"The testimony of John Bryant, that the duty with the waggon boiler was say fourteen millions, and that of Mr. Lean, giving twenty-one millions with the new Trevithick boiler, bear much the same proportion as the charges for coals in the respective periods above given.
"In the year 1816 a new 76-inch single engine was erected in the place of the old Boulton and Watt 63-inch double engine with Trevithick's cylindrical boilers. The average duty performed during the year 1817 was 43¾ millions. This same engine is still at work, and her regular duty is from thirty-six to thirty-eight millions.
"I am, Sir,
"Your most obedient servant,
"Charles Thomas.
"Francis Trevithick, Esq."
Captain Charles Thomas, who was one of the most experienced of Cornish miners, for many years the manager of Dolcoath, and in youth the acquaintance of Trevithick, states that the new high-pressure boilers were made in the mine in 1811, and gave their first supplies of strong steam to the three large pumping engines in April, 1812, with such good effect that the increasing water which had threatened to drown the mine was speedily removed, and that with a saving of nearly one-half of the coal before consumed. Prior to their use Dolcoath Mine paid 1000l. monthly for coal; but for the latter nine months of the year, in consequence of the new boilers, the cost was reduced to 612l. a month. This saving in the pumping cost of one mine crowned with success the high-pressure steam engineer, who had been steadily gaining ground during his fight of twelve or fourteen years on the battle-ground chosen by Watt thirty-three years before.
The low price of tin and copper, which caused so many engines to cease working about the close of the last century, had changed for the better, and the present century opened with an increasing demand for steam power. Trevithick's high-pressure portable engines had worked satisfactorily for several years; and as a means of making public the relative duty performed by Cornish pumping engines, and of solving conflicting statements on the rival systems of low and high pressure steam, it was determined that an intelligent person should examine and give printed monthly reports of the amount of duty done by the different engines, and in 1810 Captain Andrew Vivian was requested to take this work of engine reporter in hand; on his refusal it was offered to Trevithick. In August, 1811, Mr. Lean commenced such monthly reports, showing that the duty of twelve pumping engines at the end of that year averaged seventeen millions, exactly the duty done by the Boulton and Watt engines thirteen years before, as reported by Davies Gilbert and Captain Jenkin in 1798, proving the small inherent vitality of the Watt engine.
In 1814 the Dolcoath pumping engines, with Trevithick's cylindrical boiler and high steam expansion, are thus reported:—"The Boulton and Watt, Dolcoath great double engine, 63-inch cylinder, did a duty of 21½ millions; the Shammal 45-inch cylinder, single engine, did 26¾ millions; and the 63-inch single, Stray Park engine, 32 millions." Shammal engine, nearly 100 years old, beat the Watt engine of more than half a century later; and so did Stray Park 63-inch, which Watt had laughed at when he first tried his hand as an engineer in Cornwall in 1777.[82]
The marked change in these three engines, while for two or three years under Trevithick's guidance, becoming more powerful and economical, raised the usual swarm of detractors, and in 1815 a special trial was made, which lasted for two days, to test the reported increased duty by the cylindrical boilers and expansive working.
The unbelievers were then convinced, and agreed to throw out the Boulton and Watt great double engine 63-inch cylinder, together with its neighbour, the worn-out old 45-inch, and put in their stead one engine with a cylinder of 76 inches in diameter, with expansive valve and gear, and parts strong enough and suitable to the high-pressure steam, on Trevithick's promise that it should do more than the combined work of the other two with one-half the coal.
In 1816 this new engine commenced work, and did forty millions of duty, increasing it during the next two or three years to forty-eight millions, being three times the duty performed by the Watt 63-inch double engine before it was supplied with steam from Trevithick's boilers, and twice as much as it performed when so supplied. Lean says, "This was the first instance of such duty having been performed by an engine of that simple construction." The other mines followed Trevithick's advice, but never paid him a penny. On this Lean again says, "The engines at work in the county in 1835 would have consumed 80,000l. worth of coal over and above their actual consumption yearly, but for the improvements that had been made since 1814."