| Inches | |
| 4000 | in a 72-inch cylinder, single. |
| 10 | lbs. to the inch. |
| 4000 | 8 feet stroke. |
| 320000 | 9 strokes per minute. |
| 2880000 | lbs. lifted one foot high per minute. |
"Suppose a 33-inch plunger-pole, 10-feet stroke, ten strokes per minute (which is not so fast by three or four strokes per minute as this engine will go, because she will have no heavy beam to return, neither will she have to wait for condensing, like B. and Watt's, which, when loaded, hangs very long on the injection).
| 855 | square inches in a 33-inch plunger-pole. |
| 10 | lbs.to the inch. |
| 8550 | |
| 10 | feet stroke. |
| 85500 | |
| 34 | lbs. to the inch, real duty. |
| 342000 | |
| 256500 | |
| 2907000 | lbs. lifted one foot high per minute. |
"I should judge that less than 50 lbs. to the inch above the atmosphere would be quite enough to do the work of a 72-inch cylinder single, which is but a trifle for those wrought-iron tubes to stand. This engine, everything new, house included, ready for work, will not exceed 700l. Two months are sufficient for erecting it. The engine of Woolf's, at Wheal Vor, which is but two-thirds the power of a 72-inch cylinder, single power, cost 8000l., and was two years erecting. I would be much obliged to you for your opinion on this business.
"I remain, Sir,
"Your very humble servant,
"Rd. Trevithick.
"I am sorry to say that the mines in general are very poor."
He shows that with steam of 34 lbs. to the inch, his Herland pole puffer steam-engine of 33 inches in diameter would be equal in power to the Watt low-pressure steam vacuum engine, with a 72-inch cylinder. Herland, like Dolcoath and Wheal Treasury, was the chosen battle-ground of rival engineers; fifty years after Newcomen had there erected his famously large 70-inch cylinder engine, Watt surpassed it in size by a cylinder 2 inches more in diameter, and, after personally superintending its erection in 1798, declared that "it could not be improved on." Mr. Davey, the mine manager, considered that it did twenty millions of duty, though Mr. Watt had made it twenty-seven millions with a bushel of coal.[31] This difference is probably explained by the then Cornish bushel weighing 84 lbs., while Watt generally calculated a bushel at 112 lbs.
Trevithick declining to believe Watt's prognostication, a public test of Watt's engines in the county was demanded; Mr. Davies Gilbert, with Mr. Jenkin, were requested to report on their duty, and gave it in 1798 as averaging seventeen millions.[32] During the same year the adventurers in Herland Mine engaged Trevithick and Bull, jun., to erect a 60-inch cylinder Bull engine to compete with Watt's 72-inch cylinder. The result of this fight is not traceable, nor what took place there during the succeeding fifteen years; when in 1814 Woolf erected in Cornwall his double-cylinder engine to compete with Watt's engine, and Trevithick attacked them both with his Herland high-pressure pole puffer in 1815, when he erected at his own risk and cost a 33-inch pole-engine, engaging that it should, both in power and economical duty, equal the Watt 72-inch engine. The boilers were similar in form to those used a year before in Wheal Prosper high-pressure steam vacuum pole-engine, being two wrought-iron tubes, each 45 feet long and 3 feet in diameter, made of plates half an inch thick. The fire was in external flues. The engine was fixed directly over the pump-rods in the shaft, using neither main beam nor air-pump.
Trevithick's rough hand-sketch shows the steam-ring in the stuffing box and the steam-vessel; the particular use of the latter he has not described: probably it was because Cornish pumping engines, not having the controlling crank to limit the movement of the piston, are obliged to trust to the very admirable, but little understood, steam-cushion, without which the ascending piston would inevitably strike and break the cylinder-cover, while in the pole puffer-engine this danger was during the descent of the pole, and therefore the discharge-steam valve was closed, while the steam in the pole-case was still of ten or more pounds to the inch, so that by the time the pole reached the finish of its down-stroke, it had compressed this steam-cushion, filling also the steam-vessel, with a pressure approaching to that in the boiler, and equal to the weight of the pole and pump-rods. A comparatively small supply of steam from the boiler into the steam-vessel brought it up to the boiler pressure, sending the pole and pump-rods upwards with a spring. The steam-valve then closed, allowing the momentum of the great weight of pump-rods, together with the expanding steam, to complete the up-stroke. The discharge-valve was then opened for a moment, allowing a blast of steam to escape, reducing the pressure say to one-half. The weight of the rods caused their downward movement, raising the load of water in the plunger-pole pumps, and at the same time compressing the steam from the pole-case into the steam-vessel, equal at the finish of the stroke to the support of the pole and pump-rods. This most simple steam-engine combined in the greatest degree the two elements of expansion and momentum.
The up-stroke began with a much higher pressure of steam than was necessary to raise the load; having given momentum to the rods, the supply of steam was cut off, and the stroke was completed by expansion. The down-stroke began with a comparatively low pressure of steam under the pole. The unsupported pump-rods fell downwards, setting in upward motion the column of water in the plunger-pole pumps. The discharge-valve was closed long before the completion of the down-stroke, and the momentum of the moving mass of rods and water compressed the steam driven from the pole-case into the steam-vessel up to a pressure equal to the support of the pole and pump-rods. The pole was, therefore, continually floating or rising and falling in steam of ever-varying pressure.