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| Here a zoom on such a double-face ruler, which will be plastified to give stability and protection. These rulers were invented by the French railroad engineer Henri Genaille in 1891; the mathematician Edouard Lucas proposed to put the rulers on the side of a prism, similar to Napier's bones. Their usage is much faster and easier than Napier's bones, as the digits of the result are simply read out, without any need for mental subtotals. | |
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