At the period of those high-pressure pole-engine experiments, Trevithick had devoted twenty years of constant labour to the improvement and extended use of the steam-engine, causing it to assume every variety of form except that of the Watt patent engine, an approach to which was unusual, as evidenced in the high-pressure steam Kensington model of 1796, without beam, parallel motion, air-pump, or condenser, having no one portion either in principle or detail similar to the Watt engine, being portable and not requiring condensing water, with single and double cylinders, placed vertically or horizontally. Having during twelve busy years constructed over a hundred high-pressure steam-engines, scarcely any two of which were exactly alike, he departed if possible still further from the Watt type, and went back apparently, though not in reality, to the Newcomen engine, simplifying it by the omission of the great bob, and use of condensing water, as in the nautical labourer and steamboat engine of about 1810,[43] and the South American mine engines of 1816,[44] which had open-top cylinders, more like a Newcomen than a Watt, but if possible even more simple and primitive-looking than the former. Again, compare the thrashing engine of 1812[45] with the Newcomen of 1712:[46] the great and all-important difference being that one was a high-pressure steam-engine, the other a low-pressure atmospheric engine. Then came the varieties of high-pressure steam pole-engines, working very expansively either as puffers or condensers, retaining the same dissimilarity to the Watt engine: and lastly, the combination of the high-pressure pole with the Watt patent engine, thereby causing the old Watt engine to do more than double the work it had done when new from the hands of the maker, and also to perform this increase of work with a decrease in the consumption of coal. [Footnote 43: See vol. i., p. 336.] [Footnote 44: See chap. xxi.] [Footnote 45: Vol. ii., p. 37.] [Footnote 46: Vol. i., p. 5.] The following chapter will trace the adaptation of high-pressure expansive steam, from cylindrical boilers, to the form of pumping engine still in general use. CHAPTER XX. THE WATT AND THE TREVITHICK ENGINES AT DOLCOATH. Having up to 1816 traced the progress of the steam-engine in Cornwall through a century, during the latter half of which Trevithick, sen., and his son were among its most prominent improvers, the latter having devoted a quarter of a century to the work, the effect of which is shown in the skeleton outlines of a few classes of engines, one important feature still remains for examination before a correct judgment can be formed of the events of this period and their prime movers. The use of an increasing pressure of steam gave increased force and value to the improved steam-engine, but the power of constructing engines and boilers to render the increased pressure manageable was the result of a lifetime of labour.