DAVIS. — CERTAIN THERMAL PROPERTIES OF STEAM. 269 



present procedure to errors in C P . The result of such a reversal of their 

 reasoning is the formula which was suggested two years ago, namely, 



H=H 1M + 0.3745 (t - 100) - 0.000990 (t - 100) 2 . 



This formula belongs only to the range between 100° and 190° C. 

 Within this range its accuracy is believed to be of the order of one 

 tenth of one per cent. 



When the new formula was announced, there were no direct experi- 

 mental determinations of H or L above 100° by which it could be 

 checked except Regnault's, but more recently Henning 13 has published 

 a continuation of his admirable research to 180°. The extent of the 

 agreement of this with the formula will be discussed later. 



As has been indicated, the computations leading to the new formula 

 involve two different sorts of experimental data. The first of these, 

 namely, the throttling experiments of Grindley, Griessmann and Peake, 

 have been sufficiently discussed in a previous paper. 14 The second, 

 the direct determinations of Cp mentioned above, will be discussed in 

 the next section. 



2. On the C p Values Available for the Present Purpose. 



There are three direct calorimetric determinations of the variation of 

 C P with pressure and temperature, namely, those of Lorenz, 15 of Knob- 

 lauch and Jakob 16 and of Thomas. 17 That of Lorenz was' the earliest 

 of the three and was, as he himself says, a preliminary survey for the 

 sake of those engineers who could not afford to wait for more accurate 

 work. It is not ordinarily considered comparable with Knoblauch's. 



Both Knoblauch's and Thomas' results were obtained by determin- 

 ing the electrical energy necessary to increase by a known amount the 

 temperature of previously superheated steam. In Knoblauch's appa- 

 ratus the original superheating took place in an electrical preheater. 

 The steam was then still further heated in a separate calorimeter, the 

 energy added being the object of a direct measurement. In Thomas' 

 case the separate preheating and calorimetric coils of Knoblauch's 

 apparatus were replaced by a single coil, by means of which initially 

 wet steam was brought, first just to dryness, and in a later experiment 



13 Wied. Ann., 1909, 29, 441. 



14 These Proceedings, page 241. 



15 Zeitsch. Ver. d. Ing., 1904, 48, 698; Phys. Zeitsch., 1904, 5, 383; and 

 Forschungsarb., 1905, 21, 93. 



16 Zeitsch. Ver. d. Ing., 1907, 51, 81 and 124 ; Forschungsarb., 1906, 35, 109. 



17 Proc. Am. Soc. Mech. Engs., 1907, 29, 633. 



