English, c. 1820, in boxwood and brass, double sided with numerous finely divided logarithmic scales and early form of cursor, time side with scales marked Half Sum, Lat, Diff, Time PM, Time AM, cu graduated cursor engraved 'Declination', lunar side Hor Par, App Dist, App Alt, Corr, Comp Corr, signed 'D Thomson Invt' and 'Bate London no 320', length 62.8cm. Note: for determining longtitude at sea. The lunar distance method continued to be used well into the 19th century until marine chronometers became more affordable. This is the only two-foot example we have found, the two other examples we have found being three-foot (see National Museums of Scotland).