The hygrometer is a device for measuring the relative humidity of air. 

  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.