How great an advantage she was upon their first ship will be seen by the following comparison:

Britannia.Persia.
Coal necessary to steam to
New York		570 tons1,400 tons
Cargo carried224 „ 750 „
Passengers 90 250
Indicated power7103,600
Pressure per square inch 9 lbs. 33 lbs.
Coal per indicated horse-power per hour 5.1 „ 3.8 „
Speed 8.5 knots 13.1 knots

Thus, for two and a half times the quantity of coal nearly three and a half times the cargo was carried, and nearly three times the number of passengers. This result was due partially to increased engine efficiency, and partially to increased size of ship; and thus to a relative reduction of the power necessary to drive a given amount of displacement.

The Scotia was almost a sister ship to the Persia, slightly exceeding her in size, but with no radical differences which would mark her as an advance upon the latter. She was the last of the old régime in the Atlantic trade, and the same year in which she was built saw the complete acceptance by the Cunard company of the newer order of things, in the building of the iron screw steamer China, of 4,000 tons displacement, with oscillating geared screw engines of 2,200 indicated horse-power, with an average speed of 12.9 knots on a daily expenditure of 82 tons of coal. She was the first of their ships to be fitted with a surface condenser. The Scotia had been built as a paddle steamer rather in deference to the prejudices of passengers than in conformity to the judgment of the company, which had put afloat iron screw ships for their Mediterranean trade as early as 1852 and 1853.

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]

Longitudinal Section of the Warship Duilio.

[Larger image] (157 kB)

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.