Eighteenth-Century Meteorological Theory and Experiment

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Eighteenth-Century Meteorological Theory and Experiment

Overview

Eighteenth-century meteorological study in Europe marked interest in methodical observations on a larger scale, providing a more systematic and cumulative database to formulate early theories of atmospheric movement and phenomena. With innovative instrumental designs, laboratory experiment and resulting new fundamental laws of physics helped the progress of meteorology into field application. Expanding theory included further delving into the general circulation of the atmosphere, the nature of lightning, and some delineation of the basic physical extension of atmospheric parameters or indicators (pressure, temperature, and humidity). Perhaps the most significant and fundamental advance marking eighteenth-century meteorology was early attempts at mathematical correlation through the study of fluid flow.

Background

While seventeenth-century interest in the atmosphere had mainly dealt with basic conception of atmospheric parameters and devising experimental instrumentation to record these parameters, methodical data gathering had not spread widely enough. An almost faddish interest in meteorological measurement pervaded the eighteenth century, and now well-established scientific academies from the previous century turned to more concerted daily observations and made attempts at cooperative networks of atmospheric data recording from city to city. Meteorological data collecting at sea proceeded in much expanded fashion from the seventeenth century, with wind data of particular interest for nautical charts. Comprehensive enough meteorological data prompted some attempts at dealing with theory of the general circulation of the atmosphere, initially by the otherwise obscure Ralph Bohun (d. 1716) and later astronomer Edmond Halley (1656-1742). Halley had the right idea about the trade winds of the tropics arising from upward flow of equatorial air with north and south air flow induced to replace it, but he could not explain the easterly orientation of this air flow.

The seventeenth century had initialized the empirical and instrumental inspection of the atmosphere. Galilei Galileo's (1564-1642) profound experimental acumen had included questions dealing with atmospheric pressure and credit for the first glass-bulb thermometer. England's Francis Bacon (1561-1626) with his dedication to inductive philosophy used his own fascination with the wind as the one completed subject for his method. Rene Descartes (1596-1650), who introduced the corpuscular theory of matter and motion, applied it to investigating the phenomena of the atmosphere, particularly the rainbow.

Groundwork in researching the nature of gases was one hallmark of the seventeenth century. And important correlation in methodology between laboratory experimentation and instrumentation was followed by meteorological applications probing the parameters of the atmosphere. Galileo's student Evangelista Torricelli (1608-1647) invented the glass tube mercury barometer (there were also water barometers) for measuring atmospheric pressure. Robert Boyle (1627-1691) would coin the term "barometer" and use the instrument in the laboratory and in weather observation along with various hygrometers (indicating atmospheric humidity) in observations of changing weather. He and other familiar names—fellow Englishman Robert Hooke (1635-1695) and Dutch Christian Huygens (1629-1695)—all studied relevant temperature scales centering on freezing and boiling points of water. Hooke and architect/scientist Christopher Wren (1632-1723) invented rain gauges and the first attempted multiple functioning weather instruments, or weather clocks, using a clock as a drive to register remote readings—still 200 years in the future. Gottlieb Wilhelm Leibnitz (1646-1716) conceived the aneroid or vacuum-chamber barometer, and Pierre Daniel Huet (c. 1721) developed the idea of the pressure-tube anemometer for measuring wind speed.

Impact

Why did the tropical winds move with a easterly slant toward the equator, not north and south as Halley had said? In 1735 a London lawyer who dabbled in natural philosophy and made regular weather observations, George Hadley (1685-1768), was the first thinker to understand that the rotation of the Earth was the cause, and set it down in a paper titled "Concerning the Cause of the General Trade Winds." Hadley thought in terms of equatorial atmospheric air cycling by warming and rising then flowing out and sinking. The flow became known as the Hadley Cell. And his effort was the first limited but correct explanation of tropical circulation of the atmosphere.

Wind observations with respect to storm movement prompted ideas about the scale of weather patterns. Again, seafarers were well acquainted with changing weather and the bad winds that accompanied the worst. They had logs of observations on these and the rarer great whirling storms in tropical oceans: hurricanes, cyclones, typhoons. American political philosopher/scientist Benjamin Franklin (1706-1790), who included weather studies among his polymathic interests, was an acute observer. On observing the passage of a storm in October of 1743 at Philadelphia not reaching Boston to the northeast until the next day, he studied other observations of the storm and other similar storms and reasoned correctly that the prevailing trajectory of east coast weather was from the southwest to the northeast. Investigating storm movement and differentiating between local surface winds and wider scale storm winds would become a meteorological focus of the next century.

Atmospheric phenomena had always prompted curiosity and certainly superstitious fears. Nothing in the atmosphere put more fear into people and conjured more allusions to myth and an avenging God as lightning and thunder. The true nature of lightning became known before the middle of the eighteenth century, prompted by the century's mania for demonstrating electrostatic phenomena, whether as a study in the laboratory or as a diversion in the parlor. One of the most dedicated researchers was the celebrated Abbé Jean Antoine Nollet (1700-1770), whose electrical research, starting about 1730, had included collaboration with fellow Frenchmen chemist Charles Francois Dufay (1698-1739) and technologist René Antoine de Réaumur (1683-1757). Dufay had discovered the attractive-repulsive nature of electricity and believed that this indicated the existence of two separate electrical fluids moving in opposite directions and inducing charge on bodies. Nollet agreed.

