In 1759 John Bird, commissioned by the Board of Longitude, produced an instrument taking up both suggestions, a sextant of 20 inches radius with a brass frame May, William Edward, A History of Marine Navigation, G. T. Foulis &amp; Co. Ltd., Henley-on-Thames, Oxfordshire, 1973, and 1/3 the weight of the 16 inches reflecting circle, which served as a prototype of the marine sextant still used today.