Physics of the Terrestrial Environment, Subtle Matter and Height of the Atmosphere. Eric Chassefiere

Française, a (short) majority of whose members did not look favorably on this initiative, which was in competition with that of the institution. In addition to the content of the 1690 edition, a comparison of this content with the enriched 1727 edition provides interesting outlooks on the evolution of the meaning of words, and of the underlying knowledge, during the first quarter of the 18th century. The Encyclopédie of Diderot and d’Alembert, published in the middle of the 18th century, provides a much larger compendium, which reflects the rapid development of scientific thought in the first half of the Enlightenment. We limit ourselves in this chapter to this particular period, to the 18th century, during which reflections on the height of the atmosphere, and on the subtle matters invoked to explain the rapidly emerging discrepancies between the estimates made by the different methods, are developed. The progression of knowledge between 1727, the date of the publication of the fourth edition of Le Furetière, and 1751, date of the first edition of the Encyclopédie by Diderot and d’Alembert, is very significant, and we will try to outline the primary paths of this evolution. In this chapter, we will note DUF-1690 and DUF-1727, the editions of the Dictionnaire Universel Furetière of 1690–1701 and 1727.

      The first English alphabetical encyclopedia is the Lexicon Technicum or, An Universal English Dictionary of Arts and Sciences: Explaining not only the Terms of Art, but the Arts Themselves (Harris 1704), referred to simply as the Lexicon here. Although the emphasis is on mathematical subjects, it does not only contain terms of a scientific or technical nature, but also includes entries relating to law, commerce, music and the arts in general. It is the work of Pastor John Harris, who claimed for his dictionary not only the function of giving the meaning of scientific and artistic words, but also elements of knowledge about the sciences and arts referred to by words, in a universalist approach that would be that of Diderot and d’Alembert’s Encyclopédie half a century later. We have limited ourselves in this chapter to the first edition of the Lexicon dating to 1704, composed of a first volume, a second volume consisting of additions, composed mainly of mathematical and astronomical tables, being published in 1710. Three other editions of the first volume were created in a short period of time, in 1708, 1716 and 1725, with the two volumes being published together only in 1736. In his preface, Harris insists on the fact that the contents of his dictionary do not come from other dictionaries, but are collected from the original works of the best authors, thus making it a source of information in its own right, with contents distinct from the Furetière, and otherwise specific to the English scientific context, differing in many respects from the French context, as we will see.

In section 1.2, we examine the definitions of air and atmosphere, as these two terms are far from being equivalent. Sections 1.3 and 1.4, respectively, deal with vapors and exhalations, as well as with subtle matter, two essential components of the atmosphere, as the treatment of subtle matter also requires attention to the definition of “normal” (“coarse” as opposed to “subtle”) matter. Section 1.5 is devoted to the triptych made up of the three subtle matters heat, fire and light, closely linked by multiple relations, whether demonstrated or conjectural. Section 1.6 deals with the question of the meaning given to the word ether, in its complex relationship with the atmosphere. We end with a conclusion, which is also the introduction to the main body of the work, which deals with subtle matters, particularly in relation to the question of the height of the atmosphere. Naturally, in this analysis, we seek to investigate the scientific concepts underlying the words used, since they partly determine their meaning, the nascent scientific approach contributing to inflecting this meaning according to the characters, proven or supposed, of the designated environments and phenomena. However, this investigation is only minimal, in the very spirit of the dictionaries used, as the scientific contents specific to the primary subject of the work, namely the height of the atmosphere, are to be developed in the following chapters. We have not presented all of the matter considered in the various articles analyzed, focusing on those that are directly related to the theme of this book.

1.2. Air and the atmosphere

      The word ATHMOSPHÈRE (ATMOSPHERE) is only briefly defined in the DUF-1690:

      It is the part of the air that is charged with vapors, or clouds, and that does not have the purity of the ethereal region: this is what causes the refraction of the light of the stars. The Moon appears larger at its rising, because of the vapors of the Atmosphere.

This definition expresses that the atmosphere is only a part of the air, the one in which vapors and clouds are mixed. Purity is here a criterion used to distinguish the atmosphere from the ether, or ethereal region, and the phenomenon of atmospheric refraction seems to be associated with the presence of vapors and clouds. The second sentence, which refers to what is known today as “the lunar illusion”, that is, the fact that the Moon appears larger when it is close to the horizon, long attributed to refraction, confirms the link that the first sentence seems to establish between the presence of vapors and atmospheric refraction. The definition of the atmosphere in DUF-1727 is a little more detailed. It further notes that the part of the air charged with vapors and clouds is “the coarsest and heaviest”, an idea that we will see plays an essential role in the representation that scientists, especially French scientists, had of our atmosphere and some of its characteristics, such as atmospheric pressure. The following sentence about the atmosphere was also added in the 1727 edition: “It ends at a certain distance, and forms like a globe that surrounds and encloses that of the Earth.” Thus, the idea of a physical top of the atmosphere, located “at a certain distance”, became more clearly apparent at the turn of the 18th century. The attribution to vapors of the perceived enlargement of the Moon near the horizon remained in the 1727 edition. The entry RÉFRACTION (REFRACTION) from DUF-1727 is instructive in this respect. It provides information about atmospheric refraction and about the light during the twilight hours that is reflected from the sky (the coarse air charged with impurities, as was the vision held at that time) once the Sun sets, or before it rises:

      and if the coarseness of the air, which seems to cause this great refraction, also gives longer twilight hours, as it would appear: in the longest periods of darkness, the six-month nights experienced at the poles, there will still be a fairly great twilight even without the Moon, and this utility compensates them [the inhabitants of the polar zone] for the inconvenience of the coarse air they breathe.

      Thus, with respect to both the refraction of the light of the stars and the reflection of the light of the Sun during twilight, it is necessary to attribute these to the coarseness of the air in its lower part, thus to the impurities it contains (vapors, exhalations, clouds), to which it owes precisely its name, the “atmosphere“ (“sphere of the vapors”). But what exactly is air? The DUF-1690 tells us, with regard to air:

      A fluid and light element that surrounds the globe, the sea and the Earth. The air is divided into lower, middle, and upper regions. Water resolves and evaporates into air. One cannot live without breathing air. We cannot live on air. The ancients did not know the gravity of the air. We know the gravity of the air from using a barometer, its heat from a thermometer, its dryness from a hygrometer. We found the invention of pumping air to make vacuum, by Mr. Boyle’s machine. Mr. Mariotte, in his Essays on Physics, says that air can expand more than four thousand times more than it is near the Earth before it is in its natural expansion, as it has it at the top of the atmosphere, where it is not loaded with any weight. Its height, according to his calculation, is only 20 leagues [≈80 km]: and it would not be 30 when it is eight million times more rarefied than the one near the Earth.

      In the 1727 edition, it is not the element that is described as fluid and light, but the matter, or substance, and a more precise definition of the vertical stratification of air is provided:

An element; fluid, light, transparent matter that surrounds the Earth; fluid, moist substance used for breathing. The mass