Engraving from 'Theatrum Arithmetico Geometricum' by Leupold, published in 1724. John Napier (1550-1617), the inventor of logarithms, also invented this aid to calculation known as 'Napier's Bones'. The 'bones' consisted of a set of rectangular rods, each marked with a counting number at the top, and the multiples of that number down their lengths. When aligned against the row of multiples as shown, any multiple of the top number can be read off from right to left by adding the digits in each parallelogram in the appropriate row.
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