=359. Improvement of the means of steam navigation.=—The early steamers were moved by paddle-wheels, which offer special advantages for use in shallow water, but which are not so efficient as the screw propeller in the open sea. They require heavier and bulkier engines which must be placed in the best part of the ship, they waste power, and they show the effects of wear and tear more quickly. The _Great Britain_, which made its first voyage in 1845, was noteworthy on two accounts: it was the first large steamer (over 3,000 tons) to be built of iron, and it was the first to introduce the screw in ocean navigation. These two improvements were adopted by the Inman line (1850) and were gradually accepted by other builders. In the second half of the century various improvements have added still more to the efficiency of the ocean steamer. Early steamers ran under such a low steam pressure that we find recorded in the log-book of one, “Broke the larboard steam-pipe, lapped it with canvas and rope-yarn and proceeded”! Higher pressures were introduced, and after about 1870 the steam was more fully utilized by compound engines, of which some have three or even four sets of cylinders. The introduction of twin screws, first applied to the _City of New York_ (1889), has added rather to the safety than the speed of a passage, by permitting further development of the system of water-tight compartments. =360. Gains resulting from increase in size.=—Another most important factor in the development of efficient steamers has been mere growth in size. A ship’s carrying power varies as the cube of her dimensions, while the resistance offered by the water increases only a little faster than the square of her dimensions. Large ships, therefore, consume less coal per ton of cargo, and as large boilers and engines consume coal more efficiently than small ones, there is a double gain. Here again, as in the case of railroads, the introduction of cheap steel has been of immense importance, and may fairly be said to have revolutionized the art of ship-building since 1875. While in 1880 nine tenths of British steamers were still constructed of iron, the proportion had sunk in 1890 to less than one twentieth, and the employment of steel is now almost universal. From steel are constructed the great cargo-carriers and the fast express steamers of the modern oceanic service. Some conception of the progress that has been made can be got by a comparison with earlier conditions. In 1841 the total steam tonnage of the British Empire was 188,000; nowadays a single steamer (_Leviathan_, _Majestic_) has a tonnage in excess of 50,000. The horse-power of British steamers in 1841 was estimated at 75,000; nowadays a single steamer, has an indicated horse-power almost equal to that total. The boilers of a modern express steamer (_Teutonic_) were required to evaporate 120 _tons_ of water every _hour_, yet so thoroughly is the heat utilized that it was said of a steamer some