The hygrometer is a device for measuring the relative humidity of air. While there are many physical effects dependent on the amount of water vapor in the air which have been used to construct hygrometers, the three below are psychrometers that use the evaporative cooling of a thermometer bulb. Once the temperature of this bulb and a similar dry bulb are known, the relative humidity can be found using tables.

   The hygrometer at the middle, by J. A. Seitz of Boston, has a built-in table. When this table is elaborated into a nomagraph, the device is called a Hygrodeik. At the left is a hygrometer marked "U. S. Weather Bureau", and the instrument at the right is Mason's Hygrometer, made by Green of New York. The wick around the wet bulb was supplied with water from the bottom of a [missing] glass tube, which was held by the two clips in the center of the instrument.
             Wittenberg University                                    Middlebury College                              United States Military Academy

B. Daniell's Hygrometer
 

John Frederic Daniell (1790-1845) was the Professor of Chemistry at King's College, London and is best known for his  Electro-Chemical Cell that he devised in 1836. In 1827 he published a description of the hygrometer which bears his name.     Both of these examples are at Denison University in Granville, Ohio. The hygrometer in the middle column was purchased from Eimer and Amend in June 1894.

The closed glass system contains ether, which is condensed in the left-hand bulb, into which a thermometer dips. Ether is then poured over the right-hand bulb, which is normally covered with a muslin bag. The cooling effect causes ether vapor in this bulb to condense and lower the vapor pressure inside the tube. The liquid ether in the left-hand bulb then starts to evaporate, and its temperature falls.     At a certain point dew begins to form on this bulb, and the temperature of the ether is read; this is the dew point temperature for the surrounding atmosphere. The dew-point temperature is a well-known function of the relative humidity of air.

                 Colby College                                     Smithsonian Institution

This example is at the physics department of the University of Texas. Since the University opened in 1890, it probably dates from the 1900-era.     A similar device sold for $16.00 (including a fitted case) in the 1888 catalogue of Queen of Philadelphia.