Hydraulic 'Amorini'.
Gaspar Schott, Technica curiosa, sive, Mirabilia artis, Würzburg 1664

Measuring atmospheric pressure gives one of the basic pieces of information for weather-forecasting. The barometer, which derives directly from the torricellian tube, has in fact been for more than two hundred years the fundamental instrument for forecasting and measuring climatic changes and the resulting variations in temperature.
A low-pressure zone pulls in a rush of air from neighbouring regions at normal pressure, making changeable weather. In a high-pressure zone, on the other hand, the weather tends to be unchangeable. Therefore, when the level of mercury in the barometer goes down, it means that changeable weather is on its way, whereas if it goes up, it means fine weather.
Pressure, measured in millibars (mb), corresponds, at sea level, to an average of 1013 mb, going down to 700 mb at 3,000 metres. At the cruising altitude of most airliners (9,000 metres), atmospheric pressure goes down to 300 mb, while at 21,000 metres it's down at 40mb.

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