PHYSIOGRAPHY. 



[chap. 



ments used for this determination are based on the principle 

 to which reference has just been made. Thus, the instru- 

 ment designed by the late Prof. Daniell, and represented in 

 Fig. 13, consists of a glass tube bent twice at right angles, 

 and terminated at each extremity by a bulb : one of these 

 bulbs. A, contains ether, while the other, B, is empty and 

 inclosed in muslin. If a few drops of ether be poured 

 upon this muslin, the ether-vapour within the tube is con- 

 densed, and the liquid in A rapidly evaporates ; but this 



Fig. 13. — Daniell's h^'gronieter. 



evaporation is attended with reduction of temperature, and 

 consequently the bulb A is rapidly cooled. When the 

 temperature of the surrounding air is sufficiently lowered, 

 the dew-point is reached, And a film of moisture is then 

 deposited on the outside of the bulb A. The temperature 

 at which this takes place is indicated approximately by the 

 thermometer inside the bulb, while the temperature of the 

 air outside is given by the thermometer placed on the 



