Q. In the "On the House " column in the October 2000 issue, we described a method for finding the radius of a circle when only an arc of the circle is known. This problem needs to be solved when laying out an arch-top window opening or a barrel-vaulted ceiling.

A.Two readers, Warren Barstad of Fertile, Minn., and Peter Hueber of Truckee, Calif., wrote to suggest that the problem can be solved a different way, using a mathematical formula. Here is the formula to find the radius (r) of a circle, when presented with an arc measuring w inches wide and h inches high: