Hydraulic Architecture, Volume One, Part Two

HYDRAULIC ARCHITECTURE, BOOK III.

...between the two remains empty, that is to say, deprived of coarse air. This once again results from the fact that the external air presses upon the surface FG of the water in which the pipe is immersed. Finding no other way to escape the pressure it is under except through the vacuum the piston has created in the pipe, the water rises until the action of the air's weight has the force to sustain it, after which both remain in equilibrium.

Method of determining the weight of the atmosphere.

791. Having established the height of a column of water when it is in equilibrium with the air, it will be easy to judge the weight of the air in its current state; for if the column of water is, for example, 31 1/4 feet high with a base of one square foot, it will comprise 31 1/4 cubic feet. Since a cubic foot of water weighs 70 pounds The livre (pound) used here is the French poids de marc, approximately 489.5 grams, making it slightly heavier than the modern imperial pound. (340), one can say that the column of air will then weigh 2205 pounds.

Method of determining the weight of a certain volume of air.

792. If one has a barometer at the foot of a mountain, and the mercury is suspended there at a height of 28 inches, it is evident that the weight of the entire column of air will be equal to that of 28 inches of mercury. If the barometer is then carried 10 toises A toise is an old French unit of length equal to about 6.4 feet (1.949 meters). higher, and the mercury at that height has dropped by one line A "line" (ligne) is a traditional unit of measurement equal to 1/12th of an inch., as will indeed happen, the remaining column—which will now be only 27 inches 11 lines—will be in equilibrium with the column of air whose base begins 10 toises above the horizon. Consequently, the weight of the line of mercury that dropped is equal to the weight of a column of air 10 toises (60 feet) high at the foot of the mountain, having the same base as the mercury in the tube.

If a second observation is made 10 toises above the first, and the mercury there has dropped by 1/10th of a line, for example, one could conclude that the column of air corresponding to that height is equal to the weight of the mercury suspended in the barometer—that is, 27 inches 10 lines and 9/10ths of a line—and that the weight of the 10-toise column of air between the first and second observations is 1/10th of a line. One can therefore use this instrument to measure the weight of the same volume of air at 60 feet of height, taken at different distances from the earth, and determine the ratio of the weight of a given volume of air to that of an equal volume of water.

Since a cubic foot of mercury weighs, on average, 946 pounds (343), dividing this number by 144 Bélidor is calculating the weight of a "slice" of mercury one line thick with a surface area of one square foot. There are 144 square inches in a square foot. yields 6 pounds 9 ounces for the weight of one line of mercury with a square foot base; consequently, this is also the weight of a column of air of the same base and 60 feet in height. Dividing 6 pounds 9 ounces again by 60, it results in one ounce 6 drachms A "drachme" (drachm or dram) was a unit of weight equal to 1/8th of an ounce. for the weight of a cubic foot of this air, assuming it to be uni- The text cuts off here at the end of the page; the word is likely "uniforme" (uniform).