We live submerged at the bottom of an ocean of the element air, which by unquestioned experiments is known to have weight. The discovery helped bring Torricelli to the conclusion: Previously, the more popular conclusion, even for Galileo, was that air was weightless and it is vacuum that provided force, as in a siphon. This validated his belief that air/gas has mass, creating pressure on things around it. The weight of the mercury would pull it down, leaving a partial vacuum at the far end. He would dip a glass tube, closed at one end, into a bowl of mercury and raise the closed end up out of it, keeping the open end submerged. In the 17th century, Evangelista Torricelli conducted experiments with mercury that allowed him to measure the presence of air. This was akin to how gasses really do become less dense when warmer, more dense when cooler. More condensed air made colder, heavier objects, and expanded air made lighter, hotter objects. He could observe water evaporating, changing to a gas, and felt that this applied even to solid matter. For much of human history, the pressure of gasses like air was ignored, denied, or taken for granted, but as early as the 6th century BC, Greek philosopher Anaximenes of Miletus claimed that all things are made of air that is simply changed by varying levels of pressure.
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