Although the tendency at first in building iron and steel vessels, especially for the navy, was towards an entire metal structure, later experience resulted in a more composite style, using wood in some parts, where found best adapted by its capacity of lightness, non-absorption of heat and less electrical conductivity, etc., and at the same time protecting such interior portions by an iron shell or frame-work. One great improvement in ship-building, whether in wood or metal, thought of and practised to some extent in former times, but after all a child of this century, is the building of the hull and hold in compartments, water-tight, and sometimes fire-proof, so that in case of a leakage or a fire in one or more compartments, the fire or water may be confined there and the extension of the danger to the entire ship prevented. In the matter of _Marine Propulsion_, when the steam engine was made a practical and useful servant by Watt, and men began to think of driving boats and ships with it, the problem was how to adapt it to use with propelling means already known. Paddle-wheels and other wheels to move boats in place of oars had been suggested, and to some extent used from time to time, since the days of the Romans; and they were among the first devices used in steam vessels. Their whirl may still be heard on many waters. Learned men saw no reason why the screw of Archimedes should not be used for the same purpose, and the idea was occasionally advocated by French and English philosophers from at least 1680, by Franklin and Watt less than a century later, and finally, in 1794, Lyttleton of England obtained a patent for his “aquatic propeller,” consisting of threads formed on a cylinder and revolving in a frame at the head, stern, or side of a vessel. Other means had been also suggested prior to 1800, and by the same set of philosophers, and experimentally used by practical builders, such as steam-pumps for receiving the water forward, or amidships, and forcing it out astern, thus creating a propulsive movement. The latter part of the eighteenth century teemed with these suggestions and experiments, but it remained for the nineteenth to see their embodiment and adaptation to successful commercial use. The earliest, most successful demonstrations of screw propellers and paddle wheels in steam vessels in the century were the construction and use of a boat with twin screws by Col. John Stevens of Hoboken, N. J., in 1804 and the paddle-wheel steamboat trial of Fulton on the Hudson in 1807. But it was left to John Ericsson, that great Swedish inventor, going to England in 1826 with his brain full of ideas as to steam and solar engines, to first perfect the screw-propeller. He there patented in 1836 his celebrated propeller, consisting of several blades or segments of a screw, and based on such correct principles of twist that they were at once adopted and applied to steam vessels.