254 



APPENDIX B. 



third, or a quarter of the stroke. The piston com- 

 pletes its stroke, therefore, under a constantly 

 diminishing pressure. The first engines working on 

 this principle date from 1778. Watt conceived the 

 idea of them in 1769, and took out a patent in 1782. 

 We give here the Table appended to Watt's 

 patent. He supposed the steam introduced into 

 the cylinder during the first quarter of the stroke of 

 the piston; then, dividing this stroke into twenty 

 parts, he calculated the mean pressure as follows: 



Portions of the descent from the 

 top of the cylinder. 



Decreasing pressure of the 

 steam, the entire pressure 

 being 1. 



11 583 

 Mean pressure - -- = 0.579. 



