This wall barometer was manufactured in Madrid by Grasselli y Zambra between 1849 and 1863. It consists of a rectangular wooden box that supports a brass plate with two scales and various inscriptions. Fixed onto this plate is the glass tube that connects with the mercury container at the bottom.

The barometer has two scales for measuring atmospheric pressure. The scale graduated from 5 to 80 expresses the measurement of the barometric pressure in millimetres of mercury (mm Hg). There is also a secondary vernier scale for greater precision. In the meantime, the second scale, graduated from 2 to 29.4, provides an equivalent measurement in inches of mercury (in Hg).

Besides the scales, the barometer also has a movable pointer for weather forecasts. This allows the day’s meteorological conditions to be predicted in accordance with the level reached by the mercury in the glass column. The face bears the following inscriptions: Muy Seco (Very Dry), Sereno Fijo (Set Fair), Buen Tiempo (Fine Weather), Variable, Lluvioso (Rainy), Muy Lluvioso (Very Rainy) and Borrascoso (Stormy).

To adjust the barometer, the indicator of the vernier scale is placed at the height of the meniscus of the mercury. The weather outside is then observed, and the atmospheric weather scale is adjusted with a screw. As the weather improves or becomes more anticyclonic, the atmospheric pressure increases, leading to an increase in the height of the mercury in the column. By contrast, on rainy or stormy days, associated with low pressures, the column of mercury falls.

In this type of barometer, atmospheric pressure is measured by taking the height in millimetres reached by the column of mercury (mm Hg). The reference is established by the value of the pressure at sea level, which is 760 mm Hg, equivalent to one atmosphere. For this reason, some cities located at an altitude close to sea level, like Saint Petersburg, London, Venice and Paris, are indicated at the top of the barometer. These cities represent higher values on the scale, since they experience greater atmospheric pressure owing to their proximity to sea level. On the lower part of the barometer, on the other hand, are the names of higher locations, like Madrid, as well as well-known mountains, peaks or passes, such as the Pyrenees, the Saint-Gotthard Massif and the Saint Bernard Pass. These locations register lower values on the scale because they are at higher altitudes, resulting in a lower atmospheric pressure. As a curiosity, the lowest reference on this barometer corresponds to the highest peak in the Himalayas, Mount Everest, with a reading of 255 mm Hg.

Rosa María Martín Latorre