Daniell Hygrometer
From the collection of National Museum of American History. Free to download, adapt and use — no permission needed.
John Frederic Daniell, a young English natural philosopher, described this type of instrument 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. These instruments, Daniell said, were "accurately constructed, and packed in a box for the pocket, by Mr. Newman, Lisle-Street." The reference was to John Frederick Newman, a noted instrument maker in London.
National Museum of American History’s own description
