Napier’s rods

These wooden reckoning rods were probably made during the late 17th century in England or Scotland. The numbers and symbols are scored into their surface. This set comprises 28 rods, one of which is wider and is used for finding square roots. Together with an accompanying board they all fit into a leather folder. John Napier, Baron of Merchiston, developed his rods, which first became known through his book Rabdologia published in 1617, from the well-known “Gelosia Method“ (“multiplicare per gelosia“), which was very popular for multiplying numbers with several digits. Napier adapted this paper-and-pen method to one with movable rods. Each of these has the multiples of a particular digit inscribed on one face. In order to multiply two numbers, the corresponding rods are repeatedly juxtaposed in a certain way, giving the partial results of the product which are then added separately by the user who also has to deal with any tens-carry operations.