The hull of the transatlantic steamer is now always of iron, and is divided into a number of "compartments," each of which is water-tight and separated from the adjacent compartments by iron "bulkheads," in which are fitted doors which, when closed, are also water-tight. In some cases these doors close automatically when the water rises in the vessel, thus confining it to the leaking portion. Thus we have already seen a change in transoceanic lines from steamers like the Great Western (1837), 212 feet in length, of 35-1/2 feet beam, and 23 feet depth, driven by engines of 450 horse-power, and requiring 15 days to cross the Atlantic, to steamships over 550 feet long, 55 feet beam, and 55 feet deep, with engines of 10,000 horse-power, crossing the Atlantic in 7 days; iron substituted for wood in construction, the cost of fuel reduced one-half, and the speed raised from 8 to 18 knots and over. In the earlier days of steamships they were given a proportion of length to breadth of from 5 to 6 to 1; in forty years the proportion increased until 11 to 1 was reached. The whole naval establishment of every country has been greatly modified by the recent changes in methods of attack and defense; but the several classes of ships which still form the naval marine are all as dependent upon their steam-machinery as ever. [Illustration: H. B. M. Iron-Clad Captain. H. B. M. Iron-Clad Thunderer. U. S. Iron-Clad Dictator. U. S. Iron-Clad Monitor. H. B. M. Iron-Clad Giatton. French Iron-Clad Dunderberg. FIG. 145.--Modern Iron-Clads.] It is only recently that the attempt seems to have been made to determine a classification of war-vessels and to plan a naval establishment which shall be likely to meet fully the requirements of the immediate future. It has hitherto been customary simply to make each ship a little stronger, faster, or more powerful to resist or to make attack than was the last. The fact that the direction of progress in naval science and architecture is plainly perceivable, and that upon its study may be based a fair estimate of the character and relative distribution of several classes of vessels, seems to have been appreciated by very few. In the year 1870 the writer proposed[99] a classification of vessels other than torpedo-vessels, which has since been also proposed in a somewhat modified form by Mr. J. Scott Russell.[100] The author then remarked that the increase so rapidly occurring in weight of ordnance and of armor, and in speed of war-vessels, would probably soon compel a division of the vessels of every navy into three classes of ships, exclusive of torpedo-vessels, one for general service in time of peace, the others for use only in time of war. [99] _Journal Franklin Institute_, 1870. H. B. M. S. Monarch. [100] London _Engineering_, 1875. "The first class may consist of unarmored vessels of moderate size, fair speed under steam, armed with a few tolerably heavy guns, and carrying full sail-power.