the Admiralty seriously took the question up. The _Rattler_, of 777 tons, and 200-horse-power, was lashed stern-to-stern with the paddle-yacht _Electro_ of the same displacement and horse-power. Both ships were driven away from each other at full speed, and the _Rattler_ succeeded in towing the _Electro_ after her. After this, in 1844, a screw frigate, the _Dauntless_, was ordered to be constructed; but as late as the year 1850, steam was merely regarded as an auxiliary, and received little or no consideration outside that. The use of iron instead of oak as a material for shipbuilding was first seriously considered about the year 1800. In 1821, an iron steamer was in existence, and in 1839 the _Dover_ was ordered to be built for Government service as a steam packet. In 1841, the _Mohawk_ was ordered by the Admiralty for service on Lake Huron, but the first iron warship for the Royal Navy proper was the _Trident_, of 1850 tons and 300 horse-power, built at Blackwall, by Admiralty orders, in 1843. Iron, as a material for warship construction, was looked on with considerable suspicion, both in England and in France. Experiments were conducted at Woolwich with some plates rivetted together like the sides of an iron ship, these plates being lined inside with cork and india-rubber (the first idea of a cofferdam). It was expected that this preparation, which was known as “kamptulicon,” would close up after shot had passed through and prevent ingress of water. This was found to be quite correct, but the egress of shot on the other side had quite the opposite result. The plates were sometimes packed with wood and sometimes cased with it, but the general result of the experiments was held prejudicial to the use of iron, which was supposed to splinter unduly compared to wood. The importance of deciding whether warships should be built of iron or wood was accentuated by the necessity of replacing those heavy warships which had been converted to auxiliary steam vessels. All such proved to be cramped in stowage and bad sea boats. So long ago as 1822 shell-guns had been adopted. Consequently, in the experiments as regards iron, shell-fire had to be taken into consideration.