.146 THE WATT AND THE TKEY1THICK ENGINES. 



in the usual way, as at other times. Neither of the engines 

 have done so much duty as on the first trials, as they have not 

 been so strictly attended to. The average of the trial at this 

 time stands 26 cwt. for a bushel of coals to Boulton and Watt's 

 engine ; mine, 83 cwt. for a bushel of coals. 



" If I do not remain in Cornwall to attend next Dolcoath 

 account, I shall be in town about the 15th instant, otherwise 

 about the 20th instant. I shall call on you immediately on my 

 arrival. In this time I should be glad to hear from you again. 

 The Trinity business will answer exceedingly well ; I have two 

 engines ready for that purpose to put to work on my arrival in 

 town. 



" I am, Sir, 



" Your very humble servant, 



" RD. TREVITHICK. 



" P.S. I would try the evaporation of water by both boilers, 

 but Boulton and Watt's engine is so pressed with work, and 

 being on the best part of the mine, they will not stop it a 

 moment. A boiler of 8 feet diameter and 30 feet long will 

 have as much fire-sides in the tube as there is now in Dolcoath 

 great boiler. The fire-tube in this boiler would be 5 feet 

 diameter, and a fire-place 6 feet long in it would be 30 feet of 

 fire-bars. In the whim-engines 1 find that a fire-place 14 inches 

 long and the tube 2 feet 3 inches diameter would, being forced, 

 burn 1 bushel per hour. At this rate the great tube would 

 burn near 12 bushels per hour, which is above the quantity 

 that the great engine boiler can consume, now at work. Small 

 tubes would have an advantage over large ones. Two boilers 

 would not cost much more than one large one, and be much 

 stronger." 



The battle-ground of the fight between low and high 

 pressure from 1806 to 1812 had also served for the 

 personal encounter of Trevithick, sen., and Watt a 

 quarter of a century before, when the Dolcoath great 

 pumping engine was erected to compete with the two 



