34 



On the other hand, experiments made by the author* prove 

 that steam, when expanded spontaneously, is superheated steam, 

 being a verification of Eegnault's discovery, that the total heat of 

 steam increases with its pressure. 



When steam is expanded behind a working piston the excess of 

 free heat is first absorbed, or changed into dynamical effect, and 

 if that does not suffice, partial condensation must take place. It 

 appears, however, to the author, that Messrs. Rankine and 

 Clausius undervalue .the amount of free heat, and, therefore, over- 

 estimate the amount of condensation during expansion, by taking 

 the specific heat of steam at 0*305 (Regnault's co-efficient of 

 increasing heat), which there seem good reasons to believe is far 

 too low, because : 



1st. The specific heat of an elastic fluid must be proportionate 

 to its rate of expansion by heat. It has been shown, however, in 

 experiments instituted by the author above referred to, that the 

 rate of expansion of steam near its point of saturation is about 

 three times greater than that of air at the same temperature, 

 which would make its specific heat 3 x -267 = ' 801, diminishing 

 however rapidly with the increase of temperature. 



2nd. The actual forms of diagrams, taken from the best ex- 

 pansive steam engines, do not show the effect of condensation : 

 the ordinates of the lower portion of the curve are indeed higher 

 than those given by Watt's law, starting from the same point. 

 This is shown by Fig. 2, Plate 5, which is a diagram taken from 

 the Old Ford Engine, by Mr. W. Pole,t in which a a a is the 

 actual curve, b b J-the curve representing Watt's law, and c c c 

 the curve of equal heat. The rise of the actual curve towards the 

 end may in part be owing to the generation of steam from the 

 heated sides of the cylinders ; but it could not be supposed that 

 the effect of such generation would equal that of spontaneous 

 condensation throughout the body of the steam. Moreover the 

 actual curve proves to be almost the perfect dynamical curve, as is 

 proved by the equal areas, or fields of power, obtained by drawing- 

 lines from points of the curve of equal progression of temperature. 

 If the limits of the sheet of diagrams had admitted of a con- 



* Vide Proceedings of the Institution of Mechanical Engineers, for June 1, 

 1852, p. 24, ante. 



f Vide Treatise on the Cornish Engine, by W. Pole, Part III. 



