182 THE WATT AND THE THEY! THICK ENGINES. 



effects, that by gradual increase the engines in 1815 lifted 

 twenty-one millions and a half, taking the average of thirty-three 

 engines. In 1816, Stray Park, a 63 -inch cylinder, 7 feet 9 inches 

 stroke, single-acting, being one of the three engines on the vast 

 Dolcoath Mine ; its performance in four different months was 

 thirty-one, thirty-one and a quarter, twenty-eight, and twenty- 

 eight and a half millions." 



This statement reveals a source of error in estimating 

 the relative values of the Watt and the Trevithick 

 engine ; that of the latter was the Welsh locomotive, 

 compared in duty with the large Watt pumping engine, 

 pointed out in Trevithick's letter 1 of that time, as an 

 unfair comparison ; the small high-pressure puifer, in 

 1804, is admitted to have done seventeen and a half 

 millions of duty with a bushel of coal of 84 Ibs., while 

 in Rees' calculation of the engines, he .gives Watt 

 94 Ibs. of coal to a bushel ; and having stated that the 

 Watt pumping engines in Cornwall, in 1811, averaged 

 but thirteen and a half millions of duty, draws the 

 false conclusion -that the high-pressure cannot com- 

 pete with the low-pressure where coals are dear ; yet 

 he agrees with other writers that the great increase in 

 the duty of the Cornish pumping engines commenced 

 from 1811 (when Trevithick first gave them his high- 

 pressure steam); and states that in 1816 the Stray 

 Park 63-inch cylinder single-acting engine, 2 being one 

 of three then working in Dolcoath, did thirty-one 

 millions. 



The * Encyclopaedia Britannica' on the question of 

 duty states : 3 



"The duty of the best of Smeaton's engines was, in 1772, 

 9,450,000 foot pounds per cwt. of coal. On the expiration of 



1 See vol. i., p, 166. 2 See Watt's statement, vol. ii., p. 115. 



3 See " Steam-Engine," published 1860. 



