180 



THE WATT AND THE TBEYITHICK ENGINES. 



expansion of the steam until we could get men than could work 

 it, as a few tons of coal per year was less expensive than having 

 the work stopped. In some of the mines a few hours' stoppage 

 was a serious matter, as it would cost the proprietor as much as 

 701 per hour." l 



Pole expresses the same view, intimating that Watt 

 only used steam of 1 or 2 Ibs. pressure to the inch. 



" In Watt's engine, as is well known, the pressure of steam in 

 the boiler very little exceeded the pressure of the atmosphere. 

 He recommended that when the engine was underloaded, this 

 excess should be equal to about 1 inch of mercury ; and when 

 full loaded, ought not to exceed 2 inches ; adding, ' It is never 

 advisable to work with a strong steam when it can be avoided, 

 as it increases the leakages of the boiler and joints of the steam- 

 case, and answers no good end.' 2 



" Mr. Watt's engines with such boilers " (which will not 

 retain steam of more than 3 Ibs. per square inch above the 

 atmosphere) " cannot be made to exert a competent power to 

 drain deep mines, unless the supply of steam to the cylinder 

 is continued until the piston has run through more than half its 

 course. 3 



" In 1801-2 Captain Trevithick erected a high-pressure 

 engine of small size at Marazion, which was worked by steam 

 of at least 30 Ibs. on the square inch above atmospheric pressure. 

 In 1804, as Mr. Farey admits, 4 the same gentleman introduced 

 his celebrated and valuable wrought-iron cylindrical boilers, 5 now 

 universally used in this county. 



" To these, everyone at all acquainted with the Cornish 

 improvements ascribes a great part of the saving we have 

 obtained. This will further appear from an extract from a 

 valuable work edited by John Taylor, Esq., F.E.S. 6 



" The monthly consumption of coal in Dolcoath Mine was, 



1 Smiles' 'Lives of Boulton and 

 Watt,' p. 228. 



2 Appendix A to Tredgold, 'Pole 

 on Cornish Engines,' p. 49. 



3 'Phil. Mag. and Annals,' N.S., 



vol. viii., p. 309, by W. J. Henwood. 



4 Ibid., p. 313. 



5 Ibid., vol. i., p. 127. 



6 Records of Mining,' p. 163. 



