Institute and Museum of History of Science, Florence, ITALY

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Torricelli's barometric experiment The void within a void The Magdeburg Hemispheres

Variations on the ordinary Torricellian tube for the experiment of the void within a void.
Pascal, Traitez de l'equilibre des liqueurs, et de la pesanteur de la masse de l'air, Paris 1663


One of the decisive confirmations of the influence of atmospheric pressure was given by the so-called "void within a void" experiment, performed in 1648 in a variety of ways by Roberval, Adrien Auzout, Pascal, the members of the Accademia del Cimento and Robert Boyle, successively.
They discovered, in fact, that if the torricellian experiment was performed within a vacuum, the mercury was not held up in the tube, but descended completely into the basin. It was observed, too, that when air was allowed back in, the mercury went back up the tube.
To confirm Torricelli's experiment, Blaise Pascal thought up a definitive proof, in 1648. He had a barometer carried to the top of the Puy de Dme, in the Massif Central in France, where the level of the column of mercury fell to a few inches lower than at normal ground level. Pascal correctly interpreted this variation as a result of the lowering of air-pressure at altitude.




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