Sec. 4] SUMMARY OF PART III 309 
ment that the capital is a mean proportional between its 
past cost and its future return. 
§ 4 
Another use which may be made of the diagrammatic 
representation is to exhibit in a compact manner the sum- 
mation of both the capital and the income of any given 
enterprise or community. This may be done simply by 
  
  
fi ———C 
Ev 
v 
  
Fia. 21. 
adding together the corresponding ordinates or vertical 
lines in any number of capital curves, representing any 
number of articles of wealth or property. Let Figure 18, 
for instance, represent one capital curve, and Figure 19 an- 
other. Figure 20, formed by combining Figures 18 and 19, 
then represents the sum of the capital and income of both. 
Figure 20 is derived from Figures 18 and 19 in such a 
manner that the ordinate B is the sum of the individual 
ordinates b’ and b”, and in like manner any other ordinate, 
C, is the sum of the corresponding individual ordinates ¢ 
and ¢’. From the rule by which Figure 20 is constructed, 
it is evident that every tooth in the constituent curves, 
such as a’ and a”, will be reproduced in the combined dia- 
gram. In Figure 20, therefore, the ordinates represent the 
combined capital-values at various points, while the two 
teeth a” and a’ represent the total income accruing in that 
time interval which includes them. Thus, Figure 20 epito- 
mizes the summation both of capital and income.