Atmospheric pressure. Torricelli’s experiment
The Earth is surrounded by an air shell — the atmosphere. Air has mass (its density at 0 °C is about 1.29 kg/m³), so it presses on all bodies on Earth and on the Earth’s surface: this is atmospheric pressure. The Italian scientist Evangelista Torricelli (1608–1647) turned a mercury-filled tube closed at one end upside down into a dish of mercury and showed that the mercury column stops at about 760 mm: the pressure of the column equals atmospheric pressure. At sea level and 0 °C normal atmospheric pressure is 760 mmHg = 101 325 Pa ≈ 101.3 kPa; 1 mmHg ≈ 133.3 Pa. Higher up, air is thinner and pressure falls: near the ground, about 1 mmHg for every 12 m of climb. Pressure is measured by a barometer (mercury or aneroid), the pressure of a gas or liquid in a vessel by a manometer. With water instead of mercury the column would be about 10 m tall. Atmospheric pressure lets us suck a drink through a straw; the Magdeburg hemispheres (Guericke, 1654) could not be pulled apart even by two teams of horses once the air was pumped out.
Experiment: fill a glass with water, cover it with a stiff paper, press the paper with your palm and turn it over above a sink; remove the palm. Why does the water not spill? Safety: do it over a sink or basin and use a plastic cup rather than glass. Never experiment with mercury: its vapour is poisonous.