The French revolutionary period brought a precipitous decline to meteorology. The correspondence of the Société royale de médecine ceased with the onset of the Revolution in 1789; along with all the Old Regime academies the Society was suppressed in 1793. The Palatine Society, already in decline after the death of Hemmer in 1790, collapsed in 1795 when the French army crossed the Rhine, occupied Mannheim, and closed the Academy of Sciences. In Germany the number of stations observing the weather dropped during the 1790s to a third of its peak value in the previous decade.
Meteorological contributions to the Royal Society of London's Philosophical transactions had already diminished in the second half of the 1780s.
No doubt other factors besides the physical disruption of war and revolution contributed to the decline. Politics distracted natural philosophers; van Swinden, for example, became involved in administrative and educational reform. When in 1800 Cotte requested some earlier observations from him, van Swinden wrote in regret of "the oblivion to which I have consigned them, regretting all the while that so much research produced so few useful results." His remarks suggest a further reason for the loss of interest in meteorology. Meteorologists like Cotte had promised "to prove the usefulness of meteorological observation to those who deride [it]." Nothing positive seemed to have come from the expenditure of so much effort on what many considered a tedious and trivial activity.
The program of late Enlightenment climatology had failed. Meteorology had not brought "the perfection of the sciences of agriculture and medicine." No useful correlations had been discovered between the weather and agriculture or disease, and correlations like the lunar influence that might lead to predictive rules remained in doubt. "All attempts at rules governing the weather have been in vain," remarked the astronomer Bode. There were a number of causes for this failure. The meteorological projects of the late Enlightenment still confronted some of the limitations of the earlier decades of the century. Instruments had neared perfection but remained unavailable in the more remote French provinces and German states. Rigorous standards had been established for observers but were not always followed outside scientific centers. Institutional arrangements were weak in many areas; this was especially true in central and eastern Europe, where projects suffered from the small size and political fragmentation of the German states. And editors of
meteorological ephemerides retained the practice of publishing summaries rather than the full record of observations, to the frustration of generations of meteorologists.
Cotte and his colleagues no doubt believed that they might have reached some of their goals if the Revolution had not interrupted them. They were mistaken. Their search for correlations between weather patterns and agriculture and disease belonged to the "classical" episteme , as Foucault called it, the episteme of natural history and nosology. Meteorologists working within this episteme associated the weather with diseases and agriculture according to superficial correlations, without penetrating to the interior forces that govern their interactions. But climatology (parallel with the rest of Western knowledge, if Foucault was right) was moving in the last decades of the 18th century away from this approach toward the perception of these interior relations.
It was approaching this goal through quantification. Quantification here did not include mathematical theories of the weather. Other parts of meteorology did develop mathematical theories. In hygrometry, thanks to a new hygrometer and the techniques of exact experimentation, de Saussure and others announced Dalton's law of partial pressures for the special case of aqueous vapor: the total pressure of moist air at a given temperature is the sum of the pressures of its components, water vapor and dry air. From this elementary arithmetical relation de Saussure calculated the relative weights of dry and saturated air, using nothing more complicated than simple proportions. More sophisticated mathematical laws were worked out in the 1790s governing the relation of vapor pressure to temperature. In barometry Lambert, working in the tradition of mixed mathematics, had applied the integral calculus to the variation of air pressure with height in the 1760s. Studies of the distribution of
heat over the earth employed trigonometry to calculate the effect of the sun's heat in different latitudes. All these results lay in areas of contact of meteorology with the numerous disciplines discussed in the introduction to this chapter. Except for one or two isolated examples, meteorology proper—the study of weather phenomena such as rain and snow, winds, and storms, as well as climatology—lacked any mathematical theory until well into the nineteenth century.
In climatology quantification meant precise instruments and a rigorous discipline of observation. The example of the Société royale de médecine shows how closely tied this discipline was with the bureaucratizing impulse of enlightened absolutism. Once observations were collected, meteorologists analyzed them using the most elementary statistical techniques: counting (the number of days of rainfall, the number of days on which the wind blew from different quarters), taking averages and ratios (means of temperature, pressure, etc., the proportion of rain falling in each season, the proportion of changes in the weather occurring within three days of the moon's syzygies and quadratures), and establishing correlations (between the weather and diseases, among the weather at different locations and times, the weather rule). It is no accident that a discipline so close to public health and administration should rely on statistical techniques.
Limited as it was, this type of quantification pushed climatology to the verge of a new existence. The "discipline" of regular observation and intensive calculation led toward the "discipline" of climatology: the practice of deriving a standard set of variables representing the typical weather of a location. By enforcing a finer coverage in time and space, this discipline led meteorologists' perception (or "gaze," as Foucault called it) toward the creative functions of time and space in weather and climate.
Even so, there is a tremendous gap between the climatology of the late Enlightenment and that of the early years of the 19th century. The most impressive result of late 18th-century climatology, the diurnal barometric variation, remained a correlation. The discovery of the great extent of barometric variation did not capture the full role of space in modifying weather and climate. Steiglehner's "early in the west, late in the east" still sounds like a weather rule.
The climatology of the first years of the 19th century looks vastly different. Alexander von Humboldt may stand as a representative of the new point of view. Humboldt's isotherms (lines of equal temperature), presented in an essay of 1817, are syntheses of mean annual temperatures across the globe, which present immediately to the eye the role of space in the development of climate. In the same essay Humboldt speaks naturally and comfortably of the climates of the different regions of the earth; develops comparisons among the climates of eastern and western shores of continents and among coastal, continental, and island climates; and arrives at the notion of a "climatic system"—a region in which the different factors of climate vary in a continuous manner, so that in specifying for example the mean annual temperature one has fixed by implication the range of other climatic factors such as winter and summer temperature. Foucault argued that between 1775 and 1795 naturalists found the key to the new episteme —the notion of organic structure, in the case of biology—but still applied it to the old task of classification. During the same decades exact experimental physics had provided the key to an appreciation of the role of space and time in climate and the weather, but meteorologists still applied that key to the search for correlations. A mature climatology awaited the age of Humboldt.