"In 1819, Dolcoath engine performed the best during this year, and at one time reached forty-eight millions.
"In 1820, Treskerby engine, to which Trevithick's high-pressure pole had been adapted, reached 40·3 millions.
"In 1816, Sims also erected an engine at Wheal Chance, to which he applied the pole adopted by Trevithick in his high-pressure engines. This engine attained to forty-five millions.
"In 1828 public attention had now been attracted to the improvements which Captain Grose had introduced into his engine at Wheal Towan. The duty of this engine, in the month of April this year, equalled eighty-seven millions.
"This again gave rise to suspicions of error in the returns. This engine was accordingly subjected to a trial (as Stray Park engine had been in 1815), which was superintended and conducted by many of the principal mine agents, engineers, and pitmen of other mines.
"The quantity of coal consumed in 1835, compared with the quantity that would have been consumed by the same engines in the same time, had they remained unimproved from the year 1814, shows that the saving to the county amounts to 100,000 tons of coal, or 80,000l. sterling per annum."[94]
Lean seems to have calculated on a bushel of coal as 94 lbs. In 1798, when Trevithick was about to give increased pressure of steam to the Cornish engines, his friend Davies Gilbert reported the average duty of the Watt engine in Cornwall to be seventeen millions.
In August, 1811, the reported duty averaged 15·7 millions. This was the month and year in which Trevithick, after twelve years of working evidences of the reasonableness of his promises of increased power and economy from using high-pressure steam, was allowed to erect his cylindrical boilers for the large pumping engines in Dolcoath Mine.
Has the reader realized that the 45-inch atmospheric Carloose engine, of nearly 100 years before,[95] had in 1775[96] become the Bullan Garden engine of Trevithick, sen., which was improved and re-erected by Trevithick, jun., in 1799,[97] when the name was again changed, this time to Shammal, because it was linked to another engine, no other than the Watt 63-inch double engine? This Shammal 45-inch took steam from the globular boiler, using a pole air-pump[98] and a Watt condenser, though retaining the beam with the arched head and chain connection; and again in 1811 took still more highly expansive steam from the cylindrical boilers with a new beam and parallel motion, enabling it in 1814 to beat its rival, the Watt Dolcoath great double engine.[99] The old 63-inch Gons, under the name of Dolcoath Stray Park engine, with Trevithick's improvements, did sixty-seven per cent. more work than the Watt 63-inch with an equal quantity of coal.
This startling fact was disbelieved by the advocates of low-pressure steam, and as the visible change in the Dolcoath engine from Newcomen to Watt, and from Watt to Trevithick, had been gradual and not very striking, and the public were careless of principles, the one most puffed was most thought of; but the money saved was tangible, and in 1815 a special trial was made, which lasted two days, to discover if it was really true that Trevithick's appliances could so increase the duty of the engine. The 63-inch cylinder, then called Stray Park engine, was selected; the result proved that the large saving reported from Trevithick's boilers and expansive working during the last three or four years, was an incontrovertible fact.