A psychrometer determines humidity by measuring the cooling effect of evaporation. This example has two mercury in glass thermometers, Each mounted a silvered brass plate, and the whole mounted on a wooden board. The plate holding the wet-bulb thermometer is marked "No. 1868 SIGNAL SERVICE U.S. ARMY" and the plate holding the dry-bulb theremometer is marked "H.J. GREEN, N.Y" and "No. 3070 SIGNAL SERVICE U.S. ARMY."
The Signal Service maintained a national weather service from 1870 until the establishment of the U.S. Weather Bureau in 1891. This instrument was made after 1885 when James Green retired and his nephew, Henry J. Green, went into business on his own; and it was probably made before 1890 when H.J. Green relocated to Brooklyn. The Weather Bureau transferred it to the Smithsonian in 1904.
Ref.: Henry J. Green, Meteorological and Scientific Instruments (Brooklyn, ca. 1890), p. 30.
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