INTRODUCTION 5
Figure 3b
a point M of the first (Fig. 3b). To do this it suffices to move radius OM onto
radius O M (Fig. 3b), which is possible since the two segments are equal.
8. A portion of a circle is called an arc (ApB, Fig. 4).
From the fact that two circles can be superimposed in infinitely many ways,
it follows that it is possible to compare circular arcs from the same circle or in
two equal circles, in the same way in which we compare line segments. For this
purpose one transports the two arcs to a position where they have the same center, a
common endpoint, and they lie on the same side of this common endpoint. Suppose
that the two arcs in this positions are AB, AC; we will say that the first arc is
greater than the second if, starting from the point A and moving along
arc13
AB
we encounter point C before point B (Fig. 5); the first arc is said to be smaller, if
on the other hand the order is ABC (Fig. 4).
Figure 4
Figure 5
8b. We can also define the sum of two arcs AB, BC (Fig. 4) on the same
circle (or on equal circles) by moving them end-to-end.
A circular arc AB can be divided into two or more equal
parts,14
just as a line
segment can.
13It
is essential here to specify the direction of motion (which was not necessary in the case
of a straight line), because the points A, B divide the circle into two arcs, and the order in which
we encounter the points B, C depends on the direction of motion.
14The
remark made about straight line segments in footnote 10 holds here as well.
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