pascal (Pa)		Blaise Pascal (1623–1662)

Blaise Pascal was a French scientist, mathematician and philosopher. One of his early interests was the study of fluids. This led him to design an experiment using a barometer like the one invented by Torricelli in 1644. In this experiment, carried out in 1648, the level of mercury in a barometer equipped with a scale was measured at the base of Puy-de-Dôme, and again at the top, some 1000 metres (3300 feet) higher (Descartes had attached the first such scale to Torricelli's barometer in 1647). The account of Pierre Florin, who carried out the experiment, records that the "quicksilver" reached a height of 26 inches plus 3 ½ lines at the base of the hill, compared to only 23 inches plus 2 lines at the top. This meant that the pressure exerted by the atmosphere decreased with height, consistent with the idea that the pressure was due to the weight of the atmosphere in the column above the barometer. Pascal later repeated the experiment in Paris, where he measured the pressure difference between the base and the top of a church bell tower.

To honour his scientific work with atmospheric pressure, Pascal's name was given to the SI (International System of Units) unit of pressure. One pascal is equal to one newton per square metre. Modern meteorologists often refer to atmospheric pressure in hPa (hectopascals). A typical sea-level pressure would be around 1000 hPa.

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