T1IK WATT AND TIIK TIIKVITII K 'K KXdIXKS. 115 



for increasing the elasticity of the steam, and reducing 

 the size of the boiler." J 



In 1775 Richard Trevithick, sen., removed the flat 

 top of a Newcomen boiler, and substituted a semi- 

 circular top, enabling it to contain stronger steam, 

 and at the same time he improved the mechanical part 

 of the engine by finding a better resting-place for the 

 steam-cylinder than the top of the large boiler. Pryce 

 gives a drawing of this engine as the best at that time 

 in Cornwall. 2 



"It is known as a fact that every engine of magnitude 

 consumes 3000Z. worth of coal every year. 



" The fire-place has been diminished and enlarged again. 

 The flame has been carried round from the bottom of the boiler 

 in a spiral direction, and conveyed through the body of the 

 water in a tube (one, two, or three) before its arrival at the 

 chimney. 



" Some have used a double boiler, so that fire might 'act on 

 every possible point of contact, and some have built a moorstone 

 boiler, heated by three tubes of flame passing through it. 



"A judicious engineer does not attempt to load his engine 

 with a column of water heavier than 7 Ibs. on each square inch 

 of the piston." 3 



While Pryce's book was being printed, Watt in 1777 

 wrote of the Cornish steam-engines : 



" I have seen five of Bonze's engines, but was far from seeing 

 the wonders promised. They were 60, 63, and 70 inch cylinders 

 at Dolcoath and Wheal Chance. They are said to use each 

 about 130 bushels of coals in the twenty-four hours, and to make 

 about six or seven strokes per minute, the stroke being under 

 6 feetea ch. They are burdened to 6, 6, and 7 Ibs. per inch." 4 



1 Bprlase's ' Natural History of Cornwall.' 



2 See drawing, vol. i., p. 25. 



' 5 Sec Trycc's * Mineralogia Cornubiensis,' published 1778. Appendix. 



4 Smiles' ' Lives of Boulton and Watt.' 



i 2 



