120 THE WATT AND THE TREVITHICK ENGINES. 



served as an internal flue, carrying the fire by a sweep 

 around the interior in the water space, and then out 

 through the side of the boiler into the external brick 

 flue. It may be said that there was nothing new in a 

 circular form of boiler, or in an internal tube ; but it 

 will be admitted that this repaired engine, in this its 

 third stride in the inarch of advancement, made pub- 

 licly known those principles which in a few years more 

 than doubled the power, the economy, and the appli- 

 cability of the steam-engine. His patent drawing of 

 1802 shows this form of boiler applied to a small port- 

 able engine, in which, for the sake of simplicity of 

 structure and cheapness, cast iron was used instead of 

 wrought iron, and the internal tube omitted. 1 



The full detail estimate, from which the following 

 items are extracted, of the cost of alteration was written 

 by Trevi thick, jun., in the book and on the page ad- 

 joining that containing the account of the former 

 alteration and re-erection of the same engine by Trevi- 

 thick, sen., in 1775. 



"A 45-inch cylinder engine, working 20 Ibs. to the inch : 



s. d. 



Boilers, 8 tons at 42Z 336 



Iron about ditto, 6 cwt. at 42? 12 12 



Castings about ditto, 15 cwt. at 42s 18 



Safety-valve and cocks 100 



Wood about bob, 200 ft. at 6s 60 



Cast iron about ditto, 45 cwt. at 25s 5650 



Brass about ditto, 60 Ibs. at 2s 600 



Piston-rod, 4 in., 14 ft, long, 550 Ibs. at Is. .. 27 10 



T-piece, 10 cwt. at 25s 12 10 



Cover, and bottom, and piston, 35 cwt, at 32s. .. 56 



Nozzles, 6 cwt. at 32s 9 12 



Steam and perpendicular pipe, 10 cwt. at 25s. . . 1 2 10 



Receiver, 2 ft. 4 in. long, and bottom, 15 cwt. at 25s. 18 15 



Air-pump, bottom, and case, 10 cwt. at 25s. .. 12 10 



Plunger, 22 in., 6 ft. long, 12 cwt, at 40s 24 



Force lift 5 



Engineer 66 0" 



1 See vol. i., p. 128. 



