Fig. 13.

From C as a centre, describe a circle whose diameter will be equal to the width of the boiler outside of the wire, and draw the line A B perpendicular to the line E F, having it pass through the point D, which is one-half of the length of the boiler; then mark the point J one quarter of an inch or more as you wish, for the pitch of the cover, and apply the corner of the square on the line A B, allowing the blade to fall on the circle at H, and the tongue at the point J; then draw the lines H B, B J, G A and A J, which completes the description.