Page:Popular Science Monthly Volume 12.djvu/552

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three feet of water. It attained a speed of ten miles an hour, and towed an American packet-ship, the Toronto, four and a half miles an hour on the Thames. This was a splendid success.

Ericsson built several screw-boats, and finally, meeting Captain Robert F. Stockton, of the United States Navy, that gentleman was so fully convinced of the merits of Ericsson's plans that he ordered an iron vessel of seventy feet length and ten feet beam, with engines of fifty-horse power.

The trial of the Stockton, in 1839, was eminently satisfactory. The vessel was sent to America under sail, and the designer was soon induced to follow her to this country, where his later achievements are well known.

The engines of the Stockton were direct-acting, the first examples of engines coupled directly to the crank-shaft without intermediate gearing, that we meet with after that of John Stevens.

99. Soon after Ericsson arrived in the United States, he obtained an opportunity to design a screw-steamer for the United States Navy, the Princeton, and, at about the same time, the English and French Governments had screw-steamers built from his plans, or from those of his agent in England, the Count de Posen.

In these ships—the Amphion and the Pomone—the first horizontal, direct-acting engines ever built were used. They were fitted with double-acting air-pumps, having canvas valves and other novel features.

From 1840 the screw gained favor rapidly, and finally began to displace the paddle for deep-water navigation. Progress in this direction was at first somewhat slow.

In 1840, and during the following ten years, many experiments were instituted between the performance of screw and paddle steamers without definitely settling engineering practice.

100. The reason was, probably, that the introduction of the rapidly-revolving screw, in place of the slow-moving paddle-wheel, necessitated a complete revolution in the design of their steam-engines. And the unavoidable change from the heavy, long-stroked, low-speed engines, previously in use, to the light engines, with small cylinders and high piston-speed, called for by the new system of propulsion, was one that necessarily occurred slowly, and was accompanied by its share of those engineering blunders and accidents that invariably take place during such periods of transition.

Engineers had first to learn to design such engines as should be reliable under the then novel conditions of screw-propulsion, and their experience could only be gained through the occurrence of many mishaps and costly failures. The best proportions of engines and screws for a given ship were determined only by long experience, although great assistance was derived from the extensive series of experiments made on the French steamer Pelican. It also became