"Your obedient servant, "RICHD. TREVITHICK.
A 4-horse-power portable high-pressure steam-puffer engine cost 105_l._, with internal fire-tube and machinery attached to the boiler, ready for work in half an hour after lighting the fire, consumed 21 lbs. of coal and 11 gallons of water for each hour's work, at a cost of threepence.
The reader's attention has been very imperfectly drawn to the numerous subjects touched on in these remnants of Trevithick's correspondence between the years 1804 and 1816; among them may be traced the portable high-pressure steam-engine, the tubular cylindrical boiler of wrought iron, the economy of expansive working with steam of 100 lbs. on the inch, but limiting it to 20 lbs. when not in the charge of experienced workmen, and testing boilers by water pressure to four times the intended working pressure.
The economy of heat in smelting furnaces and in the aërated steam-engine were bold means to large results. The cheap 100l. steam-engine of 1812, with open-top cylinder and rigid simplicity of gear, resembling Newcomen's first atmospheric engine, was really a high-pressure steam expansive engine, with the germ of surface condensation, as ready to convey itself from mine to mine or from farm to farm, and to join in performing labourer's work, even to boring and conveying rock by land or sea, as the most perfect of modern engines; and yet this unadorned engine, as seen in the agricultural engine of the following chapter, followed the excellent mechanism of the double-acting Kensington model of 1798, and the still more beautiful engine of the 1802 patent and London locomotive.
The late Mrs. Trevithick said "that during the difficulties in London in 1808 and 1810, when Trevithick was overwhelming himself with new experiments and the cost of patents, and law expenses, lawyers and bailiffs took everything worth having from her house, including account-books, drawings, papers, and models, which she never saw again."
His earlier account-books left in safety in his Cornish home, though very disconnected, give trustworthy traces of his work up to 1803. From that time only detached accounts or papers are found until 1812, when the unused pages in two old mine account-books of his father served as his letter (rough-draft) books; and judging from their number and style, his correspondence was most extensive and varied.