A wooden wall plaque supports a brass outer casing which protects the glass barometer tube and wood and leather cistern. A brass screw provides the means to raise or lower the level of mercury in the cistern to a fiducial point in the form of an ivory pointer. Air pressure on the free surface of the mercury in the cistern supports the column of mercury, the length of which is read by a vernier operated by a rack and pinion over a graduated scale marked on the casing. A mercury thermometer attached to the casing gives the temperature of the barometer, and allows corrections to be made.
The Fortin Barometer was commonly used at meteorological stations to measure atmospheric pressure. The advantages of this type of barometer are its portability (inverted), and that it permits the inspection of both free surfaces of mercury whose difference in level have to be measured. The major disadvantage is the cistern and the mercury it contains require frequent cleaning to maintain the instrument's accuracy. This particular Fortin Barometer was in use in the second year laboratory until 1994. Restored recently, it would still be operational, however it contains no mercury.
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