Various simple electrical apparati were used in studying the nature of static electricity from conductive materials to containers to hold electric charge, the Leyden Jar, and measuring gauges. Charging these jars and discharging them in ever more spectacular ways became a favorite demonstration. Nollet, one of the first, and others constructed "electroscopes" or electrometers—crude gauges which would indicate the presence of electric charge by the separation of suspended pith balls or strips of metal foil. It was not long before the spark of discharging static electricity prompted thinkers to conjecture that lightning might also be electricity.

Franklin, best known for his electrical experiments, began research in 1746, deciding (1751) that there was one fluid of electricity leaving surfaces positively or negatively charged. Franklin also decided that lightning was but a huge electric spark. With the basic equipment being used, he proposed (1752) a method for carrying out an experiment with lightning—never realizing sufficiently how dangerous that was. During a thunderstorm Franklin flew a kite—even more dangerous—high enough that electric charge traveled down the string and charged a Leyden Jar. Lightning was an electric discharge. The experiment was actually carried out by several researchers in differing circumstances before Franklin.

Probing the atmosphere in the field was yet another interesting aspect of eighteenth-century meteorological investigation, starting with much weather observing at the systematic level. The serious eighteenth-century observer realized the importance of some sort of systemization to accurate recording of weather observations. Coordinated weather observing was carried out by Britain's Royal Society, France's Academie des Sciences, in central Europe, and Russia. In 1780 under the auspices of the Societas Meteorologica Palatina of Mannheim, Germany, a network of 39 weather-observing stations began operations with the first standardizations of mounting and reading instruments, which was later followed and imitated by other groups.

There were many observer/instrument enthusiasts taking extended observations but largely known for other accomplishments. There was John Dalton (1766-1844), famous for his atomic theory and interest in the properties of gases (partial pressure), who was an avid weather observer. He acquired the interest from his Quaker schoolmaster Elihu Robinson, who observed and also built weather instruments. Dalton kept daily weather records from his childhood until his death and from this developed an acute interest in forecasting weather "with tolerable precision" as important "advantages" to humanity. He published his ideas about observing and making instruments in Meteorological Observations and Essays (1793). Benjamin Franklin and fellow Americans George Washington and Thomas Jefferson were weather observers. More central figures in meteorology were Jean-André Deluc (1727-1817) and Horace-Benedict Saussure (1740-1799). Saussure, the inventor of the hair hygrometer (1783), disputed theories of evaporative cooling with De Luc, who invented a gut hygrometer and shared with Joseph Black the discovery of the latent heat property.

Other observing went further afield. And this was important to the early conceptions of the spacial delineation of atmospheric parameters: pressure, temperature, and humidity. Climbing mountains was one way to ascend higher to appraise characteristics of the atmosphere. The only problem was that with few exceptions, prior to the mid-seventeenth century, no one wanted to climb mountains, which were considered anomalies of nature. But mountaineering interest and meteorological field research came together about the same time late in the eighteenth century. Mont Blanc was climbed in 1786. Saussure followed suit with his own ascent of the highest mountain in the Alps the next year with scientific equipment and a large contingent. Taking barometers, hygrometers, and thermometers on mountain excursions became a popular pursuit. In the process, the fact that atmospheric pressure, humidity, and temperature decreased with height prompted the urge to venture into the atmosphere itself.

Humanity was already heading into the free atmosphere in 1783 with the first hot air balloon flights. Among other intrepid balloonists was physicist Jacques Charles (1746-1823), who would add the factor of temperature (Charles Law, warming/expanding and cooling/contracting of a gas) to Boyle's inverse law of pressure and volume. Experimenting with hydrogen-filled balloons, he finally made a flight at the end of 1783 in his own designed balloon, taking a barometer to use as an altimeter. Two years later American Dr. John Jeffries (1744-1819) took a barometer, hygrometer, and thermometer on a balloon trip up to 9,000 feet (2,743 m), recording those parameters. On subsequent balloon flights by others, more would lie in store than was bargained: freezing temperatures and violent storms. The atmosphere was being observed—sometimes painfully—but at the source and with accurate instruments.

Interpreting meteorological data mathematically was a slow process, but a string of brilliant mathematicians put the process underway in the eighteenth century through the development of the science and mathematics of hydrodynamics. The first of these was the most celebrated of the Swiss Bernoulli family, Daniel (1700-1782), whose greatest published work was his Hydrodynamica (1738). He explained both hydrostatics and dynamics via the mechanics of Newton with many examples of forces in fluid flow. Following him was the one Frenchman of the group, Jean le Rond D'Alembert (1717-1783), whose ideas centered on analyzing problems of dynamics by static means (bodies at rest and acted on by forces). He applied this method to coming up with rather involved equations for showing the planar motion of fluids, further applying this to the ocean and the atmosphere. The latter was an important confirmation of what many had suspected since the seventeenth century: the air behaved like a fluid. The most advanced ideas were those of Swiss Leonhard Euler (1707-1783). He was the first to elucidate on the force of pressure in fluid flow. In three papers between 1753 and 1755, he formulated the basic equations and concepts of fluid mechanics.

The study of weather and basic meteorological parameters stimulated advances in understanding the workings of the atmosphere—and also some fundamental scientific method. In the latter case, Swiss physicist/mathematician Johann Heinrich Lambert (1728-1777) included among his varied research the first ordering of recorded atmospheric measurements with the use of graphical plots to interpret the data, a fundamental method of interpreting the relationships of parameters in modern science.

WILLIAM J. MCPEAK