Bill Potts wrote in USMA 12300: >However, the inch is defined as 25.4 mm (or, if you prefer, 2.54 cm). Feet >and yards are derivations of that definition. > >An inch is not legal unless it measures exactly 25.4 mm. Only since 1959 when the USA and the British Commonwealth countries agreed on that definition, which they got from the International Organization for Standardization, and perhaps from ISA before that. The American incn was defined in 1893 as 39.37 inches = 1 meter. The British yard was defined as 0,914 399 m or the British inch was 25,399 972 mm. Ä conference of English-speaking countries in 1959 agreed to adopt the ISO definition of 1 inch = 25.4 mm. This had the great advantage that any machine tool with a gear wheel with 127 teeth could work in either system. The American Geodetic Survey refused to alter its maps because it was in the process of changing them anyway from feet to meters, and the old unit became known as the "survey foot". Joseph B. Reid 17 Glebe Road West Toronto M5P 1C8 Tel. 416 486-6071
