Investigation into the design and accuracy of old-time instruments is always of interest. The Nocturnal probably used from the sixteenth century onwards for estimating the approximate local time at night is well known; with this primitive instrument the Pole Star was viewed through a central hole in a pair of wooden or brass discs. A pivoted arm turned to coincide with the ‘Guards’ of the Lesser Bear, the local time being deduced from an engraved scale. A seventeenthcentury improvement included an extra scale for determining the time by aligning the pointer with the Great Bear ‘Pointers’.