Riding the wave


A barometer measures atmospheric pressure. It consists of:

- a tray, which contains mercury on which atmospheric pressure is acting;

- a tube, with the mercury column, whose weight balances the atmospheric pressure. The tube is made of glass, usually about 80 cm long, with an internal diameter of approximately 7 mm. Above the mercury in the tube there's an absolute vacuum. Next to the column, a thermometer measures the temperature of the mercury;

- a scale used to measure the barometric height, the difference in level between the two free surfaces of mercury in the tray and in the pipe.

Barometer Petrali
The barometer represented here, owned by the Hydrographic Institute of the Navy , is signed "Petrali & Cordella, London.
A brass plate bears the engraving "Museum No. 11,", while the brass capsule that contains the tray is engraved with the number 113.
It is a gimbal suspended barometer including a thermometer and a hair tension hygrometer.
By Paola Presciuttini