POLE STKAM-ENGINE. 83 



for work, will not exceed 7001. 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 

 8000Z., 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, 



"KiCHARD TREVITHICK. 



" I am sorry to sa^ that the mines in general are very 

 poor." 



He shows that with steam of 34 Ibs. to the inch, his 

 Herland pole puffer steam-engine of 33 inches in dia- 

 meter 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 per- 

 sonally 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. 1 This difference is pro- 

 bably explained by the then Cornish bushel weighing 

 84 Ibs., \vhile Watt generally calculated a bushel at 

 112 Ibs. 



Trevithick declining to believe Watt's prognostica- 

 tion, a public test of Watt's engines in the county 

 was demanded ; Mr. Da vies Gilbert, with Mr. Jenkin, 

 were requested to report on their duty, and gave it in 



Lean's ' Historical Statr-nunt of Hie Steam- Kngine,' p. 7. 



ft 2 



