Atmospheric pressure can be measured with the mercury barometer (Torricelli tube). Torricelli (1608-1647) proposed a method to measure atmospheric pressure by inventing the mercury barometer in 1643.
Mercury barometer is a long glass tube which was filled with mercury and then inverted in a vat of mercury.Atmospheric pressure was determined very easy so that it is equal with:
p=rho*g*h
Actual pressure, at a point in a fluid, is called absolute pressure. Relative pressure (manometric) are given either above or below atmospheric pressure.
Although it is very dense and hard, the mercury does not pass completely into the boat, staying 760mmHg at normal pressure, and going up even more when atmospheric pressure increases.
To the question "Why use mercury barometers (highly toxic liquid metal) and no water barometers?", the answer is very simple:
If water would be used in barometer, barometer tube should have the huge height of 10.3m! Mercury is 13.6 times more dense than water and a column weight of only 760mmHg equals the forces generated by atmospheric pressure, so the tube can be much shorter.
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