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 unbroken
 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