The material employed for shipbuilding was almost entirely wood. This was displaced in the 'sixties and 'seventies by iron, which in turn was displaced by steel, so that at the present time, except for special reason, no material other than steel is thought of for this purpose. With the gradual displacement of wood by iron in the mercantile marine, Ericsson's relation was only indirect. Some of the earlier mercantile vessels in which he was interested were of wood and some of iron. In the field of warship construction, however, his influence through the "Monitor" was more direct, especially as to the value of metal armor as a protection against great gun-fire. Still, it is no more than justice to say that with the change from wood to iron which took place during the active part of his life, Ericsson had only an indirect relation, and the change would doubtless have come about at the same time, and in much the same general way as it did, independent of any influence which his work may have had upon the question. Turning to the means of propulsion, we find sails as the main, or almost only, reliance during the early years of the century. The steam-engine operating paddle-wheels had come to be recognized as a possibility, and under certain conditions as a commercial success. The screw-propeller as a means of propulsion was known only as a freak idea, and was without status or recognition as a commercial or practical means for propelling ships. So far as the screw-propeller was thought of as a means of propulsion, it lay under a suspicion of loss of efficiency due to the oblique nature of its action, and this was supposed to be such as to render it necessarily and essentially less efficient than the paddle-wheel. Ericsson lived to see the use of sails almost entirely discarded for war purposes, and for mercantile purposes relegated to ships for special service and of continually decreasing importance. He lived to see the steam-engine take its place as the only means for supplying the power required to propel warships, and attain a position of almost equal relative importance in the mercantile marine. He lived to see the paddle-wheel grow in importance and estimation as a means of propulsion only in turn to be supplanted by the screw-propeller, which gradually increased in engineering favor from the days of its obscure infancy until it became the only means employed for the propulsion of ships navigating the high seas, while it had become a most serious rival to the paddle-wheel even for the purposes of interior and shallow-water navigation,--long a field considered as peculiarly suited to the paddle-wheel and to the engines adapted to its operation.