KODUCTi 19 



will, howevi T, give an approximate solution directly, and an exact solution 



BterpoiatioiL 



External Work during Adiabatic Expansion. Sin* r no beat is trans- 

 mitted during an adiabatic expansion, all of the intrinsic energy lost is 

 /ed into external work, so that 



W ,- 7| - 0, + x |P| - xvv 



For fxamplf, the external work of one pound of dry steam in expanding 

 illy from 100 pounds to 15 pounds absolute is 



W - 778 (298.1 -- 181.3 + * X 802.4 - 0.8935 X 89.30) 

 W - 121.3 X 778 - <M,.$7 foot-pounds. 



:i should be called to the unavoidable defect of this method of 

 tali ulation of external work luring adiabatic expansion, in that it depends 

 on taking the difference of quantities which are of the same order of 

 magnitude. For example, the above calculation appears to give four 

 places of significant figures, while, as a matter of fact, the total heat // 



from whit h p is derived i* affected by a probable error of - or perhaps 

 more. Both the quantities 



q, + Xl p, and q, + 



a numerical value somewhere near 1000, and an error of- - is 



500 



nearly equivalent to two thermal units, so that the probable error of the 

 above calculation i> nearly t\\<> per i ent. For a wider range of tempera- 

 be error i> less, and for a narrower range it is of course larger. This 

 r .should be borne in mind in considering the use of approximate 

 methods of calculation, for example, by aid of a diagram like the tem- 

 perature entropy diagram. 



Heat Contents. The heat required to raise one pound of water from 

 iig-point to a given temjwrature / corresponding to a pressure p, 

 and to vaporize a part x at that pressure is represented by 



^ f I 



iiiantiiy may be called the heat conu 



