The British ironclad Pallas, completed in 1866, was remarkable for having the first successful naval engines on the compound principle, in which the steam is admitted at high pressure to a small cylinder, and passes thence to a larger one which it fills by its expansion. To Great Britain the world owes also the development of triple expansion, i. e., the use of steam successively in three cylinders. This system was inaugurated in naval engines by the British, in 1885–86, and is now universally employed. Prior to 1879, the boilers of all modern war-vessels had been those of the Scotch type, in which the flame passes through tubes fixed in a cylindrical shell containing water. In that year, however, France began a revolution in the steam generators of navies by equipping a dispatch-vessel with the Belleville tubulous boiler, in which the water to be evaporated is contained within tubes surrounded by flame confined in an outer casing. The water-tube principle, also, bids fair to become of universal application. It has had its most noteworthy naval installation in the British cruisers Powerful and Terrible, of 14,200 tons and 25,886 horse-power, completed in 1895. [Illustration: PLATE IV. ENGINE OF U. S. S. ERICSSON.] The use of more than one screw for propulsion dates back to 1853. During our Civil War multiple screws figured, to a small extent, in the “tin clads” and larger monitors. The application of twin screws, in the modern era, begins with the British ironclad Penelope of 1868. France, in the years 1884–85, blazed the way for another naval advance of much importance in conducting a series of trials with the launch Carpe, equipped with triple screws. The system, however, although of much value, from engineering and tactical points of view, was not adopted in large, high-powered vessels until the advent of the French armored cruiser Dupuy de Lôme in 1890, and the protected cruisers Columbia and Minneapolis of the United States navy in 1893. It has now won full approval in the navies of continental Europe, and triple-screw ships, aggregating 500,000 tons, are built or building there.