This wooden instrument is an early form of mathematical calculator called Napier's bones or Napier's rods. Their invention followed the creation of logarithms in 1614 by John Napier (1550–1617), the use of which had improved the speed and accuracy with which mathematicians could compute. Motivated by the ‘desire to reduce the tedium and difficulty of calculation’, which he believed put people off mathematics, Napier published 'Rabdologiae, seu Numerationis per Virgulas' (1617).

In this work he described three instruments designed to make calculations easier. The simplest was a set of rods: on each face was the multiplication table, from one to nine, of a digit. The different rods could be used to perform mathematical functions such as multiplication and division. Subsequent improvements to the initial design included using a box like this that contained cylinders, each divided into strips, so that the numbers could be ‘dialled’. This design was being made in London by the late 17th century. In 1710, for example, Zacharias von Uffenbach purchased from an instrument maker in Westminster Hall ‘two conventional kinds of Bacillus Nepperiansis, one of which does not consist of specially placed pieces of wood but of round rods which are turned in front by means of little knobs’. This particular set was probably intended for use at sea, since the box cover includes a tide table for a range of British ports as well as a perpetual calendar.