84 POLE STEAM-ENGINE. 



1798 as averaging seventeen millions. 1 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-engjne, 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 con- 

 trolling crank to limit the movement of the piston, are 

 obliged to trust to the very admirable, but little un- 

 derstood, 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, 



1 Lean's ' Historical Statement of the Steam-Engine,' p. 7. 



