This is an unusual and quite rare barometer by Negretti & Zambra, London known as a Hypsometer Aneroid. The dial is graduated in both inches of mercury and and degrees Fahrenheit. The pointer indicates the barometric pressure together with the temperature of free steam (ie, the the boiling point of water). In good working condition, crisp engraved dial and set into an Oak surround with an overall diameter of 160mm.
This instrument is featured in the Negretti trade catalogue and can be dated to around 1939- see extract.