312 NATURE OF CAPITAL AND INCOME [Cuar. XVII 
certain amount of logs, the turning out of which is credited 
to the camp, but debited to the mill. In the diagram, 
the space between the base line and the first curve above 
it represents the capital curve of the logging camp, and 
the space above this curve represents the capital curve of 
the sawmill. At the time of the transfer of logs from 
the one category to the other, there is a corresponding 
diminution in the capital-value of the logging camp, but 
an increase in the capital-value of the sawmill. 
The characteristic of such an interaction or couple is 
that it leaves unbroken the upper curve of final summation. 
  
  
N 
B 
Sapling Tree 
Cc D 
Fie. 24. 
There is no carrying forward of teeth, as by the dotted lines 
in the previous diagram, — or rather, the carrying forward 
results in a cancellation. The interaction is merely a 
sacrifice of one capital for the benefit of another, and does 
not disturb the total. 
If the interaction BC is greater than we have represented 
it in the diagram, so that C is lower and B higher than in- 
dicated, the discount curve AB will be nearer coincidence 
with MN, and CD nearer coincidence with XY. We may 
suppose a case in which coincidence is reached. This case 
is represented in Figure 24. Here BC represents such an 
interaction as occurs when one. capital good is completely 
transformed into another, as when the “sapling” becomes a 
‘““tree’” at a certain definite point of time. The capital-value