It is an instrument consists of
exterior copper tube and interior glass tube sealed at one end and filled with mercury.
The open end is inserted into a fixed cistern. The basic principle is that the pressure
of the atmosphere is balanced against the weight of the column of mercury insides
the glass tube. As a result, the length of the mercury is read for atmospheric pressure.
An attached-thermometer is also hanged besides the copper tube with its bulb well
attached to the tube in order to measure the temperature of the mercury.
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