Galileo Galilei was interested in ordinary water pumps and how they function. At that time, people used suction pumps to draw water from wells. Galileo once noticed that a well functioning pump failed to lift water more than 34 ft above the free water level in the well.
Evangelista Torricelli*, who invented the vacuum and the barometer, was able to explain the failure of the pump. He took a glass tube about 3 ft long that was sealed at one end and filled it with mercury. He put his finger on the open end, and turned the tube over so it stood in a vessel containing mercury. Torricelli removed his finger carefully, so that no air got in the tube, and noticed that the mercury in the tube sinks and comes to rest about 30 in. above the mercury in the open vessel.
Since there is a vacuum in the space above the mercury in the tube, no atmospheric pressure can be acting on the top of the mercury column. However, there is atmospheric pressure acting on the mercury in the vessel, and that is equivalent to the weight of the mercury in the tube.
It should be noted that mercury is around 13.6 times heavier than water. So:
13.6*30 inches=408 inches
408 inches is 34 feet of water, and this explains the failure of a suction pump to lift water above this height.
*The SI unit for pressure is in atmospheres. 1 Atmosphere=760. Millimeters of Mercury=760 Torr. The unit Torr is named after Evangelista Torricelli.