1824.J On Steam- Engines. 353 



the same time it must be admitted that the improvements in the 

 construction of the steam vessels have materially lessened this 

 serious evil. 



The preceding statements respecting the Cornish steam- 

 engines, is taken from the reports published by Dr. Tilloch, in 

 the Philosophical Magazine ; and the more recent English En- 

 cyclopaedias confirm the facts which have been stated. 



Experiments made in France support the truth of these re- 

 ports. MM. Girard and Prony have made separate compara- 

 tive experiments on the power of low pressure engines, and the 

 condensing engines of mean pressure on Woolfe's system, as 

 improved by Edwards. They find that the latter deserves the 

 preference, as to economy of fuel, though their results do not 

 exactly agree as to the extent of the saving in this respect ; 

 their conclusions, however, tend to the same end, and their dis- 

 crepancies are referable to particular circumstances. 



Instead of estimating the power of a steam-engine by assum- 

 ing the vague and ill- defined power of a horse as unity, it would 

 be better to assume a given weight, raised to a given height in 

 a given time ; as 100 weight raised one yard in a second, which 

 might be called a power. The working force of the engine 

 would thus be indicated by the number of powers it is equal to, 

 which may easily be ascertained by loading the piston with a 

 sufficient determinate weight, and marking the space it passes 

 through, so loaded, in one second of time. 



The tension of the vapour being measured by its relation to 

 the pressure of the atmosphere, taken as unity, it must always 

 be referred to the standard barometrical pressure of 30 inches, 

 and the temperature of 32°. 



According to the preceding details, it may be assumed as 

 incontestable, that it is most economical to employ steam at 

 such a temperature, that its tension shall be equal to that of 

 several atmospheres ; but it is not so easy to decide to what 

 exact tension it should be raised ; or what is the mathematical 

 law which expresses the product of steam-engines' powers in 

 the function of the temperature, and the tension resulting 

 from it. 



" We have hitherto," says the report, " compared low pres- 

 sure engines only with those of mean pressure ; we now proceed 

 to compare them with high pressure engines, which, as is well 

 known, act without condensation of the vapour. 



Mr. Trevithick, in England, and Mr. Oliver Evans, in Ame- 

 rica, are the persons who first made high pressure engines. 



In 1814 Mr. Trevithick exported to Peru nine of these 

 engines, for the purpose of clearing the mines of water, from 

 the accumulation of which many of the richest had been aban- 

 doned : so effectual were the engines, that the treasurer of the 

 province proposed to erect a silver statue to Mr. Trevithick, as 



New; Series, vol. vim. 2 a 



