The American Civil War then gave a great opportunity for practical experiments in torpedo attack; but the difficulty of wholly submerged navigation not having been yet solved, the boats used were not true submarines, but submersibles. Their propulsion was by steam, and their dimensions small. A more ambitious invention was put forward in 1869 by a German, Otto Vogel, whose design was accepted by the Prussian Government. His submersible steamship was to be heavily armed, and was ‘considered the equal of a first-class iron-clad in defensive and offensive powers.’ These powers, however, never came into operation. Inventors now returned to the designing of true submarines; and after the Frenchman, Constantin, the American, Halstead, and the Russian, Drzewiecki, had all made the best use they could of the hand-winch or the pedal for propulsion, three very interesting attempts were made in 1877-8 to secure a more satisfactory engine. Olivier’s boat, patented in May 1877-8, was to be propelled by the gases generated from the ignition of high explosives, the massed vapours escaping through a tube at the stern. This ingenious method was, however, too dangerous for practical use. Surman’s design of 1878 included a propeller, rotated by compressed air. But the English boat of the same date, Garrett’s _Resurgam_, was much the most noteworthy of the three, and introduced a method which may in the future be brought to perfection with great results. In this boat, the motive force was steam, and propulsion under water, as well as on the surface, was aimed at and actually attained. In her trials, the vessel showed herself capable of navigating under water for a distance of 12 miles, by getting up a full head of steam in a very powerful boiler, with the aid of a blower, before diving; then by shutting the fire-door and chimney, and utilising the latent heat as long as it would last. When the heat was exhausted, it was, of course, necessary to return to the surface, slow up the fire again and recharge the boiler with water. The vessel was remarkably successful, and had the great merit of showing no track whatever when moving under water. She was lost by an accident, but not until she had impressed Nordenfelt, the Swedish inventor, so strongly that he secured the services of her designer, Garrett, for the building of his own submarine boats. The first of these appeared in 1881.