A specially graduated arc attached to the vertical circle of an alidade or transit to simplify the computation of elevation differences for inclined stadia sights (without the use of vertical angles). The arc is so graduated that each division on the arc is equal to 100 (0.5 sin 2A), where A is the vertical angle. Named after William M. Beaman (1867- 1937), U.S. topographic engineer, who designed it in 1904.