Finding a circle's centre from a chord and its sagitta
A worked rule: chord 8, sagitta 2, giving the centre of the whole circle
The pen heading 'Of chord and arc' sits above a compass-drawn circle whose vertical diameter and horizontal chord are marked off in dotted divisions. The text sets out a general rule for locating a circle's centre from a chord and its sagitta (versine): with a chord of 8 and a sagitta of 2, half the chord is multiplied by the sagitta to give 8, then the sagitta's 2 is added to reach 10, and half of that, measured along the sagitta line, fixes the centre. Faint, indeterminate red-chalk sketches fill the lower part of the leaf.
On this page
Circle with diameter, chord and marked sagitta
A compass circle is crossed by a vertical diameter and a horizontal chord, both stepped off in dotted divisions. The construction supports measuring inward along the sagitta to locate the circle's centre.
General rule for the centre from chord 8 and sagitta 2
Half the chord (4) is multiplied by the sagitta (2) to make 8; this 8 is drawn along the sagitta line, then the sagitta's own 2 is added to give 10. Half of 10, taken along that line, is the centre of the whole circle, which the note calls a general rule.