"The engine you refer to is still occasionally used here; when first erected there was a large quantity of corn thrashed by it, but of late years it has not been much used except for chaffing, bruising, &c. "I remain, dear Sir, "Yours truly, "WM. TRETHNOY. "F. TREVITHICK, Esq." Trevithick's Trewithen engine, which sixty years ago was more manageable than horses going momentarily faster or slower at the will of a common labourer,[22] remains in use unchanged. [Footnote 22: See vol. ii., p. 38.] His preparations for South America, and application of high steam in the large Cornish pumping engines, interfered with the perfecting the smaller agricultural work. CHAPTER XIX. POLE STEAM-ENGINE. When in the autumn of 1810 Trevithick returned to Cornwall, the experience of ten busy years had established the practicability and usefulness of the high-pressure engine. The principles of the invention were now to be applied on engines of the largest size. In 1811, the late Captain S. Grose, a young pupil of Trevithick's, was employed to erect at Wheal Prosper Mine, in Gwythian, the first high-pressure steam pole condensing engine. It was placed immediately over the shaft and pump-rods, requiring no engine-beam. The air-pump, feed-pump, and plug-rod were worked from the balance-bob. The pole was 16 inches in diameter, with a stroke of 8 feet. The boilers were two wrought-iron tubes, 3 feet in diameter and 40 feet long. The fire was external. Shortly after Captain H. A. Artha erected several of those pole-engines for Trevithick. The drawing shows the simplicity of parts of this highly expansive steam-engine, beginning the up-stroke with steam of 100 lbs. to the inch above the atmospheric pressure, expanding it during the stroke down to a pressure of 10 lbs., and then condensing to form a vacuum for the down-stroke. It cost 750 guineas. The drawings of this expansive pole condensing engine are from the dimensions given by Captain Grose who erected it, and by Captain Artha who knew it well. [Illustration: WHEAL PROSPER HIGH-PRESSURE EXPANSIVE STEAM-CONDENSING POLE-ENGINE, 1811. _a_, cast-iron pole, 16 inches diameter, 8-feet stroke; _b_, pole-case, a small bit larger in internal diameter than the pole; _c_, cross-head, fixed on top of pole; _d_, guides for cross-head; _e_, side rods connecting the two cross-heads[**typo beads corrected]; _f_, bottom cross-head; _g_, pump-rod; _h_, balance-beam, with box for weights; _i_, connecting rods from balance-beam to bottom cross-head; _k_, guides for air-pump cross-head; _l_, cross-head and side rods for working air-pump; _m_, air-pump, condenser, and water-cistern; _n_, feed-pump worked from air-pump cross-head; _o_, plug-rod worked from balance-beam; _p_, exhaust-valve; _q_, steam-valve; _r_, exhaust-pipe; _s_, steam-pipe; _t_, bracket for carrying working gear; _u_, expansive steam-horn and tappets; _v_, handles for working valves.]