Fixing the Sky by James Rodger Fleming Page B

Philadelphia in 1817. There he supported himself by teaching mathematics and classics part time at the Franklin Institute while devoting his free time to meteorological research. From 1834 to 1838, he served as the chairman of the Joint Committee on Meteorology of the Franklin Institute and the American Philosophical Society. He won the latter’s Magellenic Prize in 1836 for his theory of hail. Working with the scientific societies of Philadelphia, Espy gained the support of Pennsylvania’s legislature to equip weather observers in each county in the state with barometers, thermometers, and other standard instruments to provide a larger, synoptic view of the weather, especially the passage of storms. He also maintained a national network of correspondents and volunteer observers. During this period, he invented a “nephelescope,” an early cloud chamber, which he used in his popular lectures and, in his technical work, to calculate the amount of heat released by condensing water vapor.
    Espy moved to Washington, D.C., in 1842. In his first government appointment, as professor of mathematics in the navy, he developed a ventilator for ships and expanded his network of meteorological correspondents. He also held a joint appointment as the “national meteorologist” in the U.S. Army Medical Department, a position that boosted his storm studies by providing him access to the meteorological reports of the army post surgeons. From 1847 to 1857, his salary was provided by annual appropriations from Congress. With Joseph Henry, he established the Smithsonian meteorological system of observers and experimented with telegraphic weather reports. Several of his major reports on meteorology appeared as U.S. Senate executive documents. 8
    Espy viewed the atmosphere as a giant heat engine. According to his thermal theory of storms, all atmospheric disturbances, including thunderstorms, hurricanes,
and winter storms, are driven by steam power. Heated by the Sun, a column of moist air rises, allowing the surrounding air to rush in. As the heated air ascends, it cools and its moisture condenses, releasing its latent heat (this is the “steam power”) and producing rain, hail, or snow. Espy emphasized, correctly, the importance of knowing the quantity of vapor in the air, “for it is from the latent caloric [or heat] contained in the vapor that all the force of the wind in storms is derived. It is only when the dew-point is high that there is sufficient steam power in the air to produce a violent storm; for all storms are produced by steam power .” 9 His theory was well received by many scientists of his time, including a committee of the French Academy of Sciences chaired by François Arago. The convective theory is now an accepted part of meteorology, and for this discovery Espy is well regarded in the history of science.
    Espy strayed from the scientific mainstream when he promoted his idea that significant rains of commercial importance for agriculture and navigation could be generated by cutting and burning vast tracts of forest. He believed the heat and smoke from these fires would create huge columns of hot air, producing clouds and triggering precipitation, much like the effects of volcanic eruptions. He listed five scientific reasons why setting large fires should produce rain: (1) experiments showed that expanding air cools dramatically, and (2) under certain conditions of high humidity forms both a visible cloud and significant amounts of precipitation; (3) chemical principles indicated that the “caloric of elasticity” (a venerable term for latent heat) released in the condensation of this vapor is immense, equal to about 20,000 tons of anthracite coal burned on each square mile of cloud extent. Espy’s convective theory further held that (4) this release of heat would keep the cloud buoyant, lower the barometer, and “cause the air to rush inward on all sides toward the