DAVIS. — CERTAIN THERMAL PROPERTIES OF STEAM. 287 



We now know that the specific heat of water decreases with in- 

 creasing temperature from 0° to above 25°. There is some difference 

 of opinion between Barnes and Dieterici, the two leading investigators 

 of the subject, as to the exact shape of the curve of variation, but it 

 is near enough to the truth to take, as in the steam tables already 

 mentioned, a mean curve between that of Barnes and that of Dieterici, 

 giving the former twice the weight of the latter. 



Regnault's values of C p have been recomputed from the data in his 

 memoir, using his own value for the coefficient of expansion of sheet 

 iron, modern data for the density of water, and the mean curve just 

 mentioned for the specific heat of water. The new results are given in 

 the fourth column of Table IV. They are somewhat lower than his 

 original values and are thereby brought nearer to the corresponding 

 values obtained by Knoblauch and Jakob, which are given in the fifth 

 column of the table. 



In the present unsettled state of our knowledge of C p , Regnault's 

 work should have considerable weight. 



The only other important direct experimental determination of C p is 

 that of Holborn and Henning. 34 Their work, like Regnault's, was only 

 at atmospheric pressure, but, unlike his, it covered a very wide temper- 

 ature range, reaching 1400° C. It is certainly to be regarded as standard 

 in the region of high superheats. It shows that in that region C p in- 

 creases with increasing temperature, but not as rapidly as Knoblauch's 

 curves would indicate. 



In a "Memorandum by the Chief Engineer for the year 1906 to the 

 Executive Committee of the Manchester Steam Users Association," 35 

 the National Physical Laboratory at Teddington, England, is said to 

 have found C p = 0.532 at saturation at 4.3 atmospheres (147° C). 

 This value lies remarkably close to Knoblauch's saturation curve. 



Ba. Throttling experiments. — The failure of even the best throt- 

 tling experiments to give satisfactory values of C P by the ordinary 

 methods has already been mentioned. A new method elaborated by 

 Dodge 36 is much more promising, but no thoroughly reliable results 

 have yet been obtained by it. 



Bb. Characteristic equations : — If a sufficiently accurate character- 

 istic equation, f (p, v, t) =0, were known for superheated steam, much 

 useful information about G P could be obtained from Clausius's equation 



\dp) T T \d?) t 



34 Ann., 1907, 23, 809. 35 Manchester, June 4, 1907. 



36 Proc. Am. Soc. Mech. Engs., 1907, 28, 1265 and 1908, 30, 1227. 



