Napier Bones
"John Napier, the inventor of logarithms, also invented this aid to calculation known as 'Napier's Bones' in 1617.

The 'bones' consist 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. Multiplication is thus reduced to addition." National Museum of Sciences, London, "Computing Then and Now."

Picture: David Bauman

X-Number World