The most prominent feature in Trevithick's numerous modifications of the steam-engine was the boiler. In the 'Life of Watt,' though his commentators have been numerous and eminent, little or nothing is said about the boiler or the steam pressure. He left that all-important part of the steam-engine just as he found it, resisting the increase of steam pressure, which was the mainspring of Trevithick's engine. The boiler of the high-pressure engines of 1796[176] sheltered the steam-cylinder from cold; and the used steam from the cylinder circulated around the exterior of the boiler, on its way to the blast-pipe, while the condensed portion was returned as feed-water in the patent engine of 1802.[177] In 1811 he proposed to force air into the fire-place, hoping thereby to reduce the amount of heat lost by the chimney.[178] His various forms of tubular boilers, as at the Herland Mine,[179] and at Dolcoath,[180] and the upright multitubular boilers patented in 1815.[181] followed up in 1828. "I shall have a small portable engine finished here next week, and will try to heat steam independent of water, in small tubes of iron, on its passage from the boiler to the cylinder, and also try cold sides for condensing." In 1829 a simple boiler and condenser composed of three tubes was made, the inner or fire-tube being 2 feet in diameter and 15 feet long, "for the express purpose of experimenting on the working the same steam and water over and over again;"[182] and on the same subject, "By making the condenser of 4-inch copper tubes 1/32nd of an inch thick, it would stand in one-twentieth part of the space of the boiler:"[183] and finally the sketch of the tubular boiler and tubular condenser of 1830, in its boiler portion similar to the best portable boilers of the present day, and the patent specification of 1831. Surely therefore to him belongs the credit of having invented and perfected the tubular boiler and surface condenser. [Footnote 176: See vol. i., p. 104.] [Footnote 177: See patent specification, vol. i., p. 128.] [Footnote 178: See Trevithick's letter, 13th Jan., 1811, vol. ii., p. 6.] [Footnote 179: See vol. ii., p. 71.] [Footnote 180: See chap. xx.] [Footnote 181: See Trevithick's letters, 8th July, 1815, vol. ii., p. 80; and 7th and 16th May, vol. i., pp. 364, 370.] [Footnote 182: See vol. ii., p. 332.] [Footnote 183: See vol. ii., p. 336.] Smiles has written:[184]-- [Footnote 184: See 'Life of George Stephenson,' by Smiles, p. 279; published 1857.]