Since the foregoing was written, the following has been received:--
"DEAR SIR, "TREWITHEN, PROBUS, _May 17th, 1872_.
"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.
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.
His preparations for South America, and application of high steam in the large Cornish pumping engines, interfered with the perfecting the smaller agricultural work.
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.]