1654, Gehrig made a famous experiment in Madeborg, Germany, which strongly proved the existence of atmospheric pressure and made people have a deep understanding of atmospheric pressure, but people still don't know how big it is. 1 1 years ago, Italian scientist Torricelli filled a 1 meter long thin glass tube with mercury and inverted it in a water tank filled with mercury. He found that the mercury column in the glass tube dropped by about 24 cm and stopped falling. There is no air in this 24 cm space, it is a vacuum. Torricelli concluded that atmospheric pressure is equal to the length of mercury column, which is the famous Torricelli experiment.