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

      Introduction

This book presents the methods developed in the 17th and 18th centuries for estimating the height of the atmosphere, methods essentially based on the observation of the light refracted and reflected by the atmosphere, and of certain meteors. As a result, these methods gave rise to a number of contradictions and advances. This question of determining the height of the atmosphere is inseparable from how the atmosphere is represented, with the atmosphere passing from the status of an idealized and purely mathematical object up to the middle of the 17th century, to that of a complex, eminently variable physical object, whose nature was the subject of a multitude of hypotheses, and which 18th-century scientists tried to reconcile within a coherent overall vision. The quest to determine how high the atmosphere reached, which the different phenomena studied did not all place at the same height, proved a powerful motivator of new research and debate during this time, as well as for the necessary syntheses that resulted. Rather than addressing, one by one, the different methods used to estimate the height of the atmosphere, in this book, we adopt a transverse approach by examining the various matters of a more subtle nature introduced by the scientists of the time to explain the phenomena from which they derived this height – the milestones in the construction of the representation of the atmosphere as a physical object. These subtle matters, most of which are part of the legacy of René Descartes, offer a broad field of investigation, which allows us to set the stage for the evolution of the scientific thinking of the time in relation to the understanding of atmospheric phenomena. There is a strong overall coherence between the hypotheses formulated at that time, far beyond the question of the validity of these hypotheses with respect to today’s knowledge, showing to what extent their abundance and the confrontation of points of view allow knowledge to mature, until the conceptual leap that overturned old ideas and allowed for an objective advance took place. These processes occurred over a considerable period of time, on the scale of over a century. It is this work of progressive maturation – the product of a permanent tension between the legacy of old ideas and the new ideas resulting from ever more numerous, rich and precise observations – which we present in this book.

      Chapter 1 provides an analysis of the meaning of words used in the 18th century to characterize air, atmosphere, ether and, more generally, subtle matters. Understanding the literature of the time on the atmosphere and subtle fluids requires a good understanding of the exact meaning of the terms used, which often differs from the meaning given to them today. For example, the term vapor, which we apply today