The Fortin barometer is named after its inventor, Nicolas Fortin. It features a cistern partly made of glass, in which one can observe the lower level of the mercury and determine the zero point of the barometric scale. Before reading the height of the mercury column in the barometric tube, it is necessary to adjust the mercury level in the cistern so that it coincides with the tip of an ivory reference screw. This adjustment is performed by modifying the position of the cistern's leather bottom by means of a screw. The tube, which is lost, was enclosed in a brass tube with small windows to observe the mercury level. The tube is fitted with a vernier for readings and carries three engraved scales. There is also a mercury thermometer with a centigrade scale.