131 



" These obvious facts and inferences could not have es- 

 caped the notice of the Comte de Pambour, and accordingly, 

 in his Treatise on Locomotive Engines, he has made some re- 

 marks on the connexion betM^een the area of the steam pipe 

 and the pressure of steam in the cylinder. In his Treatise on 

 the Steam Engine, however, — the best, and, as far as I know, 

 the only systematic work on the subject based on correct 

 principles, — the author has not only omitted all reference to the 

 effect of the magnitude of the steam pipe on the pressure of 

 steam in the cylinder, but has made use of some expressions 

 which might lead casual readers to form incorrect notions on 

 this point. Thus, having determined that the maximum use- 

 ful effect, with a given expansion, is obtained when the load 

 of the engine is the greatest possible, and that this takes 

 place when the pressure P' is greatest, he says :* ' The maxi- 

 mum useful effect will be given by the condition P'= P, or 



S I 



a{n + qP) ' I \ c 



This is, then, the velocity at which the engine must work, in 

 order to obtain the greatest effect possible ; and the equation 

 P - P shows reciprocally that when this velocity takes place 

 the steam enters the cylinder at full pressure, that is, nearly 

 at the same pressure which it had when in the boiler.' And 

 so also in his determination of the absolute maximum of use- 

 ful effect,! he supposes -j variable, but always connected with 

 the velocity by the above equation ; the velocity must, there- 

 fore, also vary, but as long as this equation is satisfied he con- 

 siders the engine to work at ' full pressure.' 



Now this equation is the same as equation (5) given above, 

 and, as 1 have shown, is merely a statement that if the velo- 

 city V be ivithin the limits assigned by equations (3) a7id (6), 

 the engine will work with uniform velocity, and at the full 



Page 125, English edition. f Pages 134, 135. 



