A vertical tank of length greater than 0.76 m has its top end closed by a tightly fitting frictionless piston of negligible weight. The air inside the cylinder is at an absolute pressure of 1 atm (1 atm = 101,325 Pa). The piston is depressed by pouring mercury on it slowly, so that the temperature of the air is maintained constant. What is the height of the air column when mercury starts to spill over the top of the cylinder?