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| aneroid barometer |
The air also has its weight, and like all other bodies exerts a pressure, by the effect of gravity, on the earth's surface. Many scientists came up with the idea of measuring this pressure, but the first to do so was Galileo Galilei, using a very long tube closed at one end. He filled it completely with water, and put the open end into a container full of water: the liquid in the tube descended, stopping at a height of ten meters. Some years later, Evangelista Torricelli, Galileo's pupil, wanted to repeat the experiment with a liquid much heavier than water, that is, with mercury. The mercury rose through the tube up to 76 centimetres. The new device was called a barometer (in Greek, baros means weight and metron means measurement). Torriceli soon realized that the column varied in height according to pressure variations. More modern is the aneroid barometer (a = no; neros = liquid), consisting of a steel box in which a vacuum has been made: the external pressure displaces one of its faces inwards or outwards, acting on a hand that indicates the displacements in a graduated sphere, thus indicating the variations in pressure and its intensity. This type of barometer, also known as a metallic barometer, is less cumbersome, although it is also less accurate than mercury. In addition, before being used, it has to be adjusted with a mercury barometer.
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