In the Fortin barometer, the level of mercury in the glass at the bottom of the barometer cistern is adjusted to a scale zero, known as the fiducial point, each time a reading is to be taken. The level of mercury in the column is then read against the scale, using a vernier adjustment for extra accuracy.
The Observatory bought 11 barometers of this type in 1899 in order to equip the Greek meteorological stations.

Fortin, French physicist (1750-1831). Established in Paris, he began his career as engineer and globe maker. Probably after Lavoisier’s advice, he became physics instruments maker and constructed a “gazomètre” in 1784. In 1788 he made the great precision Lavoisier’s balance. On November 10, 1794, after Lavoisier’s execution he made the inventory of his laboratory. During the Empire, he continued to make balances but he made also instruments for gas measurements for Gay Lussac (1806).

This barometer is made in France