104 POLE STEAM-ENGINE. 



pole for the up-stroke, after which it was expanded in 

 the old and much larger cylinder on the top of the 

 piston causing the down-stroke ; it then, by its passage 

 through the equilibrium valve, allowed the piston in the 

 large cylinder to make its up-stroke, by equalizing 

 the pressure of steam on its top and bottom, while a 

 fresh supply of strong steam from the boiler admitted 

 under the pole gave power to the up-stroke ; and finally, 

 the comparatively low-pressure steam under the large 

 piston passed to the condenser and air-pump to form a 

 vacuum for the down-stroke, as in the Watt engine. 



Sims, the engineer at Wheal Chance, one of the mines 

 in the eastern or Watt district, was converted and became 

 in 1815 or 1816 a partner with Trevithick, and erected, 

 at Treskerby Mine, Trevithick's high-pressure pole of 

 36 inches in diameter, as an addition to the old Watt 

 engine working with a cylinder 58 inches in diameter. 



Watt, then, within a year or two of his death, was too 

 old to any longer take part in the contest ; his engine 

 in the hands of others was converted and became a 

 high-pressure expansive engine. 



Trevithick, as a further proof that he could do with- 

 out the Watt patent air-pump bucket, with its piston 

 and valves, removed it from a Watt engine at Wheal 

 Alfred Mine in 1812, replacing it by one of his poles, 

 answering the same purpose, but different in construc- 

 tion. Many other mines used them ; one remained at 

 work in Old Wheal Damsel in 1860. They have also 

 been used in steamboat air-pumps. 



Having traced during a period of five or six years 

 the rise and progress of the high-pressure expansive 

 pole condensing-engines, the high-pressure expansive 

 pole puffer-engine, and the combined pole and cylinder 

 high-pressure engine, their value in a commercial sense 



