1820 Daniell Hygrometer

Invented in 1820 by John Frederic Daniell, an English natural philosopher. This unique and scarce Hygrometer consists of a glass ball partially-filled with a low-boiling point liquid (likely sulfuric acid); connected via a long, thin glass tube from which air has been evacuated, to a second ball which is then covered in muslin . When water is placed on the muslin-covered ball, the evaporation chill will cause the vapor ("ether") inside to condense, forming a visible dew-point on the opposite balls' surface. 

A wooden stand and base houses the instrument. An outer white porcelain thermometer is mounted on central wooden rod, graduated every degree and numbered every ten degrees, ranging from -20 to +50 degrees Centigrade. The wet-bulb apparatus consists of a U-shaped glass tube, each end terminating in a ball. The longer tube houses a glass thermometer with the base of the thermometer emersed in a liquid.  

Later Model Featured in The Smithsonian (National Museum of American History).

Diameter: 4.5"

Height: 13"

Glass Width: 7.5"

Weight: 12 oz.