AND SUKFACE CONDENSKIL 331 



on its passage through numbers of small tubes, and 

 decreasing its expansive force on the other side of the 

 piston by cooling it in passage through similar tubes 

 exposed to cold, is partly effected in steamboats, but 

 has not yet been attempted in engines on the road. 

 After a month's further consideration he wrote : 



" Wheal Towan engine is working with three boilers, all of 

 the same size, and the strong steam from the boilers going to 

 the cylinder-case ; the boilers are so low as to admit the con- 

 densed water to run back from the case again into the boiler : 

 they find that this water is sufficient to feed one of these 

 boilers without any other feed-water, therefore one-third of the 

 steam generated must be condensed by the cold sides of the 

 cylinder-case, and this agrees with the experiments I sent to 

 you from Binner Downs. Wheal Towan engine has an 80-inch 

 cylinder, and requires 72 bushels of coal in twenty-four hours, 

 therefore, the cylinder-case must, in condensing high-pressure 

 steam, use 24 bushels of coal in twenty-four hours. Boulton 

 and Watt's case for a 63-inch cylinder working with low-pres- 

 sure steam, condensed only 4J bushels of coal in equal time, the 

 proportions of surface being as 190 to 240 in Wheal Towan. 

 Nearly five times the quantity was condensed of high steam 

 than of low steam, proving that there is a theory yet unac- 

 counted for." 1 



These apparent facts are, in the case of steamboats, 

 more culpably overlooked now than when he wrote 

 forty -two years ago ; engines have been examined and 

 reported on by eminent scientific men, but it was left 

 for Trevithick to point out that cold on the surface of 

 the steam-case of a Watt low-pressure steam vacuum 

 engine condensed about one-fifteenth of the steam given 

 from the boilers, and that the loss from exposure to cold 

 was nearly five times more from high-pressure steam 

 than from low-pressure. Within a few more months he 



1 SL-O Trevithiek's letter, January 24th, 1829, vol. ii., p. 368. 



