122 THE WATT AND THE TREVITHICK ENGINES. 



pressure powder, so that 12 inches of iron armour- 

 plates cannot resist its force; but this knowledge has 

 taken nearly as long in growing to perfection as did 

 the mastery of high-pressure steam, and its use in the 

 much more complicated steam-engine. 



Watt's engine, as described a quarter of a century 

 after the expiration of his patent and the advent of the 

 high-pressure steam-engine, still derived its gross force 

 from 14 Ibs. of vacuum and 2 or 3 Ibs. of steam, re- 

 sulting in a net force of 8i Ibs. Trevithick's engine 

 of 1799, which heralded the last hours of the Watt 

 patent authority, and may be taken as the first distinct 

 evidence of comparatively high-pressure steam in large 

 Cornish pumping engines, derived its power from 14 Ibs. 

 of vacuum and 6 Ibs. of steam, being together but 2 or 

 3 Ibs. on the inch more than the Watt engine, but its 

 net force of 12 Ibs. to the inch was half again as much 

 as the net force of the Watt engine, the increase being 

 wholly from the steam pressure, which was never prac- 

 tised by Watt, and which in its almost unlimited force 

 gives the greatly increased power to modern steam- 

 engines. 



Trevithick's estimate for a new engine of the same 

 size as the old was 2000/., but as the old one could be 

 improved for 1300/., the latter course was adopted, 

 the wooden main beam with its segment head was re- 

 tained, a cover was added to the cylinder, and a new 

 piston-rod and piston ; a pole air-pump was used in 

 lieu of the more usual Watt air-pump bucket ; a feed- 

 pole forced water into the boiler, an indirect proof 

 of increased steam pressure. The new globular boiler 

 with internal tube weighed 8 tons; the engineer's charge 

 for carrying out the work was 66 



The use of strong steam as the prime mover of the 



