Mercury is 13.6 times denser than water. Atmospheric pressure can hold a vertical column of water about 13.6 times the height of the mercury column at a place. Thus, at sea level, the vertical height of the water column would be $0.76 m \times 13.6$ = 10.34 m. Thus, a glass tube more than 10 m long is required to make a water barometer. 

 

 

 

 

i. 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 needs to be only 30 inches to exert pressure equal to that of pressure.

 

ii. Mercury has a very low vapor pressure when compared to that water. So, it is more sensitive than water to changes in atmospheric pressure and rises more quickly to record the changes in atmospheric pressure.

 

iii. Mercury’s freezing point is much lower than that of water so it can record the atmospheric pressure at temperatures below that of 0 degrees centigrade.

 

iv. Mercury does not evaporate easily so very little mercury vapor enters the vacuum above the mercury in the tube.

 

v. Mercury is a metal that shines brightly and so can be used to read the markings on the tube easily.