This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
In covering a circular tower with beveled siding, as shown at (a), Fig. 13, it will be found, on bending the strip of siding around the cylinder, that the edges will creep upwards in the middle, as indicated at (b). This is owing to the slope of the back of the siding, next the sheathing, caused by its lapping over the upper edge of the strip underneath, so that it really forms part of a conical surface. It will be necessary to find the slope length of the cone, to serve as a radius to describe the curvature to which the lower edge of the siding should be cut. Draw a vertical center line a b in (c), representing the center line of
Should special siding with a straight back to hug the sheathing, as shown at (e), be used, no curvature will be required for the lower edge, as, in sheathing a cylinder, boards with straight, parallel edges can be used.