352 .TUBULAR BOILER, SUPERHEATING STEAM, 



Where water condensation was preferred the surface- 

 air condenser could be converted into a surface-water 

 condenser by a current of cold water in place of the 

 air ; in which case the air from the blowing cylinder 

 was taken direct in to the fire-place or other means 

 used for giving the necessary draught. Stearn of about 

 135 Ibs. to the inch was to be so expansively worked as 

 at the finish of the stroke, on its escape to the con- 

 denser, to be no more than atmospheric pressure, or 

 15 Ibs. to the inch just the strength with which Watt 

 preferred to commence his work in the cylinder. 



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 1 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-w'ater in the patent engine of 

 1802. 2 In 1811 he proposed to force air into the fire- 

 place, hoping thereby to reduce the amount of heat lost 

 by the chimney. 3 His various forms of tubular boilers, 

 as at the Herlaiid Mine, 4 and at Dolcoath, 5 and the 

 upright multitubular boilers patented in 1815, 6 followed 

 tip in 1828. "I shall have a small portable engine 



1 Sec vol. i., p. 104. 4 See vol. ii., p. 71. 



2 See patent specification, vol. i., p. ! 5 See chap. xx. 



128. 6 Sec Trevithick's letters, 8th July, 



3 S-ce Trevithick's letter, 13th Jan., 1815, vol. ii., p. 80 ; and 7th and 16th 

 1811, vol. ii., p. 6. May, vol. i., pp. 3(54, 370. 



