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Torricelli, a hydraulics engineer, established a number for the pressure of the atmosphere. The idea of pressure was known to many but Torricelli made that property quantitative.
He and fellow engineers (1645) were puzzled that their best pumps (water pumps - used in mining) failed completely to draw water columns any higher that 34 feet. At that height, "vacuum was created" and the flow abruptly failed.
Thinking about water and liquids, Torricelli rationalized that were mercury pumped it might fail (as did water). But with mercury being about 14 times heavier than water, he anticipated mercury would "pull vacuum" at a height of about 34/14 feet or 28.8 inches. Of course to pump mercury was out of the question. So Torricelli and his student Viviani improvised. They filled a long glass tube with mercury, inverted its bottom, then carefully inserted it into the bottom of a large bowl of water with mercury at its bottom. Next they tipped the tube slowly toward vertical. Suddenly the mercury "pulled vacuum" by showing a void in the closed top of the tube. Just as Torricelli had predicted. Upon draining the water from the bowl, they observed the remaining, sustained column of liquid mercury to stand about 30 inches tall with a clear "void" above it.
Torricelli declared, "The pressure in that void is zero!" And with that dashing, near perfect assumption (plus knowing the density of mercury) Torricelli became the first scientist to quantify pressure. The pressure of the void, though not zero, is indeed very small (~20 Pa) and sufficiently close to zero to be a serviceable "zero pressure reference" for many scientific studies. With that assumption, Torricelli calculated the first values of atmospheric pressure.