The Paper describes experiments made with the object of developing suitable apparatus for the measurement of humidity at low temperatures. The three classical methods - the wet and dry bulb hygrometer, the dew-point apparatus, and the hair hygrometer - were studied and so modified that they could be used in a low temperature room inaccessible to the observer. The common form of wet and dry bulb hygrometer is notoriousl unreliable, but the ventilated form is quite satisfactory provided the wind velocity past the bulb exceeds about 3 meters per second. A tubular form of wet and dry bulb hygrometer was developed with an electric fan for aspiring the air past the bulbs. A continuously-recording, resistance thermometer, type was also made, which had a cam device for rapidly dipping and withdrawing the thermometer bulb at periodic intervals from a reservoir of water. The dew-point apparatus was modified into a form suitable for permanent installation; the cooling of the polished metal surface being effected by pipe connections to the cold brine circulation of the stores. In another form a silver thimble, cooled by the evaporation of ether, was placed at one end of a long aluminium tube and a telescope at the other. By suitable arrangements of mirrors and lamps both the silver surface and the thermometer column were thrown into the field of the telescope. The end containing the thimble was inserted into the enclosure through a circular aperture. The instrument was portable and self-contained. A distant reading type of dew-point apparatus was constructed, in which the formation of dew was indicated by the change of reflecting power of the cooled surface. A beam of heat radiation was reflected from the polished surface and concentrated on to a minute thermopile. The thermo-junctions were connected to a pivoted indicator situated at the observation station where the observer controlled the flow of cooling fluid by means of a solenoid valve. The formation of dew resulted in a sharp falling-off of the reflecting power and was indicated by the movement of the pointer of the instrument. The hair hygrometer was studied under a variety of conditions and the changes in the calibration of this type of instrument, caused by exposure to low and high temperature, &c., were determined. A very simple form of distant reading hair hygrometer was developed. In this the clockwork drum of the ordinary recorder was replaced by a tubular rheostat. The pointer was pressed into contact with the drum when a reading of the humidity was desired. The two portions of the resistance winding then constituted the two arms of a Wheatstone's bridge. A number of other forms of hygrometers were also studied, notably the one utilising the heating effect on dry cotton when exposed to a humid atmosphere. A convenient form of apparatus for the calibration of hygrometers over a wide range of humidities is described. It consists essentially of a drum fitted with a heavy glass front making an air-tight joint. The drum contains an electric fan which circulates the air over a dish of strong sulphuric acid. This method of reducing the humidity is more effective than bubbling and does not produce acid spray. The acid dish can be cut out of communication with the chamber by a ground glass cover operated by a lever projecting out of the drum.