Napier’s bones or rabdology

This small piece of furniture was used to hold and operate two calculating devices that use carved ivory rods and several tablets that follow the treatise Rabdologiae, written by John Napier (1550-1617) of Edinburgh in 1617. It was used to simplify multiplication and other arithmetic operations. It has a sliding lid on a hidden compartment to store the portable box of the calculating device known as ‘Napier’s bones’. It was originally made up of 60 four-sided prisms and two tables of powers that were used to add, subtract, divide and find square roots. The drawers contain 200 flat rods and 100 perforated ones, that make up the second abacus – the ‘promptuary’. This was Napier’s last invention, proposed in the appendix of his treatise, which enabled complex multiplications to be solved immediately. The open doors show tables of powers and Pascal’s arithmetic triangle, while there is a tray at the base for arranging the rods and working out the calculations. Although other abacuses made using Napier’s bones have been conserved, the ‘promptuary’ is currently the only known one of its kind.