[Footnote 36: Captain Artha became the resident engineer at the Real del Monte mines in Mexico; Captain Samuel Grose, one of the first Cornish mine engineers; and Mr. Burral, the engineer of a department at the engine-works of Messrs. Harvey and Co.] The writer has had the pleasure of personal acquaintance with each of those three gentlemen, who as young engineers commenced their labours in the erection of Trevithick's engines. No sooner had Trevithick perfected the pole condensing engine and then the pole puffer-engine, than he, in conjunction with Sims, who had just taken part in the erection of one of his high-pressure steam pole-engines for working the pumps at Beeralstone Mine, combined the pole with the ordinary Watt vacuum engines, supplying them with steam from his high-pressure boilers, in other words, converting them from their original form of low-pressure vacuum engines to high-pressure expansive compound steam-engines. The old 60-inch cylinder Boulton and Watt engine, at Wheal Chance (one of Watt's favourite engines), was in 1818 transformed into a high-pressure engine, with Trevithick's pole placed between the centre of the main beam and the steam-cylinder. The high-pressure steam from Trevithick's new boilers was turned under the pole for the up-stroke, after which it was expanded in the old and much larger cylinder on the top of the piston causing the down-stroke; it then, by its passage through the equilibrium valve, allowed the piston in the large cylinder to make its up-stroke, by equalizing the pressure of steam on its top and bottom, while a fresh supply of strong steam from the boiler admitted under the pole gave power to the up-stroke; and finally, the comparatively low-pressure steam under the large piston passed to the condenser and air-pump to form a vacuum for the down-stroke, as in the Watt engine. Sims, the engineer at Wheal Chance, one of the mines in the eastern or Watt district, was converted and became in 1815 or 1816 a partner with Trevithick, and erected, at Treskerby Mine, Trevithick's high-pressure pole of 36 inches in diameter, as an addition to the old Watt engine working with a cylinder 58 inches in diameter. Watt, then, within a year or two of his death, was too old to any longer take part in the contest; his engine in the hands of others was converted and became a high-pressure expansive engine. Trevithick, as a further proof that he could do without the Watt patent air-pump bucket, with its piston and valves, removed it from a Watt engine at Wheal Alfred Mine in 1812, replacing it by one of his poles, answering the same purpose, but different in construction. Many other mines used them; one remained at work in Old Wheal Damsel in 1860. They have also been used in steamboat air-pumps.