In 1844 the _Britannia_, on arriving at Boston in February of that year in a particularly severe winter, became ice-bound. When the day came for her departure for Liverpool, the Bostonians showed their appreciation of the line and of the regularity of communication it maintained with England by cutting at their own expense a channel seven miles long and a hundred feet wide through the ice to liberate her, her sailing being only two days behind time. In 1847, even with the two extra ships, the company was unable to cope with the demands made upon it, and the commerce between the two countries had increased to such an extent that the Government felt bound when the time came for the renewal of the contract to require that the service should be doubled. It was stipulated that the company should provide a vessel of not less than 400 horse-power nominal and capable of carrying guns of the largest calibre. Its steamers were to leave Liverpool, calling at Holyhead if required, every Saturday for New York and Boston alternately, the Boston steamer touching at Halifax, and the New York one to do so also if required by the Admiralty. For these augmented sailings the subsidy was raised to £173,340 per annum, at which figure it remained to the end of 1867. This change necessitated the building of four new ships, namely, the _Niagara_, _Canada_, _America_, and _Europa_. They were 251 feet long between perpendiculars, 35 feet beam, 26 feet 3 inches moulded depth, and of 1825 tons gross register, and had engines of 2000 indicated horse-power, which gave them an average speed of 10¹⁄₄ knots. In 1850 the _Asia_ and _Africa_ were added to the fleet; they were sister ships, 266 feet between perpendiculars, 40 feet beam, 27 feet 2 inches depth, and of 2226 gross tonnage, and had engines of 2400 indicated horse-power, with an average speed of 12¹⁄₂ knots. In 1852 the _Arabia_ was built, 285 feet between perpendiculars, 8 inches more beam, with a depth of 29 feet, and a gross tonnage of 2402. Her engines developed 3250 horse-power and gave her an average speed of 13 knots. [Illustration: THE “BRITANNIA” (CUNARD, 1840).] The building of the _Arabia_ marks the close of the first period in the history of the Cunard Line for, in 1855, the company began to build iron ships. She was intended to be a reply to the steamers of the Collins Line. For some reason or other the Americans made very few attempts to enter upon the transatlantic steam-ship trade until nearly the middle of the nineteenth century. Probably they were satisfied with the performances of their sailing clippers, as they had good reason to be, for the clippers often made faster passages than the early Cunarders. From 1838 to 1847 every Atlantic liner flew the British flag, but in 1845 the United States Congress passed an Act authorising contracts to be made with owners of American vessels, steamships preferred, for the regular transportation of the United States mails. As an American writer says: