4 MK. J. H. GRINDLEY ON AN EXPERIMENTAL INVESTIGATION OF 



or, if S 2 is equal to unity, in which case the steam becomes superheated, we have 



S 1 (H 1 -/>,) + A, + g + H, = H 2 + |! + K(T t > - T 2 ) . . . (3). 



Further, if we make u l and n 2 small enough to be neglected, and ensure that Hj = 0, 

 we get instead of (2) and (3) the equations 



- K S^H, - /*,) + A, = S,(H 3 -hj + h; . (4), 



when So is not equal to unity, and 

 . , .' ". S^H, - hj + A, = H 2 + K(TV - T 2 ) (5), 



when S 2 is equal to unity. 



The second of these two equations is the one used in wiredrawing experiments in 

 which it is sought to determine the initial dryness of the steam, for this purpose a 

 value of K being assumed, which is usually REGNAULT'S determination of the mean 

 specific heat at constant pressure (atmospheric) from 248 to 428 F. approximately. 



Now from previous experiments made with superheated steam there appears to be 

 good reason for thinking that when the steam is superheated to a considerable degree 

 its condition approximates to that of a perfect gas. If in any wiredrawing experi- 

 ments, such as those here described, the amount of superheating in the wiredrawn 

 steam is sufficient to bring it to the gaseous condition, the temperature of the 

 wiredrawn steam will suffer no further diminution, however much the wiredrawing be 

 increased by lowering the pressure below the orifice. If such a condition could be 

 experimentally obtained, it would be then easily possible to obtain the value K p of 

 the specific heat at constant pressure of steam gas. But RANKINE has proved* that 

 if Hj 1 be the total heat of gasification of steam gas at temperature T 2 X from any 

 temperature T at which saturated steam is sensibly a perfect gas, the operation being 

 performed under constant pressure, then 



H 2 > = HO + K, (TV - T ) (6), 



where H is the latent heat of evaporation of saturated steam at the temperature T . 

 RANKINE assumed that saturated steam at 32 F. was sensibly a perfect gas, in 

 which case the formula takes the form 



H 2 > = 10917 + K/Ta 1 - 32) (7). 



The formula may, however, be put in the more general form 



H 2 1 = A + BT, 1 /.".''' ...... (8), 



the constants A and B being obtained from any two experiments in each of which 

 the perfectly gaseous condition is obtained by wiredrawing steam having a known 

 total heat Hj 1 in its initial saturated condition. 



* ' The Steam Engine,' p. 330, 



