THE WATT AND THE THKVITHICK KN<;i\KS. 145 



" My fire-tube is 2 feet 3 inches in diameter, and the 

 fire-bars only 14 inches long, and the fire only about 

 4 or 5 inches thick ; it raised steam in plenty, and was 

 as bright as a star." These words certainly imply the 

 use of the blast-pipe, making the fire as bright as a 

 star, and enabling the small boiler to give the required 

 supply of steam. Several high-pressure puffer-engines 

 had been ordered, and many persons were waiting the 

 conclusion of the month's public trial to enable them to 

 judge between the Watt and the Trevithick engine. 



" MR. GlDDY, " CAMBORNE, March th, 1806. 



" Sir, The day after I wrote to you the first letter, I 

 received yours, and this day I have yours of the 1st instant. 



" I am very much obliged to you for the figures you have 

 sent me. I am convinced that the pressure of steam will not 

 hold good as theory points it out, because on expanding it will get 

 colder, and of course lose a part of its expansive force after the 

 steam-valve shuts. I think there can be no risk in making this 

 trial on Dolcoath great engine, as they intend to have a new 

 boiler immediately, so as to prevent stopping to cleanse ; and a 

 boiler on this new plan can be made for one-third less expense 

 than on the old plan, when you count the large boiler-house and 

 ashes-pit, and brickwork round the boiler. It is not intended 

 to alter any part of the engine or condenser, but only work 

 with high steam from this new boiler ; and if this boiler only 

 performs as good duty as the old one, it will be a saving of near 

 '3001. to them on the erection. The vast matter this great engine 

 lias in motion will answer in part the use of a fly-ivlieel : the whole 

 of the matter in motion is near about 200 tons, at a velocity of 

 about 160 feet a minute. This I know will not be sufficient ; 

 but it will be about equal to a fly-wheel of 20 feet diameter, 

 25 tons weight, twenty rounds per minute, if weight and velocity 

 answer the same purpose. 



" Since Monday, the 18th February, being Dolcoath account- 

 day, both engines have been on trial, and are to be continued 

 until the next account, 17th instant. The engines are kept on 



VOL. II. L 



