LXx. DISCUSSION. 
grams, and it is proposed to show how the same problem may be 
represented by means of entropy-temperature diagrams. When 
a substance takes in or rejects heat, it is said to change its entropy. 
The change of entropy being expressed thus 
Pm 
8@Q represents the heat taken in or rejected, and 7 the absolute 
temperature which the substance had at the time. When an 
entropy temperature curve is drawn for a complete cycle of 
changes it forms a closed figure, since the substance returns to its 
initial state. To find the area of the figure we must integrate 
through the complete cycle thus—Let Q, and Q, represent, the 
héat received and rejected respectively, and A W the work done 
in thermal units, then— 
JrdQ=Q: -Q2 = WA, A=s45. 
For Carnot’s cycle— 
SE en et EAH ae ry 
2 Ts Q1 71 be 
and the diagram is represented by a rectangle A BC D. 
We can show by means of 
this diagram why a dry air 
machine is necessarily less 
economical than a machine 
which depends on the principle 
of evaporation and compress 
ion of a vapour such as amr 
monia, carbon dioxide, sulphur : 
dioxide etc., but in the gis 
place the cycle for a dry ar 
machine will be compared with | 
Carnot’s cycle. In the dry air 
machine the heat is not all 
abstracted at r,, but the reduc- 
ni | < : tion of temperature in the cold 
chamber is effected by intro — 
