244 Mr. W. J. M. Rankine on the Mechanical Action of Heat. 



By using Table I. according to the directions prefixed to it, 

 the volume of one pound of steam at the pressure Pj in cubic 

 feet is calculated, and thence by equation (60) the weight of 

 steam per stroke, according to theory, which is compared with 

 the weight as ascertained by experiment. 



Further to illustrate the subject, the useful effect or duty of a 

 pound of steam is computed according to the theory and the 

 experiments respectively, and the results compared. 



The followiug table exhibits the results : — 



Comparison of the Theory with Mr. Wicksteed's Experiments on 

 the Cornish Pumping -engine at Old Ford. 



This comparison sufficiently proves that the results of the 

 theory are practically correct. 



It is remarkable, that in every instance except one (experi- 

 ment E) the experimental results show a somewhat less expendi- 

 ture of steam per stroke, and a greater duty per pound of steam, 

 than theory indicates. This is to be ascribed to the fact, that 

 although the action of the steam is computed theoretically, on 

 the assumption that during the expansion it is cut off from ex- 

 ternal sources of heat, yet it is not exactly so in practice; for 

 the cylinder is surrounded with a jacket or casing communicating 

 with the boiler, in which the temperature is much higher than 

 the highest temperature in the cylinder, the pressure in the 

 boiler being more than double the maximum pressure of the 

 steam when working, as columns (2) and (5) show. There is, 

 therefore, a portion of steam of whose amount no computation 

 can be made, which circulates between the boiler and the jacket, 

 serving to convey heat to the cylinder, and thus augment by a 

 small quantity the action of the steam expended ; and hence the 

 formulae almost always err on the safe side*. 



* Another, and ))erhaps a more important cause of the excess of the 

 actual above the calculated performance of a j)oun(1 of steam is the follow- 

 ing. It has been proved experimentally by Messrs. Joule and Thomson 

 in the case of air and other gases, and by Mr. C. W. Siemens in the case 



