so GREAT MEN OF SCIENCE 



and hence that these may rightly be ascribed to the pressure 

 resuhing from the weight of the air. He immediately wished 

 to carry out a decisive observation of the barometer on a high 

 mountain, since there the less amount of air overhead should 

 result in a correspondingly shorter column of mercury. 

 Since his own neighbourhood gave him no opportunity for 

 such an experiment, he asked his brother-in-law, who lived at 

 Cleremont in the south of France, at the foot of the i ,000- 

 metre Puy-de-Dome, to carry out the experiment on this 

 mountain. He did so with great thoroughness, and found 

 the height of the mercury column diminished by several 

 inches. 



Thus in 1648 it was for the first time proved without doubt 

 that in the air-ocean of the earth the pressure diminishes as 

 we go upwards, just as it does in a mass of ordinary liquid; the 

 latter fact, by the way, being already set forth quite clearly in 

 Stevin's writings. The evidence was convincing that it is / 

 the weight of the air, already demonstrated by Gahleo, which 

 supports Toricelli's mercury column, and not Aristotle's 

 'horror of a vacuum,' in which many, including at first Pascal, 

 still believed firmly. But there are no grounds for this being 

 different high up than upon the earth. Pascal was astonished 

 at the great difference in the mercury column found by the 

 experiment on the mountain; he then ventured to make 

 experiments also with smaller differences in height, on towers 

 and houses, and found all his expectations confirmed. 



Pascal was also the author of extensive writings on the 

 equilibrium of liquids, and the hydraulic press was his in- 

 vention; but here all he did was to apply in new ways what 

 had already been mainly discovered by Stevin. The 

 religious speculations also, which occupied Pascal's last ten 

 years exclusively, did not lead him beyond ideas already 

 existing at his time. 



