"That engine at Worcester shuts off the steam at the first third of the stroke, and works very uniformly. I cannot tell what coal it burns yet, but I believe it is a very small quantity. I shall know in a short time what advantage will be gained by working expansive. I expect it will be very considerable. There are a great many engines making and ordered. Boulton and Watt and several others are doing everything to destroy their credit, but it is impossible to destroy it now that it is so well known. I have not taken any of the ground at Bristol to remove. I called on them and told them it was possible to break the ground without men, and they wish me to take a piece to clear out, but would not set but a small piece at a time; therefore it would be disclosing the business to no purpose. They were very desirous to know the plan, but I would not satisfy them, neither will I unless they pay me for it in some way or other. If you direct for me at the Dale it will find me. I am happy to find that you have a seat in the House. I wish every seat was filled with such.
"I remain, Sir,
"Your very humble servant,
"Richard. Trevithick.
Trevithick fully understood the value of the expansive principle in 1804: when working with steam of 45 lbs. to the inch, the engine went at a speed of twelve strokes a minute. On cutting off the steam at half-stroke, the speed and consequent work done fell to ten and a half strokes a minute; in other words, the work performed by the engine fell off only one-eighth part, while the quantity of steam and consequently of coal was reduced by one-half. The principle was established, but the application was practically incomplete from the want of heavier fly-wheels, to give out their momentum during the latter half of the stroke, when the expanding steam was lessening its force.
"The saving of coal would be very great by working expansively, but as coal is not an object here," Mr. Homfray was careless about the expansion. Thirty-three years after this indirect check to steam-engine economy, the writer, then living in the Sirhowey Iron Works, and within stone's-throw of Mr. Homfray's Works, recommended the removal of the Boulton and Watt's waggon boilers, to make room for Trevithick's boilers, on the plea of saving one-half the fuel, and at the same time increasing the power of the engine, and thereby the pressure of the blast in the iron furnaces. The proprietor was careless about the saving of coal, and was doubtful that an increased blast would increase the quantity of iron smelted. The promise that the wages of one-half of the number of boiler firemen would be saved, was understood. Trevithick's high-pressure boilers replaced the Watt low-pressure, resulting in a largely-increased quantity of iron from the greater power and pressure of blast in the furnaces, and at one-half the expenditure of coal in the boilers: ten men had been employed as firemen of the Watt boilers during twenty-four hours; with Trevithick's boilers, five men did the work.
The high-pressure puffer-engine, with an 18-inch cylinder, working with 45 lbs. of steam, rolled as much iron as the two larger low-pressure vacuum engines of Watt, of 24 and 27 inch cylinders, which together were more than three times the size of the high-pressure engine, and cost three times as much.
At Stourbridge, as elsewhere, everyone was against the new plan. The engineer in charge did not like it, and the carpenters, smiths, and masons saw the end of their occupation as engine erectors, if there was no longer a necessity for foundations, well-work, &c., for condensing water, and many other things, necessary to complete a Watt engine; while the high-pressure puffer was no sooner unloaded than it was ready to work.
A great charm in Trevithick's character was his freedom and largeness of view in questions of competition. He was then making three engines at Coalbrookdale, to be worked with high-pressure steam, combined with the Watt air-pump and condenser; and though smarting from the contest with his great rival, yet wrote, "I think it is better to make them ourselves, for if we do not, some others will, for there must be a saving of coal by condensing. But with small engines, or where coal is plentiful, the engine would be best without it."
Those words accurately describe the practice of the present day, though written sixty-six years ago, and were followed by others equally true in principle, though varied in form to suit special requirement. "They say at the Dale about putting two cylinders, but I think one cylinder partly filled with steam would, do equally as well as two cylinders."
These sagacious views required the untiring labour of the following twelve years to perfect and make practical, when applied to the largest engines of the time; which we shall now trace in the construction of a strong and economical boiler, supplying high-pressure steam to the cylinder during only a comparatively small portion of the stroke, completing it by expansion, so that at its finish the steam had become of low pressure when passed to the condenser. The moving parts and expansive gear were so simplified as to be applicable to the then existing low-pressure steam vacuum engines without the complication of the double cylinders of Hornblower and Woolf.
"Penydarran Place, December 26th, 1804.