His fame as a man of science does not rest solely on his labours in geometry. As an experimentalist he has earned no vulgar celebrity. He was a young man when the interesting discoveries in pneumatics were working a grand revolution in natural philosophy. The experiments of Torricelli had proved, what his great master Galileo had conjectured, the weight and pressure of the air, and had given a rude shock to the old doctrine of the schools that “Nature abhors a vacuum;” but many still clung fondly to the old way, and when pressed with the fact that fluids rise in an exhausted tube to a certain height, and will rise no higher, though with a vacuum above them, still asserted that the fluids rose because Nature abhors a vacuum, but qualified their assertion with an admission that she had some moderation in her abhorrence. Having satisfied himself by his own experiments of the truth of Torricelli’s theory, Pascal with his usual sagacity devised the means of satisfying all who were capable of being convinced. He reasoned that if, according to the new theory, founded on the experiments made with mercury, the weight and general pressure of the air forced up the mercury in the tube, the height of the mercury would be in proportion to the height of the column of incumbent air; in other words, that the mercury would be lower at the top of a mountain than at the bottom of it: on the other hand, that if the old answer were the right one, no difference would appear from the change of situation. Accordingly, he directed the experiment to be made on the Puy de Dôme, a lofty mountain in Auvergne, and the height of the barometer at the top and bottom of the mountain being taken at the same moment, a difference of more than three inches was observed. This set the question at rest for ever. The particular notice which we have taken of this celebrated experiment, made in his twenty-fifth year, may be justified by the importance attached to it by no mean authority. Sir W. Herschell observes, in his Discourse on the Study of Natural Philosophy, page 230, that “it tended perhaps more powerfully than any thing which had previously been done in science to confirm in the minds of men that disposition to experimental verification which had scarcely yet taken full and secure root.” Whatever may be the value of the fruits of Pascal’s genius, it should be remembered that they were all produced within the space of a life which did not number forty years, and that he was so miserably the victim of disease that from the time of boyhood he never passed a day without pain.