Draw the tangents at A and B, meeting at T; draw TV parallel to the axis of the parabola, meeting the arc in C and the chord in V; and M draw the tangent at C, meeting AT and BT in a and b.
These semicircles and the circles A'A' are joined by tangents and short arcs struck from the centre of the figure.
Another of Roberval's discoveries was a very general method of drawing tangents, by considering a curve as described by a moving point whose motion is the resultant of several simpler motions.
He also finished his Tabulae Directionum (Nuremberg, '475), essentially an astrological work, but containing a valuable table of tangents.
It gives tables of sines and cosines, tangents, &c., for every to seconds, calculated to ten places.
