"It is known as a fact that every engine of magnitude consumes 3000l. worth of coal every year.
"The fire-place has been diminished and enlarged again. The flame has been carried round from the bottom of the boiler in a spiral direction, and conveyed through the body of the water in a tube (one, two, or three) before its arrival at the chimney.
"Some have used a double boiler, so that fire might act on every possible point of contact, and some have built a moorstone boiler, heated by three tubes of flame passing through it.
"A judicious engineer does not attempt to load his engine with a column of water heavier than 7 lbs. on each square inch of the piston."[49]
While Pryce's book was being printed, Watt in 1777 wrote of the Cornish steam-engines:—
"I have seen five of Bonze's engines, but was far from seeing the wonders promised. They were 60, 63, and 70 inch cylinders at Dolcoath and Wheal Chance. They are said to use each about 130 bushels of coals in the twenty-four hours, and to make about six or seven strokes per minute, the stroke being under 6 feet each. They are burdened to 6, 6½, and 7 lbs. per inch."[50]
The 63-inch was an open-top cylinder atmospheric engine at Dolcoath Mine under the management of Trevithick, sen.; and shortly after, in 1777 or 1778, Watt's first engine was erected in Cornwall.[51]
In 1783 Trevithick, sen., gave Watt an order for a patent engine for Dolcoath, in size similar to the old Newcomen atmospheric, having a cylinder 63 inches in diameter, that a working trial might be made between the rival engines. The Watt engine having a cylinder-cover, with the patent air-pump and condenser, was known in the county as the Dolcoath great 63-inch double-acting engine. Three steam-engines were then at work in that mine: Trevithick senior's Carloose (then called Bullan Garden) atmospheric 45-inch cylinder, the atmospheric 63-inch cylinder, and Watt's 63-inch cylinder double-acting vacuum engine; all of which continued in operation side by side for five years until 1788, when for a time Dolcoath ceased to be an active mine. Trevithick, jun., was then a boy of seventeen years.
After ten years of idleness and rust, as if mourning the death of Trevithick, sen., in 1798 Richard Trevithick, jun., as engineer, and Andrew Vivian as manager, induced shareholders to resuscitate the old mine. Fire was again given to the voracious jaws of the boilers, and the three engines recommenced their labours and their rivalries.