[Illustration: FIG. 3]

The radius = 1.

AB, the chord, read direct on the straight scale.

AFB, the length of the arc, read direct on the back or under
surface of the plate.

FH, the versed sine, read direct on the curved scale.

ACB, the angle in the segment, read direct on the graduated
edge.

EAB, the angle between the chord and the tangent, read direct
on the graduated edge.

GAB, the tangential angle = 180 deg. - ACB.

AOB, the angle at the center = 2GAB.

AGB, the angle between the tangents = 180 deg. - AOB.

OAB, the angle between the chord and the radius = EAB - 90
deg.

AH_{2}
GF = --------- - FH.
HO