76 POLE STEAM-ENGINE. 



pressure steam-engines, a short but sufficiently close 

 calculation shows that taking Stuart's 1 estimate of the 

 effective power of the Watt engine at 8J Ibs. on each 

 square inch of the piston, and Trevi thick's engine at 

 anything approaching to 150 Ibs. on each square inch, 

 it becomes evident that the latter would be ten or 

 twenty times more powerful than the former. A few 

 figures will put the question in more practical form. 



The Wheal Prosper 16-inch pole high-pressure ex- 

 pansive steam vacuum engine commenced its up-stroke 

 with steam of 100 Ibs. on the inch, acting on the 

 122 square inches of the pole, which steam at the finish 

 of the stroke was reduced by expansion to 10 Ibs., 

 giving, say, an average steam pressure of 55 Ibs. The 

 down-stroke was caused by a vacuum under the pole of 

 14 Ibs. on the inch, reduced by, say, one-third loss in 

 working the air-pump to 9 Ibs., giving from the com- 

 pound stroke a force of 64 Ibs. on each square inch, 

 which, multiplied by the area of the pole, gives a net 

 force of 7808 Ibs. 



The Herland 33-inch pole high-pressure expansive 

 steam puffer-engine commenced its up-stroke with 

 steam of 150 Ibs. on the inch, acting on the 855 square 

 inches of the pole, which steam at the finish of the 

 stroke we will suppose was reduced- by expansion to 

 75 Ibs., giving an average steam pressure of, say, 112. 

 As this puffer-engine used no vacuum, the down-stroke 

 gave no increase of power ; its compound stroke was 

 therefore a force of 112 Ibs. on each square inch, which, 

 multiplied by the area of the pole, gives a net force of 

 95,760 Ibs. " 



To compare the Trevithick high-pressure steam 

 pumping engine, with the Watt low-pressure steam 



1 eo St'wt's 'History of the Steam-Engine.' 



