WORK OF STEAM AT EFFLUX. 257 



follows that the work of steam between any given pressures 

 P and Pj is always the same, whatever may be the nature 

 of the space through which the steam expands, and that it 

 increases with the pressure P at which the steam is gener- 

 ated ; since therefore the quantity of fuel necessary to 

 evaporate a given volume of water is always independent of 

 the pressure at which the steam is formed (Art. 135), it fol- 

 lows that it is most economical to employ steam of as high 

 a temperature as possible. 



137. Work of Steam at Efflux. Let w be the 



weight of Q cubic feet of water evaporated per second, F 

 the volume of steam from a cubic foot of water at the press- 

 ure P, which is the pressure of the steam at the point of 

 efflux, k the section of the orifice in feet from which the 

 steam is discharged, and v the velocity per second. Then 

 calling Uq the work stored in the steam at efflux, we have 



Since V = f(P), and QV = kv, we have 



w 

 ~ 62T5 



which in (1) gives 



Hence, the work of steam discharging itself from an 

 orifice varies as the cube of the water evaporated. 



COR. 1. The work stored in the steam at efflux is due to 

 the work of expansion between the pressures P and P l ; 



