"Query 12th.—A governor will be required; perhaps as good a place as any for it, out of the way, will be on the cast iron that carries the beam; you may turn the fly-wheel whichever way you please. If this engine is worked with steam of 25 lbs. to the inch above the atmosphere, and the steam shut off at one-twentieth part of the ascending stroke of the piston, the power will be as three is to two of Boulton and Watt's single engines.
"Only two pairs of stones for the present, but calculate those stones to stand in such a way that another pair may be placed, on a future day, if wanted. I have not seen Mr. Richards lately. I wish you to write a form of an order, in your next, such as you wish, and I will get him to write to you accordingly. Put the engine and drum for Lord de Dunstanville out of hand neat and well, as it will be well paid for; and make the stands, &c., in your own way.
"Rd. Trevithick."
Mr. Richards' flour-mill engine may claim to be the first practical smoke-burner: keeping the fire much thinner at the inner end of the grate-bars than at the fire-door end of the grate, allowed of the freer passage of air through the thinner layer of coal, near the fire-bridge, causing the combustion of the passing gas. This idea has, since the date of Trevithick's letter, led to several smoke-burning patents. The boiler fire-tube was oval, 2 feet 9 inches by 1 foot 11 inches. The open-topped cylinder was supplied with a heavy and deep piston serving as a counterweight, and also as a guide in the cylinder for correcting the angle of the connecting rod. Experience had taught him that the cold sides of the condenser were sufficient to work an engine a great many strokes without a supply of injection; and he had already used high-pressure steam of 25 lbs. to the inch above the atmosphere, cut off from the cylinder when the piston had performed one quarter of its course: thus both these things were as first steps leading to the modern expansive steam-engine and surface condensation.
The simplicity of the engine is remarkable—a high-pressure, expansive, condensing engine, worked by a single four-way cock, without cylinder-cover, or parallel motion.
The low first cost, and non-liability to derangement, were always kept in view; and his confirmed experience in the satisfactory working of horizontal cylinders prior to 1812 illustrates their extended application; for at that time scarcely any other engineer had constructed other than upright cylinder engines. No detail escaped his observant gaze. The fire-bars were to be 2 inches deep, 1 inch thick at the top edge, tapered to ¾ths of an inch at the bottom, giving the required strength, with free room for air, which in its passage cooled the bar, carrying the heat into the fire. Years before and after that period the fire-bar in common use by thoughtless people was a square iron bar that was always burning and bending.
The letter is descriptive of the high-pressure steam-engine in the sixteenth year of its age; and its expansive steam, made practical by Trevithick's high-pressure boilers. This engine only took steam during the first quarter of its stroke, the remaining three-quarters were by the expansion. Had it taken steam only during one-twentieth of its stroke, it would have been more powerful than Boulton and Watt's low-pressure steam vacuum engine of the same size.
[Rough draft.]
"Sir,