How was atmospheric pressure discovered?

Atmospheric pressure was discovered in the 17th century by the Italian scientist Torricelli, using a mercury barometer, Torricelln collected mercury in a glass tube, clamped the open ends of the tube with his finger and immersed it in a vessel filled with mercury. Some of the mercury poured out of the tube, the remaining mercury formed a 76 cm column. The reason for this is the atmospheric pressure, which prevents mercury from pouring out of the tube.