In 1846 the _Birkenhead_ was built at Laird’s yard to the order of the Admiralty as the first of a series of steam frigates, and was one of the largest iron steamers belonging to the Government. Her length between perpendiculars was 210 feet, breadth of hull 37½ feet, breadth over paddle-boxes 60½ feet, depth of hold 23 feet, and her tonnage 1,400 tons (carpenter’s measurement). She was followed, in 1849, by the _Simoom_, launched by Napier, at Glasgow, and by the _Megæra_, by Fairbairn, at Millwall, all being classed and armed as steam frigates. The _Birkenhead_ was to carry a 96-pounder pivot gun aft, and a similar weapon forward, and four 68-pounders on the broadside, and it was pointed out that her round stern would add to the range of fire of the “Long Tom” aft. However, the Admiralty had serious doubts of the efficacy of these vessels, and having made some experiments with an iron ship called the _Ruby_, found that the 32-pounder gun at short range could perforate the side of the iron ship, and that the projectile carried its “cloud of langrage” with great velocity into the interior of the ship, so that men could not stand against it. These experiments resulted in the _Simoom_, _Birkenhead_, and ten smaller vessels being condemned as warships. Some of them were transformed into transports, and the Admiralty returned to its beloved wooden walls once more. The loss of the transport _Birkenhead_ shortly afterwards is one of the most tragic and heroic episodes in the whole history of the British army. The Admiralty caused other tests to be made with sixteen wrought iron plates superposed, having a total thickness of 6 inches, but these also were perforated by the projectiles of the 32-pounder at 400 yards range.[33] The result was that the adoption of iron for the main structure of a ship was delayed until the discovery was made of the rolling of armour plates, in the time of the Crimean War, but it was not until 1859 that the Admiralty may be said to have definitely adopted rolled armour plates. In the year 1849 Scott, Sinclair and Co. launched, at Greenock, the first steam frigate built on the Clyde for the British navy, and thus inaugurated that association between the Admiralty and the Clyde iron-ship builders which has been maintained uninterruptedly from that day to this, to the marked advantage of both. The _Greenock_, for the vessel was named after the port, was 213 feet in length of keel and fore-rake, by 37 feet 4 inches beam, and was of 1,413 tons Admiralty measurement, and had engines of 565 h.p. Her machinery compartment measured 72 feet in length and contained the whole of the machinery, consisting of four rectangular boilers, fitted with brass tubes, and two steam engines, lying flat on the bottom, the whole being so arranged that all parts were several feet lower than the surface of the water. The screw, which weighed 7 tons, and was 14 feet in diameter, could be detached and lifted from the water.