DAVIS. — CERTAIN THERMAL PROPERTIES OF STEAM. 281 



Thomas' values of C p at saturation correspondingly too high. Only 

 recently has it become evident how difficult it is to remove the last 

 traces of moisture from apparently dry steam, and if any remained in 

 Regnault's steam, it would have made his results too low, just as they 

 seem to be. 



The mean of Henning's and Joly's values of H W o is 639.04 if both 

 are weighted alike, or 639.11 if Henning's has (as it seems to deserve) 

 twice the weight of Joly's. The final formula for H is, therefore, 



H = 639.11 + 0.3745 (t - 100) - 0.000990 (t - 100) 2 mean calories. 



The steam table of Marks and Davis, which was computed before 

 the appearance either of Henning's second paper or of Barnes' revision 

 of his value of J, was based on H ino = 639.08, which, as it happens, is 

 between the two means just found, and nearer to either of them than 

 the limit of error of the work demands. The values of //, L and L/T 

 in that table will be used in the rest of this paper as representing the 

 best available data. 



C. Extrapolation formulae for H and L. — The range within which 

 the new II formula holds has been set as from 100° to 190°. Above 

 the latter temperature no observations are available. It is often 

 important, however, both in scientific and in technical work, to have at 

 least reasonably accurate steam tables at considerably higher tempera- 

 tures. It is, therefore, desirable to develop as safe an extrapolation 

 formula as possible for either H or L. 



For this purpose the second degree H formula proposed above is 

 wholly unsuited. Within the range for which it is proposed, it happens 

 to be an unusually good three term Taylor's series development of the 

 true function but it cannot be extrapolated safely either up or down. 



That it cannot be used near 0°, is seen from Figure 6, where the 

 small circles, not previously mentioned, represent values of the deriva- 

 tive of H with respect to t, obtained from the five sets of experimental 

 values mentioned on page 268. It is evident that the graph of dHjdt 

 against t is not a straight line over the whole range from 0° to 200°. 

 No second degree formula that fitted the observations above 100° 

 could be expected to reproduce those near 0° also. 



That a second degree formula is no less unsatisfactory for a extrapo- 

 lation to high temperatures can be shown as follows. Let it be as- 

 sumed that the top of the steam dome on either the p v or the T N 

 (temperature-entropy) plane is round like Figure la and not pointed 

 like Figure lb. 29 This is the usual assumption, and it is corroborated 



29 It follows from the Clapyron equation that if the dome is round on 

 either plane, it will be on both. 



