Return to Cornwall, 1810 Wheal Prosper pole vacuum engine, 1811 Cylindrical boilers, 1811 Steam pressure, 100 lbs. Duty of engine, 40 millions — Expansive working, 1811 Herland high-pressure pole puffer, 1815 Steam pressure, 150 lbs — Boiler making — Comparison with Watt's engine — Blue — fire-steam — Patent specification Steam-ring stuffing box Engines in Lima A 33-inch pole-puffer more powerful than a 72-inch Watt engine — Description of pole engine and boilers Trevithick's cal culation Trial of Herland engines Steam-cushion Power of the pole engine — Defective workmanship Sims examines the pole-engine Opposition from shareholders Defective boilers Challenge to Woolf Davies Giddy's opinion — First cost, and cost of working one-third of the Watt engine Meeting of opposing shareholders Duty of the high-pres sure steam pole puffer-engine, 1816 Comparison with the Watt engine Combined high-pressure pole and cylinder for expansion — Wheal Alfred.