"This was a great saving of steam, so long as the valves remained as at first; but when our men left her to the charge of the person who was to keep her, he began to make, or try to make, improvements, often by giving more steam. The engine did more work while the steam lasted, but the boiler could not keep up the demand. Then complaints came of want of steam, and we had to send a man down to see what was wrong. "This was so expensive, that we resolved to give up the expansion of the steam until we could get men that could work it, as a few tons of coal per year was less expensive than having the work stopped. In some of the mines a few hours' stoppage was a serious matter, as it would cost the proprietor as much as 70l. per hour."[101] [Footnote 101: Smiles' 'Lives of Boulton and Watt,' p. 228.] Pole expresses the same view, intimating that Watt only used steam of 1 or 2 lbs. pressure to the inch. "In Watt's engine, as is well known, the pressure of steam in the boiler very little exceeded the pressure of the atmosphere. He recommended that when the engine was underloaded, this excess should be equal to about 1 inch of mercury; and when full loaded, ought not to exceed 2 inches; adding, 'It is never advisable to work with a strong steam when it can be avoided, as it increases the leakages of the boiler and joints of the steam-case, and answers no good end.'[102] [Footnote 102: Appendix A to Tredgold, 'Pole on Cornish Engines,' p. 49.] "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] [Footnote 103: 'Phil. Mag. and Annals,' N.S., vol. viii., p. 309, by W. J. Henwood.] "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. [Footnote 104: Ibid., p. 313.] [Footnote 105: Ibid., vol. i., p. 127.] "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] [Footnote 106: 'Records of Mining,' p. 163.] "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. [Footnote 107: Alteration in the boilers that year.]