326 TUBULAR BOILER, SUPERHEATING STEAM, 



those surfaces, as on the present plan, it would have done above 

 forty millions. 



<% Suppose the Binner Downs 70-inch cylinder engine, 10-feet 

 stroke, working with full steam to the bottom of the stroke, 

 when, by the experiment, the heated flues were again laid on 

 would have worked one-third expansive, by the heat of 5 bushels ' 

 of coal around the cylinder. Now one-third of the power would 

 make a 3 feet 4 inch stroke, 11 Ibs. to the inch effective power, 

 eight strokes a minute, during twenty-four hours, by the con- 

 sumption of 5 bushels of coal applied on the surface sides of the 

 cylinder, performing a duty of 324 millions with a bushel of 

 coal. Now suppose the cylinder without the heating flues had 

 the steam cut off at two-thirds of the stroke, and that it is 

 possible in a moment to heat the cylinder by the flues ; in that 

 case the steam would, by its expansion from the hot sides, fill 

 the last third of the cylinder to the bottom of the stroke ; then 

 if that steam could be suddenly cooled, so as to contract it 

 one-third, the piston would ascend one-third its stroke in the 

 cylinder ; and it appears in theory by this plan, that a cylinder 

 once filled two-thirds full of steam, by receiving the heat on its 

 surface sides from 5 bushels of coal, and again suddenly cool- 

 ing down, would continue to work for ever, without removing 

 the steam from the cylinder, and would perform a duty of 

 324 millions. This never can be accomplished in practice in 

 this way, but the effect may be obtained by partially condensing 

 in a suitable condenser, and again heating by hot sides. 



"This mystery ought to be laid open by experiment, for 

 what I have stated are plain facts from actual proofs, and I 

 have no doubt that time will show that the theory of Mr. Watt 

 is incorrect. Though there were 300 feet of cold sides, yet 

 200 feet were not condensing steam, because on the return of the 

 piston, what was condensed below, and while the engine was 

 resting, did not make against it more than what was condensed 

 above the piston on its descent; therefore you may count on 

 150 feet of cold external sides constantly condensing, that made 

 this third-part difference against the expansion of the steam. 

 " I remain, Sir, 



" Your very humble servant, 



" KICHD. TREVITHICK." 



