Tidal computer
A boxwood instrument consisting of three circular wooden plates (dia: 135 mm, 103 mm, 55 mm) pivoted at the centre, with a brass bolt which passes through brass bushes keyed into the underside of the plates, and retained by an octagonal brass nut.
The edges of the two upper plates are bevelled, the engraved graduations extending down the bevel in each case.

Graduations:
Base plate: Outer ring marks are the 32 points of the compass, each subdivided into thirds. Inner ring is marked I - XII twice clockwise.
Intermediate plate: Outer ring marked 1 - 30 clockwise (age of moon) with half divisions. Inner ring marked with 32 points of compass, divided into halves and quarters. The index is marked "Sol" and with a circle/dot, and its bevelled edge is on the N line.
Upper plate: Marked with the 32 points of the compass, divided into halves and quarters. The index is marked "Luna".
The base plate has a small shaped handle at the North point, marked with a fleur-de-lys and with a small hole.