TIIK WATT AND TIIK THKVITIIIUK KXU1NKS. 185 



ferred, either directly or indirectly, by the late Mr. Trevithick 

 on the mines of this county. I have often expressed my opinion 

 that he was at the same time the greatest and the worst-used 

 man in the county." l 



The late Sir John Rennie and other scientific persons 

 were, about 1830, associated with Mr. Henwood 2 in 

 examining the work performed by Cornish pumping 

 engines : their reports are curtailed in the following 

 comments on Wheal Towan engine, similar to Trevi- 

 thick's Dolcoath engine of 1816, except perhaps that 

 the last named was a little inferior in its detail move- 

 ments, while much less care was taken 

 to avoid unnecessary loss of heat. <jT 



Mr. Henwood also gave indicator j\ 

 diagrams of the expansion of the 

 steam, on one of which the writer 

 has marked ten horizontal lines, in- 

 dicating the position of the piston at 

 each foot of its stroke, and ten longi- 

 tudinal lines dividing the diameter of 

 the cylinder into tenths. The steam 

 pressure in the boiler was 46 * 8 Ibs. 

 on the square inch above the atmo- 

 sphere, or 4 '68 Ibs. for each of the 

 ten longitudinal line divisions, x to 

 c represents the top of the steam- 

 cylinder 80 inches diameter ; x to 

 F the length of the cylinder for a 

 10-feet stroke of the piston. By the 

 time the piston had moved through one-twentieth of 

 its course, reaching c, the expansive working had com- 



STEAM DIAGRAM OF WHEAL 

 TOWAN PUMPING ENGINE, 



ERECTED 1827. 



1 See letter of Michael Williams, chap. xix. 



2 Ik'invoud, ' Edinburgh -Journal of Science,' 10. 



