The exact definition of sand varies. The scientific Unified Soil Classification System used in engineering and geology corresponds to US Standard Sieves, and defines sand as particles with a diameter of between 0. 074 and 4. 75 millimeters. By another definition, in terms of particle size as used by geologists, sand particles range in diameter from 0. 0625 mm (or 1⁄16 mm) to 2 mm. An individual particle in this range size is termed a sand grain. Sand grains are between gravel (with particles ranging from 2 mm up to 64 mm by the latter system, and from 4. 75 mm up to 75 mm in the former) and silt (particles smaller than 0. 0625 mm down to 0. 004 mm). The size specification between sand and gravel has remained constant for more than a century, but particle diameters as small as 0. 02 mm were considered sand under the Albert Atterberg standard in use during the early 20th century. The grains of sand in Archimedes' The Sand Reckoner written around 240 BCE, were 0. 02 mm in diameter. A 1938 specification of the United States Department of Agriculture was 0. 05 mm. A 1953 engineering standard published by the American Association of State Highway and Transportation Officials set the minimum sand size at 0. 074 mm. Sand feels gritty when rubbed between the fingers. Silt, by comparison, feels like flour.
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