To construct a barometer, a tube of length 1 m is filled completely with mercury and is inverted in a mercury cup. The barometer reading on a particular day is 76 cm. Suppose a 1 m tube is filled with mercury up to 76 cm and then closed by a cork. It is inverted in a mercury cup and the cork is removed. The height of mercury column in the tube over the surface in the cup will be
In this case, we look at the pressure balance of the situation. The 76 cm is provided by the trapped air along with the weight of the liquid in balance. When we remove the cork, we are releasing the air and the pressure due to it. Now, the atmospheric pressure acts on the liquid on the column. Thus, the pressure decreases and so does the height.