Following are the reasons why mercury and not water is used in a barometer:
1. Mercury is relatively denser than water, consequently the length of the column of water would have to be about 34 feet high to exert pressure equal to that of the atmosphere while the column of mercury need to be only 30 inches to exert pressure equal to that of the atmosphere.
2. Mercury has a very low vapor pressure when compared to that of water. So it is more sensitive than water to the changes in the atmospheric pressure and rises more quickly to record the changes in the atmospheric pressure.
3. Mercury's freezing point is much lower than that of water's so it can record the atmospheric pressure at temperatures below that of 0 degrees centigrade.
4. Mercury does not evaporate easily so very little mercury vapor enters the vacuum above the mercury in the tube.
5. Mercury being a metal shines brightly and so can be used to read the markings on the tube easily.
Mercury is more dense. Its relative density is 13.534 times that of water. So, to measure the atmospheric pressure, which is 760 mm of mercury you need a barometer ,with mercury, of length slightly more than 0.76 meter, say one meter to cover the special cases of some higher pressure. If you use water, you have to have the length of barometer of length (or height) 13.534 times the length of mercury barometer, which may be more than 11 meter in length. Further mercury, being a metal has the shining quality which highlight its reading clear. Also mercury, having comparatively lower specific heat and good conductor of heat, could come to the same temperature of the atmosphere more quickly.
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:
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.