Let the square A B C D be again described (in the third figure), and in it the quadrants A B D, B C A and D A C; and dividing each side of the square A B C D in the midst in E, F, G and H, let E G and F H be connected, which will cut one another in the centre of the square at I, and divide the arch of the quadrant A B D into three equal parts in K and L. Also the diagonals A C and B D being drawn will cut one another in I, and divide the arches B K D and C L A into two equal parts in M and N. Then with the radius B F let the arch F E be drawn, cutting the diagonal B D in O; and dividing the arch B M in the midst in P, let the strait line E _a_ equal to the chord B P be set off from the point E in the arch E F, and let the arch _a b_ be taken equal to the arch O _a_, and let B _a_ and B _b_ be drawn and produced to the arch A N in _c_ and _d_; and lastly, let the strait line A _d_ be drawn. I say the strait line A _d_ is equal to the arch A N or B M. I have proved in the preceding article, that the arch E O is twice as crooked as the arch B P, that is to say, that the arch E O is so much more crooked than the arch B P, as the arch B P is more crooked than the strait line E _a_. The crookedness therefore of the chord E _a_, of the arch B P, and of the arch E O, are as 0, 1, 2. Also the difference between the arches E O and E O, the difference between the arches E O and E _a_, and the difference between the arches E O and E _b_, are as 0, 1, 2. So also the difference between the arches A N and A N, the difference between the arches A N and A _c_, and the difference between the arches A N and A _d_, are as 0, 1, 2; and the strait line A _c_ is double to the chord B P or E _a_, and the strait line A _d_ double to the chord E _b_.