Direct reading psychrometer after Lowe
The psychometer consists of a dry bulb thermometer, a wet bulb thermometer, an index knob and a table which allows the reading of relative humidity, dew point and tension of water-vapour accoring to temperature scales. The parts are fixed on a wooden frame.
In order to take a reading, the index knob is moved up or down until the upper index gives the same value on the temperature scale to the left as the dry bulb thermometer on the right. the index knob is then turned, without being moved, until the lower index gives the temperature - value of the wet bulb thermometer. The point at which the tip of the indicator is situated gives simultaneously the relative humidity, the dew point and teh tension of water-vapour.
(See also: http://www.freunde-alter-wetterinstrumente.de/61news06.htm)