Hydraulic Architecture, Volume One, Part Two

CHAPTER I. ON THE PROPERTIES OF AIR.

...where it is immersed in a deep place; one will see the mercury rise significantly above 28 inches, because the column of air being higher, and consequently heavier, is capable of supporting a greater weight of mercury in equilibrium. The opposite happens when one carries this machine to the summit of a very high mountain; as one climbs, one sees the mercury in the tube descend and mix with that of the vessel.

When it is said that a column of mercury 28 inches high is in equilibrium with the average weight of the air, it is assumed the barometer is placed at the level of the sea’s surface, which, being everywhere equally distant from the center of the earth, must be regarded as a fixed point for determining what is higher or lower.

789. To better illustrate these differences, the seventeenth figure represents the experiments performed at Clermont in Auvergne by a relative of Mr. Pascal Referring to Florin Périer, the brother-in-law of Blaise Pascal, who conducted the famous Puy de Dôme experiment in 1648 to prove Pascal's theories on atmospheric pressure.. Near this city is a mountain 500 toises A toise is an old French unit of length roughly equal to 1.949 meters or about 6.4 feet; 500 toises is approximately 3,200 feet. high, named the Puy de Dôme, where three observations were made at the same time: the first, A, in a garden in Clermont, where the mercury in the tube was found at 26 inches 3 1/2 lines A "line" (ligne) is a historical unit representing 1/12th of an inch.; the second, B, about a third of the way up the slope, where the mercury in the tube was at a height of 25 inches, having descended by 15 1/2 lines during the ascent; the third, C, at the summit of the mountain, where the mercury was found only at a height of 23 inches 2 lines, having descended a total of 3 inches 1 1/2 lines.

790. What we have just seen regarding the equilibrium of mercury with the weight of the air must be understood to apply to all other liquids; a column of water, for example, will also place itself in equilibrium with a column of air. However, as a certain quantity of water weighs 13 1/2 times less than an equal quantity of mercury (343), a column of water of the same base must be 13 1/2 times higher than a column of mercury of 28 inches—that is to say, it must be approximately 31 feet 8 inches high; but one usually counts on 32 feet.

Suction for the lifting of water in pipes immersed in it is achieved, as seen in the twelfth figure, by pulling a piston B from bottom to top in a single motion, starting from its extremity E, from which the air is expelled by several strokes of the piston, as we shall demonstrate in the third Chapter; then the water rises and follows the piston up to the height CD, of 31 or 32 feet, depending on the state of the air; and if one pulls the piston higher than this elevation, the water no longer follows it, and the interval CB which...