POLE STEAM-ENGINE. 77 



pumping 1 engine, take one of the largest of the latter, 

 made about that time, say, with an 80-inch cylinder, 

 which commenced its down-stroke with steam of, say, 

 3 Ibs. on the inch, acting on the 5000 square inches 

 of the piston, which steam at the finish of the stroke 

 the writer is describing the usage at that time, 

 for Watt himself advocated a less steam pressure 

 was reduced by expansion to, say, 1 lb., giving an 

 average steam pressure of, say, 2 Ibs. on the top of the 

 piston, whose under side was in vacuum equal to 14 Ibs. 

 on the inch, reduced by, say, one-third loss in working 

 the air-pump to 9 Ibs., which power, from vacuum 

 added to the 2 Ibs. from steam, gives a net force of 

 11 Ibs. on each square inch of the piston. As the Watt 

 pumping engine moved in equilibrium during its up- 

 stroke, it thereby gained no increase of power; its 

 compound stroke was therefore a force of 11 Ibs. on 

 each square inch, which, multiplied by the area of the 

 piston, gives a net force of 55,000 Ibs. 



The practical comparison therefore stands, Trevi- 

 thick's 16-inch pole high-pressure steam, and vacuum, 

 on each inch 64 Ibs., net force 7808 Ibs. ; Trevithick's 

 33-inch pole high-pressure steam, without vacuum, on 

 each inch 112 Ibs., net force 95,760 Ibs.; Watt's 

 80-inch piston, low-pressure steam, and vacuum, on 

 each inch 11 Ibs., net force 55,000 Ibs. As the first 

 cost was mainly dependent on the size, the Tre'vithick 

 engine was commercially much more valuable than the 

 Watt engine. 



"I saw Captain Trevithick's puffer working at the Herland 

 Mine. The steam used to blow off like blue fire it was so 

 strong. The lever on the safety-valve was about 3 feet long, 

 with a great weight on it, more, than a hundredweight. The 

 engine did not answer very well, for the packing in the pole 



