The introduction of surface condensation and of higher pressures were the two necessary elements in a radical advance in marine engineering. Neither of these was a new proposal;[3] several patents had been taken out for the former at a very early date, both in America and in England; and in 1838 the Wilberforce, a boat running between London and Hull, was so fitted. Very high pressures, from almost the very beginning, had been carried in the steamers on our Western waters; and in 1811 Oliver Evans published, in Philadelphia, a pamphlet dealing with the subject, in which he advocated pressures of at least 100 to 120 pounds per square inch, and patented a boiler which was the parent of the long, cylindrical type which came into such general use in our river navigation. The sea-going public resolutely resisted the change to high pressures for nearly forty years, there being a very slow and gradual advance from 1 and 2 pounds to the 8 and 9 carried by the Great Britain and Britannia. In 1850 the Arctic carried 18 and in 1856 25 was not uncommon. Some of the foremost early engineers favored cast-iron boilers (see evidence before parliamentary committee, 1817); and the boiler in general use in England up to 1850 was a great rectangular box, usually with three furnaces and flues, all the faces of which were planes.[4]
[Illustration: Longitudinal Section of the Warship Duilio.]
Though tubular boilers did not displace the flue boiler in British practice to any great degree before 1850, many examples were in use in America at that date, but chiefly in other than sea-going steamers. Robert L. Stevens, of Hoboken, built as early as 1832 “the now standard form of return tubular boilers for moderate pressures” (Professor R. H. Thurston). But it worked its way into sea practice very slowly; and the multitubular boiler, in any of its several forms, cannot be said to have been fairly adopted in either American or British sea-going ships before the date first mentioned, though employed in the Hudson River and Long Island Sound steamers, in one of the former of which, the Thomas Powell, built in 1850, a steam pressure of 50 pounds was used.
There had been this slow and gradual advance in ocean steam pressures, with a consequent reduction in coal expenditure, when in 1856 came a movement in the direction of economy by the introduction of the compound engine, by Messrs. Randolph Elder & Co. (later John Elder & Co.), which was soon to develop into a revolution in marine steam enginery. The Pacific Steam Navigation Company has the credit of first accepting this change in applying it to their ships, the Valparaiso and Inca. The original pressure used was 25 pounds to the inch: the cylinders were 50 and 90 inches in diameter, and the piston speed from 230 to 250 feet per minute. The idea of using steam expansively by this means was of course not new, as it dates back to Hornblower (1781), but with the low pressures which had been used at sea there was no reason for its adoption afloat. Difficulties were experienced by the Pacific company with their earlier engines, but the line adhered to their change, and for nearly fourteen years were almost alone in their practice.