Sec. 5] SUMMARY OF PART III 311 
top, as represented by the dotted lines. Similarly, a/,a”, and 
a" are transmitted to the top. The final curve at the 
top thus shows in its separate teeth all the income items 
contained in the curves of which it is the sum. 
§5 
In case two teeth in separate curves occur at the same 
instant, the combined curve will, of course, have a large 
Saw Mill 
Logging Camp 
~~ 
Fi6. 23. 
In case one tooth is positive, 
the other is negative, or outgo, 
ill result in a small tooth equal to 
difference will be zero if the two 
tooth equal to their sum. 
representing income, and 
the coincidence of these W 
their difference, and this 
items are equal. 
The most important case of this kind occurs when there 
are“interactions.” Ithas been explained that an interaction 
is an income item for one capital which at the same time 
is an outgo item for another. If the curves for these two 
capitals are superimposed, the equal income and outgo 1tems 
will cancel, and the resulting combined curve will be un- 
broken at the point representing the time of interaction. 
This is shown in Figure 23, giving the typical history of 
lumber operations. Every year a logging camp yields a