Wall Surveyors' Vernier Compass

This is an usually large compass, with an unusual skeletal design. The rim is graduated to 30 minutes. The variation arc on the north arm extends 20 degrees either way, and is read by folded vernier to single minutes. Two level vials are on the south arm. The "Thos Wall Cleveland O." signature refers to Thomas Wall (1797-1854), an instrument maker who worked in Cleveland, Ohio, in the years 1845-1854.

Natural History Museum