Air pressure

The air around the Earth is a deep sea of gas, hundreds of kilometres thick, and its weight presses on everything at the bottom of it, us included: this is air pressure, or atmospheric pressure. We don't feel it because it pushes on us from every side, and from inside too. The higher you go, the less air there is above you, so the pressure falls: it is lower on a mountain than at the seaside. Air pressure pushes a drink up a straw, crushes a closed can when the steam inside it cools, and once held two metal half-spheres together against two teams of horses. A barometer measures it: at sea level it holds up a column of mercury about 76 centimetres tall.