John Frederic Daniell, a young English natural philosopher, described this form in 1820. It consists of two glass balls, one partially filled with ether and the other covered with muslin, connected by a thin tube from which the air has been evacuated. There is a thermometer in the tube above the ball with the ether, and another on the supporting stand. When a few drops of ether are poured on the muslin, their evaporation chills the covered ball; that in turn causes the ether vapor inside the instrument to condense, thereby cooling the other ball and causing dew to form on its surface.
This example has a wood stand. The interior thermometer is mounted on a white porcelain plate that is graduated every degree C. from -9 to +50 and every two degrees F. from -24 +120. The exterior thermometer is mounted on a brass plate that is graduated every degree C. from -10 to +50 and every two degrees F. from +20 to +121. There is a gold band around the equator of the lower bulb.
Ref: J. F. Daniell, "On a new Hygrometer, which measures the Force and Weight of aqueous Vapour in the Atmosphere, and the corresponding Degree of Evaporation," Quarterly Journal of Science 8 (1820): 298-336.
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