The first barometer of the weather station of the Institute was a barometer Fortin, built according the prototype of Nicolas Fortin of 1810. The instrument consists of a barometric tube closed at upper end, filled with mercury and with the open end placed in a small container with mercury inside. In the barometric tube the height of the column of mercury varies according the atmospheric pressure balancing the hydrostatic pressure of the mercury. The level of mercury can be adjusted in a fixed position moving an ivory tip attached to the cover of the container. The barometric tube is protected by a metal sheath on which is engraved a graduation where the zero corresponds to the ivory tip. The reading of the height of the mercury column, ranging from 500 to 820 mm, is made with a sliding vernier with an accuracy of 0.05 mm. The protective metal sheath has a long rectangular window both front and rear. Behind it, a sliding mirror makes it easy to read the position of mercury. The instrument is fixed to the wall in vertical position.