METEOROLOGICAL INSTRUMENTS. 223 



Electrical thermographs must necessarily have open tubes : the 

 arrangements for record are similar to those of the electrical 

 barographs already described. 



Solar radiation thermometers are maximum thermometers with 

 the bulb and a part of the stem coated with lampblack. They 

 are then enclosed in another glass envelope, which is rendered as 

 perfect a vacuum as possible before sealing. 



Negretti introduces a mercurial pressure gauge to measure the 

 vacuum. Hicks solders platinum wires into the tube, and tests 

 the condition of the vacuum-by the electric current. 



HUMIDITY. 



Humidity is measured by the Hygrometer, and this can be 

 effected either in the direct or indirect way. Daniell's hygrometer 

 belongs to the former class, and is exhibited by Mr. Symons. 

 It consists of two bulbs connected by a tube ; one is of black 

 glass and contains a thermometer, the other of clear glass and 

 enveloped by muslin. Some ether is introduced into the instru- 

 ment before sealing. The mode of making an observation is 

 to drop some ether on the muslin ; this cools the coated bulb, 

 and the ether inside the instrument is condensed in that bulb. 

 The evaporation causes the temperature of the black bulb to fall, 

 and the moment of deposition of dew on it from the external air 

 is observed. The temperature at which this takes place is given 

 by the enclosed thermometer. 



Regnault's is a better form of direct hygrometer. In it the 

 thermometer is enveloped in a silver casing, ether is introduced 

 into the casing, and its vapour removed by an air-pump. Dew 

 forms on the silver when the temperature falls sufficiently. 



Of indirect hygrometers only two need be noticed Saussure's, 

 in which the moisture of the air is measured by the length of a 

 hair, which stretches when wet, and shrinks as it dries. This 

 motion is transferred to a hand moving on a graduated arc. The 



