186 THE WATT AND THE TREVITHTCK ENGINES. 



menced ; and when one-tenth of the stroke had been 

 run, half of a division was cut off, showing by the 

 curved indicator line the decrease in pressure of steam 

 to 44 46 Ibs. The comparatively small passage through 

 the steam-valve not giving room for sufficient steam to 

 follow up the increasing speed of the piston, led to its 

 continued expansion in the cylinder, and by the time 

 the piston had moved 2 feet, reaching D, the steam 

 pressure was reduced by two divisions or 9*36 Ibs., 

 or a pressure of 37*44 Ibs. on the piston ; at this point 

 the steam- valve was closed, and the remaining four- 

 fifths of the stroke was performed by expansion ; at 

 the fifth horizontal line, or middle of the stroke, only 

 three divisions of steam are left, giving a pressure of 

 14*04 Ibs. to the inch; at the finish of the stroke 

 there is only half a division, from E to F, or 2 34 Ibs. 

 of steam to the inch above the pressure of the atmo- 

 sphere. On the return up-stroke of the piston, when it 

 had reached within a foot of the finish of its course at C, 

 the equilibrium valve closed, causing the enclosed steam 

 of 2 * 34 Ibs. to the inch to be compressed at the finish 

 of the up-stroke shown by the curve Gr A to 9 36 Ibs. on 

 the inch, equal to its pressure about the middle of the 

 down-stroke at N. 



Trevithick's expansive engine therefore, commencing 

 its work with steam of 46 * 8 Ibs. on the inch above the 

 atmosphere, only took a full supply from the boiler during 

 one-tenth of its stroke, and none after one-fifth had been 

 performed, while at the finish of the stroke it had about 

 the same pressure as Watt began with. 



The power of the Watt low-pressure steam vacuum 

 pumping engine was increased by Trevithick from two 

 to three fold, and its economical duty in about the same 

 proportion ; in other words, he increased the effective 



