It was actually a mathematician named Evangelista Torricelli who is the first on record to demonstrate that air has weight. His experiment to prove this fact was prompted by the observation that water from a mineshaft could only be pumped upward to reach a certain height. Torricelli thought that the air pushing down on the surface of the water must play a role. To test this theory, in 1643 he placed a sealed tube of mercury upside down in a bowl of mercury. He observed that the weight of the air would keep the mercury in the tube at a certain level, and on different days he observed that the mercury would rise to different levels. We now know this is because the air pressure varies from day to day, and Torricelli’s experiment was the first barometer.