"In 1799 Henry Clark worked as a rivet boy in Dolcoath, and carried rivets to construct Captain Trevithick's new boiler, said to be the first of the kind ever made. It looked like a great globe about 20 feet in diameter, the bottom hollowed up like the bottom of a bottle; under this the fire was placed: a copper tube attached to this bottom went around the inside of the boiler, and then passed out through the side of the boiler, the outside brick flues then carrying the heat around the outside of the boiler and into the chimney.
"Captain Trevithick's first plunger-pole lifts in Dolcoath were put in at this time and worked by this engine. Glanville, the mine carpenter, was head man over the engines when Captain Trevithick was away."[55]
"Charles Swaine worked as a rivet boy in making Captain Trevithick's cylindrical wrought-iron boilers for the Dolcoath engine. Several of Captain Trevithick's high-pressure boilers were working in the mines before that, but not made exactly like the Dolcoath engine boilers. When I was a boy about the year 1804, several years before I worked on the Dolcoath engine boilers, I carried father's dinner to the Dolcoath smiths' shop, where he worked, and used to stop and watch the wood beam going up and down of Captain Dick's first high-pressure steam-whim. She was not a puffer, but a puffer-whim worked near by, called the Valley puffer. At that time most of Captain Dick's high-pressure boilers were smallish, cast iron outside, and wrought-iron tube."[56]
In 1799, shortly after the reopening of Dolcoath Mine, Trevithick, jun., selected his father's second-hand atmospheric engine of 1775,[57] to further improve it by a new boiler of uniformly globular figure, with concave circular bottom, under which fire was placed; it was of wrought iron, 24 feet in diameter, surrounded by external brick flues; a large copper tube, starting from the boiler bottom, immediately over the fire, served as an internal flue, carrying the fire by a sweep around the interior in the water space, and then out through the side of the boiler into the external brick flue. It may be said that there was nothing new in a circular form of boiler, or in an internal tube; but it will be admitted that this repaired engine, in this its third stride in the march of advancement, made publicly known those principles which in a few years more than doubled the power, the economy, and the applicability of the steam-engine. His patent drawing of 1802 shows this form of boiler applied to a small portable engine, in which, for the sake of simplicity of structure and cheapness, cast iron was used instead of wrought iron, and the internal tube omitted.[58]
The full detail estimate, from which the following items are extracted, of the cost of alteration was written by Trevithick, jun., in the book and on the page adjoining that containing the account of the former alteration and re-erection of the same engine by Trevithick, sen., in 1775.
"A 45-inch cylinder engine, working 20 lbs. to the inch:—
| £ | s. | d. | |
| Boilers, 8 tons at 42l. | 336 | 0 | 0 |
| Iron about ditto, 6 cwt. at 42l. | 12 | 12 | 0 |
| Castings about ditto, 15 cwt. at 42s. | 18 | 0 | 0 |
| Safety-valve and cocks | 1 | 0 | 0 |
| Wood about bob, 200 ft. at 6s. | 60 | 0 | 0 |
| Cast iron about ditto, 45 cwt. at 25s. | 56 | 5 | 0 |
| Brass about ditto, 60 lbs. at 2s. | 6 | 0 | 0 |
| Piston-rod, 4 in., 14 ft. long, 550 lbs. at 1s. | 27 | 10 | 0 |
| T-piece, 10 cwt. at 25s. | 12 | 0 | 0 |
| Cover, and bottom, and piston, 35 cwt. at 32s. | 56 | 0 | 0 |
| Nozzles, 6 cwt. at 32s. | 9 | 12 | 0 |
| Steam and perpendicular pipe, 10 cwt. at 25s. | 12 | 10 | 0 |
| Receiver, 2 ft. 4 in. long, and bottom, 15 cwt. at 25s. | 18 | 15 | 0 |
| Air-pump, bottom, and case, 10 cwt. at 25s. | 12 | 10 | 0 |
| Plunger, 22 in., 6 ft. long, 12 cwt. at 40s. | 24 | 0 | 0 |
| Force lift | 5 | 0 | 0 |
| Engineer | 66 | 0 | 0" |
The term "single" refers to its open-top cylinder as originally erected by Newcomen, when it was called the Carloose engine, and so it remained after its re-erection in 1775, under the name Dolcoath new engine, alias Bullan Garden; but after the last re-erection in 1799 it had a cylinder-cover, and was called the Shammal 45-inch engine; "working 20 lbs. to the inch" meant the force on each inch of the piston, including vacuum on the one side of 14 lbs. and steam on the other side of 6 lbs. to the inch.