THE WATT AND THE TREVITHTCK ENGINES. 1 17 



twenty-seven years before, but had failed to carry it 

 into practice. Hornblower had tried something 1 like it 

 in his double-cylinder expansion engine, but he did not 

 use high-pressure steam, and consequently also failed. 



The idea, therefore, of expansive steam was not new, 

 but the useful mastery of it was. Savery had tried 

 expansive steam before Watt patented it ; the latter 

 went to law with Hornblower for an infringement of 

 the idea, when neither of them had in truth constructed 

 an expansive steam-engine. The low pressure of the 

 steam from the boilers used by Hornblower and Watt 

 did not admit of profitable expansion in the cylinder ; 

 at its full boiler pressure it constituted but a compa- 

 ratively small portion of the power of the engine : to 

 reduce that power by expansion was as apt to be a loss 

 as a gain. The steam-engine was still dependent for its 

 power mainly on steam as an agent for causing the 

 required vacuum, until 1796, when Trevithick disclosed 

 his method of constructing small cylindrical boilers and 

 engines suitable for giving power from the strong 

 pressure of the steam, irrespective of vacuum. 



Lean, who favoured Watt rather than Trevithick, 

 thus records the advent of Watt's expansive engine : 



" In 1779 to 1788 Mr. Watt introduced the improvement of 

 working steam expansively, and he calculated that engines which 

 would previously do nineteen to twenty millions would thus per- 

 form twenty-six millions ; but I do not find any record of this 

 duty being performed in practice. In 1785 Boulton and Watt 

 had engines in Cornwall working expansively, as at Wheal Gons 

 and Wheal Chance in Camborne ; but in these the steam was not 

 raised higher than before, and the piston made a considerable 

 part of the stroke therefore before the steam-valve was closed. 



" In 1798, on account of a suit respecting their patent, which 

 was carrying on by Boulton and Watt, an account of the duty 

 of all the engines in Cornwall was taken by Davies Gilbert, Esq., 



