72 POLE STEAM-ENGINE. 



at the end of the stroke it was reduced to 20 Ibs. or less, when 

 the exhaust-valve allowed the steam to pass to the condenser, ^ 

 and the pole made its down-stroke in vacuum. A balance-bob 

 regulated the movement of the engine. 



" Trevithick's character in those days was, that he always 

 began some new thing before he had finished the old." l 



Captain Artha, one of his assistants, said: 



"I erected several of Captain Trevithick's pole-engines. 

 My brother Kichard worked the one at Wheal Prosper when 

 first erected. The pole made an 8-feet stroke. The case was 

 fixed over the engine-shaft on two beams of timber from wall to 

 wall. A cross-head was bolted to the top of the pole, and from 

 it two side rods descended to a cross-piece under the pole-case, 

 from which the pump-rod went into the shaft. A connecting 

 rod worked a balance-beam, which worked the air-pump, feed- 

 pump, and plug-rod for moving the valves. The steam, of a 

 very high pressure, worked expansively." 2 



The first admission of the high-pressure steam under 

 the pole was equal to a force of 8 or 9 tons, causing it 

 and its attached pump-rods to take a rapid upward 

 spring. Having travelled 1 or 2 feet of its stroke of 

 8 feet, the further supply of steam from the boiler was 

 cut off, and its expansion, together with the momentum 

 of the mass of pump-rods, completed the upward stroke. 

 The pressure of the steam in the pole-case at the finish 

 of the up-stroke would be reduced to say 10 or 20 Ibs. 

 to the inch, according to the amount of work on the 

 engine. The steam then passed to the condenser and 

 air-pump, and the engine made its down-stroke by the 

 vacuum under the pole, and by the weight of the de- 

 scending pole and pump-rods. 



Each boiler was a wrought-iron tube 3 feet in dia- 

 meter and 40 feet long, the fire-place under one end, 



1 Captain Samuel Grose's recollections. 1858. Gwinear. 



2 Captain H. A. Artha's recollections. Penzance, 1869. 



