272 



HEAT. 



and the new scale of level is analogous to the absolute scale of tempera- 

 ture. 



Since the total quantity of energy is proportional to the total number 

 of steps, and the quantity transformed is proportional to the number- 

 descended, the quantity still remaining in the water is proportional to 

 the number of steps remaining. 



If then a quantity of water is started downhill, the energy being 

 transformed as it goes by reversible wheels, the quantity 



Energy still possessed 

 level of water possessing it 

 is constant. 



This is really equivalent to saying that the quantity of water remains 



FIG. 156. Any Reversible Cycle. 



constant. 



Q 



Hence, quantity of water is analogous to the quantity 



~, which we have termed entropy. 

 6 



Reversible Cycles in General ; with Varying Temperatures 

 of Source and Refrigerator, We have found that if Q s of heat is 



taken in by a reversible engine at a definite temperature 6 S , and Q K is 

 given out at a definite temperature R , then ~? ^? = 0. 



But in many cases the heat is not all taken in at one definite tem- 

 perature and given out at another. We have, therefore, to generalise 

 this result. 



Let the working substance go through a cycle represented on the 

 indicator diagram by ABCKc&aA (Fig. 156). 



Take a succession of points ABC . . . very near together, and through 

 them draw adiabatics Aa, B&, Cc across the area. Draw the isothermals 



