Detail of model. An interactive model designed and fabricated by the Science Museum's Interactive Development Unit in 2001. John Napier (1550-1617), the inventor of logarithms, also invented this aid to calculation. The rods are marked with a 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.
© Science Museum / Science & Society Picture Library