Hydraulic Architecture, Volume One, Part Two

...since the air enclosed under the receiver The "récipient" or bell jar of the air pump where the vacuum is created. is not always the same, as it may be more or less condensed from one time to another; one can only compare the expansion Original: "dilatation." In this context, it refers to the thinning or rarefaction of air. used in this experiment, or any other, with the state of the natural air at the moment the experiment was conducted; unless one uses the manometer From the Greek manos (thin/rare) and metron (measure), an instrument used to measure the pressure or density of gases., which is an instrument devised by Mr. Varignon, with which one measures the different degrees of air expansion at different times. This instrument makes known not only how much the original air enclosed in the machine will have been expanded by a certain number of piston strokes, but also by how much an original air enclosed at a certain time would have been more or less rarefied of its own accord than that which would have been enclosed at another time; this provides an infallible means of comparing experiments that require great precision. For, as Mr. de Fontenelle Bernard Le Bovier de Fontenelle (1657–1757), the long-serving secretary of the French Academy of Sciences. remarks when speaking of the manometer, one must not expect that the barometer or the thermometer can serve in such a case; because the former marks the rarefaction that comes from the weight of the atmosphere, and the other that which comes from heat; and as these two causes both act together and modify one another, they place the air in a degree of rarefaction which is neither that marked by the barometer, nor that marked by the thermometer. One must therefore have a third instrument that can mark the degree of air rarefaction as produced at each moment by the two different causes involved in this effect, and which can perform the functions of the other two at the same time.

* Memoirs of the Academy, 1705.

How to use the barometer to expand the air in the receiver to a certain determined point.

805. One can also expand the air in the receiver to a certain determined point in a very simple manner by using a barometer arranged for this purpose; for the weight of the atmosphere being in equilibrium with a column of mercury of 28 inches Bélidor refers to the French "Paris inch," making 28 inches approximately 75.8 cm. This was the standard atmospheric pressure at sea level used in 18th-century France., if the same air were twice as expanded as in its natural state, it would support only a column of 14 inches, and only 7 inches if it were four times more expanded than usual. Since an ordinary barometer cannot be used because it is too large to be placed under the receiver, one could make one whose height is only eight inches, entirely filled with mercury, dividing the height of seven inches into an equal number of parts as usual. One will pump the air until the mercury is at a height of 7 inches above that of the orifice; then it will be four times more expanded than in its state...