Evangelista Torricelli found that his mercury barometer had a column of mercury 760 mm/Hg at his town of Faenza, Italy, which is 50 m above sea level. That same day, he took his barometer into the mountains west of town and found that the mercury column was 720 mm/Hg. When he returned to Faenza, it was raining and the column remained the same. Why? (Assume the temperature remained the same.)