The _Pacific_ was interesting in another feature, in that she generated her steam in four tubular boilers, each of which had five furnaces. Briefly the evolution of the boiler had been on this wise: As originally fitted in the _Clermont_ and _Comet_ it was simply a water-tank set in brickwork, and was nearly full of water, with the fire outside, or, to use the expression generally employed, “externally fired.” In those days the pressure of the steam was not greater than the pressure of the air, which we saw to be 15 lb. to the square inch. Then came a modification of this in which the furnace was placed inside the boiler, the advantage being that, with the water all round, the latter could be the more readily heated. This developed into the marine “box” boiler, with internal flat-sided flues and furnaces. This type continued to be fairly universal until about 1845, but the utmost pressure of steam which these were capable of enduring was not above 35 lb. or thereabouts. But tubes instead of the flat flues began to be introduced about the year 1850, owing to the suggestion of the Earl of Dundonald, and these were to be of about double the diameter of those which had been common to locomotives for the previous twenty years. The pressure was soon raised considerably, but there was a strong prejudice against using high pressures at sea, and the idea was not encouraged. [Illustration: THE “VICTORIA” (1852). _From the Model in the Victoria and Albert Museum._] [Illustration: THE “HIMALAYA” (1853). _From the Model in the Victoria and Albert Museum._] [Illustration: COASTING CARGO STEAMER (1855). _From the Model in the Victoria and Albert Museum._]