This section is from the book "The Tinman's Manual And Builder's And Mechanic's Handbook", by Isaac Ridler Butt. Also available from Amazon: The Tinman's Manual And Builder's And Mechanic's Handbook.
[Drawn for this work by L. W. Truesdell, Tinman, Owego, N. Y.] Original.
Span the dividers any distance you wish, and place one foot on the circumference AB, and describe the semicircumferences CD, EF, GH, and IK, and through the points of their intersection PQ and RS, draw two indefinite lines LM and NO; the point of their intersection T, will be the centre desired.
To find the area of a sector. Say as 360° is to the degrees, etc, in the arc of the sector, so is the area of the whole circle to the area of the sector. Or multiply the radius by the length of the arc, and half the product will be the area.