"弧的正矢"的一般定义是什么?

弧的正矢是一段弧的弦的中点与圆心的连线的延长线与弧的交点到弦中点的距离。我也看过，我自己推的。这书漏洞百出，翻得不好。

我也在看《自然哲学之数学原理》，去图书馆查了数学词典，最后在维基百科找到了大概的意思，不过时英文的。详细请在英文版维基百科里搜索“versine”。上面解释还比较详细。
"Versines" of arbitrary curves and chords:
The term versine is also sometimes used to describe deviations from straightness in an arbitrary planar curve, of which the above circle is a special case. Given a chord between two points in a curve, the perpendicular distance v from the chord to the curve (usually at the chord midpoint) is called a versine measurement. For a straight line, the versine of any chord is zero, so this measurement characterizes the straightness of the curve. In the limit as the chord length L goes to zero, the ratio 8v/2 goes to the instantaneous curvature. This usage is especially common in rail transport, where it describes measurements of the straightness of the rail tracks[6] and it is the basis of the Hallade method for rail surveying. The term sagitta (often abbreviated sag) is