The fact that expansion of steam caused reduction of heat was so evident to Trevithick that he ventured to doubt his friend's theory. The trials between the whim-engines having continued a fortnight, showed that the high-pressure steam-puffer had lifted 83 cwt., while the low-pressure steam vacuum only lifted 26 cwt. with the consumption of a bushel of coal. A suitable high-pressure boiler for the Watt low-pressure steam 63-inch pumping engine should be 30 feet long, 8 feet in diameter, with an internal fire-tube 5 feet in diameter; proportions approved of in the present day. The recommendation in 1806 to use small tubes may claim to be the first practical decision on the advantage of tubular boilers; and at the same time we read of the first hesitating step on the part of the public to use high-pressure steam in a Watt low-pressure engine, which was still deferred for further consideration, even with the limited pressure of 25 lbs. to an inch; so the large Watt pumping engines were doomed for another four or five years to struggle through their work with low-pressure steam, though at that time Cook's Kitchen high-pressure expansive condensing whim-engine had been for years at work close by.
The shareholders professed to have fear of explosion; but party-feeling and ignorance were the real causes of opposition, for working men had no dread of the new engines, while influential men leaned toward Watt's old-fashioned plans.
This fear of Trevithick's expansive plans and high steam is the more surprising, because at that time a new boiler was required for the Watt 63-inch cylinder pumping engine and Trevithick's cylindrical tubular boiler could be made for one-third less cost than the Watt waggon boiler, thus saving 300l., and in addition he promised to apply the higher pressure of steam to the Watt engine without any change in its parts or expenditure of money, and make it set in motion at the commencement of the stroke the 200 tons of pump-rods, the momentum of which would, with the expansion of the steam, when shutting it off soon after the first start in the movement of each stroke, carry it through to the end; and he practically compares this advantage from hoarded momentum in the pumping engine with his experience of the fly-wheel of the rolling-mill expansive engine in Wales.
The whim-engine with a fire-tube 2 feet 3 inches in diameter used 84 lbs. of coal per hour; and at that rate one cylindrical boiler 30 feet long, 8 feet in diameter, with internal fire-tube 5 feet in diameter, would supply steam for Watt's 63-inch cylinder; but in place of it he preferred two smaller boilers, because small tubes have an advantage over large ones, and are much stronger.
The whim trials—high-pressure puffer against low-pressure vacuum—went on for another fortnight, when high pressure, having done twice as much work as low pressure, with an equal consumption of coal, the adventurers threw aside the work that had been made for another Watt engine, ordering one in its stead from Trevithick; but they could not just then make up their minds to place the Watt 63-inch pumping engine in his hands.
"Camborne, May 30th, 1806.
"Dear Sir,
"I am very happy to find you have so far continued your agreement with the Trinity gents, and think the bargain is a good one. Must still beg leave to remind you not to proceed to show what your engine will do till the agreement is fully drawn up and regularly signed.
"Dolcoath agents, since they are informed of the accident at the iron-works in Wales, of the engine blowing to pieces, have requested me to have your opinion whether the old cylinder is strong enough for the boiler of the intended new engine, or whether you would recommend them to have a new one. Your answer to this as soon as possible, as Mr. Williams and some others are likely to make some objections.
"Mr. Sims, the engineer, has published in the Truro paper, that one of Boulton and Watt's engines at Wheal Jewell has drawn more than a ton of ore over and above that drawn by the Dolcoath engine from the same depth by a bushel of coal. On inquiry I found they had only tried for twenty-two hours. They said they left off with as good a fire as they began with. This I argued was not a fair trial. They say they are now on a trial for a month.