MR W. J. MACQUORN RANKINE ON THE DENSITY OF STEAM. 155 



V. The Volume of one Pound of the Liquid Water is itself too small to affect 

 the question. 



VI. The received value of the Mechanical Equivalent of a unit of Heat cannot 

 err by so much as aioth part of its amount. 



Conclusions. 



17. It appears, then, that none of the data from which the theoretical calcu- 

 lations are made are liable to errors of a magnitude sufficient to account for the 

 differences between the results of those calculations and the results of Messrs 

 Fairbairn and Tate's experiments, small as those differences are in a practical point 

 of view. Neither does there appear to have been any cause of error in the mode 

 of making the experiments. There remains only to account for those differences, 

 the supposition that there was some small difference of molecular condition in the 

 steam whose density was measured in the experiments of Messrs Fairbairn and 

 Tate, above 212°, and the steam whose total heat of evaporation, as measured by 

 M. Regnault, is the most important of the data of the theoretical formula, — a 

 difference of such a nature as to make a given weight of steam in Messrs Fair- 

 bairn and Tate's experiments occupy somewhat less space, and therefore require 

 somewhat less heat for its production, than the same weight of steam in M. Reg- 

 nault's experiments at the same temperature. That difference in molecular 

 condition, of what nature soever it may have been, was in all probability con- 

 nected with the fact, that in the experiments of Messrs Fairbairn and Tate, the 

 steam was at rest, whereas in those of M. Regnault it was in rapid motion 

 from the boiler towards the condenser. It is obvious, however, that in order to 

 arrive at a definite conclusion on this subject, further experimental researches are 

 required. 



VOL. xxiii. part I. 2 u 



