HYDRAULIC ARCHITECTURE, BOOK III.

responded that nature had a horror of a vacuum original: "horreur du vuide"; the belief that nature inherently resists empty space, a concept that dominated science until the mid-1600s., and that water preferred to rise into a pipe from which the air was removed rather than suffer that space not to be filled by matter. The celebrated Galileo was the first to notice that suction pumps could not raise water beyond 31 or 32 feet, even if the pipe were 40 or 50 [feet long], and the portion above 32 feet was deprived of coarse air; he only drew this conclusion: that nature had a horror of the vacuum only up to a certain point, and that the effort it makes to avoid it is limited.

787. Torricelli, his disciple, who succeeded him as Mathematician to the Duke of Florence, then performed an Experiment that has become very famous. He took a glass tube AB, 4 feet in length, hermetically sealed at one end A, which he filled with mercury; plugging the other end with his finger, he placed it perpendicularly into a vessel D, which also contained mercury. He was very astonished to see that, upon removing his finger, the mercury in the tube fell partway, leaving a vacuum AC, and remained suspended at a height CE of about 28 inches above the surface of the mercury contained in the vessel. He conceived that the horror of the vacuum was a chimera original: "chimere"; meaning a myth or a groundless fancy., and judged that air must have weight. This experiment was sent to Paris in 1644 to Father Mersenne, who made it public; this is what gave rise to all the experiments performed by Mr. Pascal, from which it was recognized that the sphere of air pressed by its weight upon the entire surface of the earth. It is true that we do not feel this weight at all, because we are pressed equally from all sides. Some Physicists The term "Physicists" here refers to "natural philosophers," the 18th-century term for scientists who studied the physical world. having calculated the pressure of the atmosphere on the body of a man of ordinary size, found that it could be two thousand pounds.

788. To explain the reason for Torricelli's experiment, one will note that if the mercury is supported at approximately the height of 28 inches, it comes from the fact that there is no air in the part AC of the tube that the mercury has abandoned, and that the outside air presses upon the surface of the mercury in the vessel, but not upon that in the tube. The mercury in the tube maintains an equilibrium with the external air, being pushed downward only by the action of its own weight; thus, the weight of a column of mercury, 28 inches high, is equal to that of a column of air of the same base, which would have the height of the atmosphere.

To be convinced that the elevation of the mercury in the tube is an effect of the weight of the air, one only has to carry it, along with the vessel,