Page:Report from the Select Committee on Steam Carriages.pdf/145



Mr. Richard Trevithick. 17 August, 1831. states, that in 1798, he made trial of Boulton and Watt's Engines in that county, and found the average duty performed in the mines was 17,621,000lbs, lifted one foot high with one bushel of coals; and in 1830, when he published his Treatise on the improvements of the Steam Engines in the Cornish mines, he says, that the improvement was so great that a duty of 75,628,000l, lifted one foot high, with the same quantity of coals, was then performing in the mines; that when compared with the duty done in 1795, the improvement exceeded Boulton and Watt's Engines, as 3.865 to 1, or 27 to 7 nearly, and exceeded the standard of the old atmospheric Engines, that were at work in 1778, as 10.75 to 1, (at present some of the best Engines have performed a duty of 90 millions with a bushel of coals,) and the result of this great improvement has been, that not one Engine on Boulton and Watt's plan remains at work in Cornwall; and it is acknowledged by all the Cornish Miners that this improvement solely has been the salvation of their deep and extensive mines, without which the mines could not have continued to work; but from this increase of power and speed, a duty and saving both in fuel and size of four to one, which has caused the saving of coals in the Cornish mines alone to exceed one million sterling, and a constant saving of above one hundred thousand pounds per year. The saving of fuel in theory, by working with high steam, is 75 per cent, every time that the elasticity of the steam is doubled, because double the quantity of coals doubles the pressure, and increases the bulk three-eighths, and by working this steam expansively three-eighths more are gained, and not only theory but practice proves that gain on all the Cornish Engines; the usual height of steam is sixty pounds above the atmosphere, but if the boilers could be made safe against explosion and work with much higher steam, the advantages would almost exceed limit; the accidents that have taken place by explosion, do not appear to be, from overloading the safety valve, but from overheating the boiler, because low pressure boilers have often