EXPERIMENTAL STUDY OF HEAT LEAKAGE. 791 



the plug, and hence that there is no heat-leakage. Under these 

 circumstances, the energy supplied per unit mass of fluid is equal to 

 fiCp-Ap, in which n is evaluated at the mean of the high and low side 

 pressures, and at the mean of the high side temperature and of what 

 would be the low side temperature if the experiment were adiabatic, 

 and Cp is evaluated at the mean of these temperatures and at the low 

 side pressure. The product fxCp is in several respects of more impor- 

 tance than n alone, and obviously a direct measurement of /iCp has 

 advantages over its determination by separate measurements of ^l 

 and of Cp, particularly as it is the manner of variation of nCp with 

 pressure and temperature which it is useful to know. 



In the present work, eleven measurements of nCp have been made, 

 all at approximately the same pressure and temperature at which 

 the /^-measurements were made. Two were made with plug A2, 

 four with plug A3, and five with plug A4. The measurements, 

 when combined with the low values of n obtained with the earlier 

 apparatus, led to values of Cp which were so much in excess of the 

 reliable results obtained by Knoblauch ^ and by Knoblauch and 

 Mollier,^° that the inference seemed unavoidable that some large 

 source of outward heat-leak must have been operative. In addition 

 to this, the measurements were discordant to a degree indicating 

 accidental errors of much greater magnitude than occur in the adia- 

 batic experiments. The results on jj. obtained with the later apparatus 

 bring the mean of these determinations of nCp into much closer 

 agreement with Knoblauch and Mollier's values of Cp, however. 

 These nCp measurements thus afford another check on the accuracy 

 of the value of /x obtained in the experiments already discussed. 

 They are not offered as representing dependable values of fxCp but 

 merely as having a certain bearing on the legitimacy of the claim 

 which has been made in respect to the reliability of fj. as determined 

 by the S-type of radial flow plug. 



If each of the several measurements of piCp is divided by the proper 

 value of Cp, taken for the conditions of the experiment from table 4 

 of the paper by Knoblauch and Mollier above referred to, there result 

 eleven values of /x. After these are reduced to 165° C, their mean 

 is 3°.26 C. cmVkgm., with an average deviation for the entire eleven 

 of 0°.09 C. cm.Vkgm. If one divides this deviation by the square 



9 Knoblauch and Jakob, Mitteilungen iiber Forschungsarbeiten des Vereines 

 deutscher Ingenieure, 35, 109 (1906). 



10 Knoblauch and Mollier, ZS. des Vereines d. Ing., 1911, p. 655. 



