Fig. 77. To describe an Octagon or Square Top or Cover.

Describe a circle, three-quarters of an inch larger in diameter than a circle that will cut each corner of the article the top or cover is for; set off the squares from B to C; take one half of the largest square; and with B and C as centres, describe arcs G and H; then with A as centre, describe the arc cutting the square at I and the arc D; where the arcs GB and HD intersect, draw the lines AE and AF, also the lines BE and CF.