CHAPTER I. ON THE PROPERTIES OF AIR.

...it may be: but that air is about twice as dilated in summer as it is in winter, much like what happens to certain liquors, where a specific measure does not fail to equal its same weight even though it has completely turned into foam. Bélidor is comparing the expansion of air to the way a liquid maintains its mass even when whipped into a high-volume foam.

809. One can conclude from what precedes that we never obtain the true weight of bodies that have a large volume, such as bales of wool, cotton, horsehair, etc., since these bales weigh less in the air than they would in a vacuum. This loss is equal to the weight of the volume of air whose space they occupy, and the weight is further reduced when the air itself is heavier This describes the principle of buoyancy in air; just as an object in water is buoyed up by the weight of the water it displaces, an object in air is buoyed up by the weight of the displaced air.; thus, it is more advantageous to buy these kinds of merchandise in winter than in summer.

810. One has a similar interest, in the case of liquids sold by measure, to purchase them in winter rather than summer, since the same vessel will contain more of them. For example, one sees in the table in article 339 that a cubic foot of Burgundy wine weighs 69 lb 9 ounces in summer, and 68 lb 1 ounce in winter, which is a difference of 24 ounces per cubic foot There is a known discrepancy in early editions of this text regarding these specific figures; physically, liquids are denser and should weigh more by volume in winter. Bélidor may be accounting for the increased buoyancy of the air in winter mentioned in the previous paragraph, or these figures may be reversed.; and as the muid A traditional French measure of capacity; a muid of Paris was equivalent to approximately 268 liters or 8 cubic feet. contains 8 cubic feet, it follows that it will contain 12 lb more wine in winter, or about 6 Paris pints more than in summer; I leave it to the reader to consider whether Physics is a science of purely curious interest.

811. The rarefaction The reduction in density; the opposite of condensation. of air can become quite considerable, judging by the conclusions drawn from several experiments. Mr. Mariotte Edme Mariotte (1620–1684), the French physicist who independently discovered Boyle's Law, describing the inversely proportional relationship between the pressure and volume of a gas., who performed more of these than anyone, reports that a certain volume of air such as that which we breathe must be dilated four thousand times to reach its natural extent; that is to say, if it were possible to carry one cubic foot of air from the surface of the earth to the top of the atmosphere, it would occupy a space of 4,000 cubic feet.

812. From all that precedes, it is evident that the spring The elastic force or pressure of the air. of the air decreases as it becomes more dilated, and it is natural to conclude that it must, conversely, increase as it is more condensed. Indeed, if the atmosphere were compressed by any cause whatsoever, the molecules of air would move closer to one another and would exert more effort than usual to return to their natural state; that is to say, they would have a greater force of spring, and would support a column of mercury higher than 28 inches. Mr. Mariotte, and several others after him, conducted experiments to see if the spring of the air increased in proportion to the weights with which it was charged, as there was reason to presume, and they found this to be the case.

Technical Terms: rarefaction (thinning of air), dilatation (expansion), Mariotte (physicist), atmosphere, mercury (used in barometers), spring of the air (air pressure), cubic foot (unit of volume), muid (large liquid measure).