A wooden chart rule, on the front of which is a protractor for laying off courses on a chart, as well as a scale in inches. On the back are several scales to aid with the mathematical calculations carried out as part of a ship's navigation. The scales include chords, sines, tans and a line of numbers.

The maker's name is inscribed on the front, as are the words 'Lady of St Kilda Yacht T.D.A. 1837'. This refers to Sir Thomas Dyke Acland (1787-1871) who bought a schooner in 1834 that he named 'Lady of St Kilda'. After Acland sold the yacht, she was sailed to Victoria, Australia, in 1841. The area where the yacht was moored became known as 'the St Kilda foreshore', and the municipal district was later named St Kilda.