Hydraulic Architecture, Volume One, Part Two

the mercury descends; regarding which it should be noted that, as it often happens that the highest particles of water, by falling very slowly, take a considerable time before joining the lower ones, the weight of the air decreases before it rains, and the barometer predicts the weather that is to come.

Air possesses elasticity and can be condensed.

795. One of the principal properties of air is its ability to be extremely condensed, and to always maintain a power of elasticity The author uses the term "ressort," literally "spring," which was the standard 18th-century way to describe what we now call air pressure or the expansive force of a gas., through which it makes an effort to push back the bodies that press upon it; for the air at the surface of the earth is far from being in its natural state. Being laden with the weight of the entire atmosphere, it is more condensed than the air at higher altitudes. To give an idea of this, let us imagine a large pile of carded wool of a considerable height; it is certain that the wool at the bottom, being laden with the weight of all that it supports, will not be as expanded as that at the top; therefore, the wool below will make as much effort to return to its natural state as the weight it bears makes to compress it. Air is in precisely the same case at whatever height it is taken; the column [of air] beneath a table, for example, makes as much effort to lift it from bottom to top as the column above the table makes from top to bottom to press it down; otherwise, if the two columns were not in equilibrium, and the action of the upper one could act alone, a table having only 20 square feet of surface area would be laden with a weight of more than 44,000 lbs, which it could not support without breaking. Likewise, the roofs of houses and the floors of apartments would never resist the immense weight with which they are laden, if they were not always situated between two columns of air, the lower of which is in equilibrium through its elasticity with the one pressing upon it.

The elasticity of air acts in every direction with equal force.

It should be noted that the elasticity of air acts in all directions with an equal force, just like liquids liqueurs In this context, the author refers to "liquids" in the general sense of fluids, following the principles of hydrostatics established earlier in the work. (343): since this force is always equal to the weight of the corresponding column of air, or to the weight of an equivalent column of mercury that would have the same base and a height of about 28 inches, or to a column of water of 32 feet, one will always know the force of this elasticity. This force will be equal to the weight of that column whose base is determined by the surface of the body against which it acts: for example, natural air enclosed in a cubic box, where each face measures one square foot internally, will push against each of these faces to separate them with an elastic force equivalent to 2,205 lbs, when the barometer is at its average height; and would separate them...