Throughout his entire career the improvement of the steam-engine occupied a large share of Ericsson's attention, and in particular was this the case in connection with his naval designs. From the "Princeton," in 1841, to the "Destroyer," in 1878, there succeeded one long series of types and forms of steam-engine, each in his opinion the best adapted to the circumstances of the case. Naturally, opinions differ, and he was brought into competition with other able engineers, and his designs were often called into question or subjected to criticism. In 1863, in competition with Chief Engineer Isherwood of the navy, engines were designed for twin ships, the "Madawaska," afterward known as the "Tennessee," and the "Wampanoag," afterward called the "Florida." This was a battle royal of types and modes of application of the power of the steam-engine to the propulsion of ships. The result was a victory for Isherwood, although the "Madawaska," which was first subjected to trial, made a speed higher than any warship at that time afloat. This was exceeded by the "Wampanoag" a short time later; but neither engine was of an enduring type, and after a time the machinery of the "Madawaska" was removed, and she was repowered with a later type of machinery, and long did service as the "Tennessee" in the list of wooden frigates of the navy. The "Florida" was too expensive to maintain in commission, and the special circumstances which had called her into existence having passed by, she was laid up at New London, and never again saw active service. Keenly as Ericsson was interested in the steam-engine, it must be admitted that he always showed a more profound interest in some form of engine which should be able to displace it with a superior efficiency; and hence his long series of efforts relating to the flame-engine, the caloric engine, the gas-engine, and finally the solar engine,--with either steam or heated air as the medium for carrying the heat. During the last years of his life some of his most patient and careful study was given to the perfection of a solar engine, or engine for utilizing directly the heat of the sun instead of that of coal or other carbon compounds. Besides this direct line of study and experimentation, he gave during these years much thought to various scientific problems connected with solar energy, the tides, gravitation, the nature of heat, etc., etc. A plan for deriving power direct from the tides, improvements in high-speed engines for electric-lighting purposes, further improvements in his hot-air engine in small sizes for commercial purposes,--these are some of the further lines of work which occupied the attention of his closing years.