"Mr. Watt's engines with such boilers" (which will not retain steam of more than 3-1/6; lbs. per square inch above the atmosphere) "cannot be made to exert a competent power to drain deep mines, unless the supply of steam to the cylinder is continued until the piston has run through more than half its course.[103]
"In 1801-2 Captain Trevithick erected a high-pressure engine of small size at Marazion, which was worked by steam of at least 30 lbs. on the square inch above atmospheric pressure. In 1804, as Mr. Farey admits,[104] the same gentleman introduced his celebrated and valuable wrought-iron cylindrical boilers,[105] now universally used in this county.
"To these, everyone at all acquainted with the Cornish improvements ascribes a great part of the saving we have obtained. This will farther appear from an extract from a valuable work edited by John Taylor, Esq., F.R.S.[106]
"The monthly consumption of coal in Dolcoath Mine was, in 1811, 6912 bushels; in 1812, 4752 bushels.[107] The alteration in the boilers was the introduction of Captain Trevithick's cylindrical boilers in the place of the common waggon boilers, which had until then been there in use.
"Mr. Woolf, as Mr. Farey states, came to reside in Cornwall about the year 1813, and his 'first engines for pumping water from mines were set up by him in 1814.'"[108]
The foregoing was read at the Philosophical Society in 1831, to refute erroneous statements on the Watt and Trevithick engines. My friend Mr. Henwood had at that time made official experiments in conjunction with Mr. John Rennie on the detail, working, and duty of high-pressure steam Cornish engines, the Watt low-pressure steam principle having been wholly given up. Rees's 'Cyclopædia'[109] also bears the following similar testimony to date of the increased duty:—
"Trevithick's high-pressure engine was erected in Wales in 1804 to ascertain its powers to raise water. The duty was seventeen millions and a half pounds raised one foot high for each bushel of coals.
"The high-pressure steam-engines require a greater quantity of coals, in proportion to the force exerted, than the engine of Mr. Watt, and consequently are not worked with advantage in a situation where coals are dear.
"From the reports of the engines now working in the mines of Cornwall, which, with the exception of a few of Woolf's engines, are all on Mr. Watt's principle, and most of them constructed by Messrs. Boulton and Watt, taking the average of nine engines—bad, good, and indifferent together—they were found in August, 1811, to raise only thirteen millions and a half. But when it was known by the engine keepers that their engines were under examination, they took so much pains to improve the effects, that by gradual increase the engines in 1815 lifted twenty-one millions and a half, taking the average of thirty-three engines. In 1816, Stray Park, a 63-inch cylinder, 7 feet 9 inches stroke, single-acting, being one of the three engines on the vast Dolcoath Mine; its performance in four different months was thirty-one, thirty-one and a quarter, twenty-eight, and twenty-eight and a half millions."
This statement reveals a source of error in estimating the relative values of the Watt and the Trevithick engine; that of the latter was the Welsh locomotive, compared in duty with the large Watt pumping engine, pointed out in Trevithick's letter[110] of that time, as an unfair comparison; the small high-pressure puffer, in 1804, is admitted to have done seventeen and a half millions of duty with a bushel of coal of 84 lbs., while in Rees' calculation of the engines, he gives Watt 94 lbs. of coal to a bushel; and having stated that the Watt pumping engines in Cornwall, in 1811, averaged but thirteen and a half millions of duty, draws the false conclusion that the high-pressure cannot compete with the low-pressure where coals are dear; yet he agrees with other writers that the great increase in the duty of the Cornish pumping engines commenced from 1811 (when Trevithick first gave them his high-pressure steam); and states that in 1816 the Stray Park 63-inch cylinder single-acting engine,[111] being one of three then working in Dolcoath, did thirty-one millions.