STEAM AND VAPORISATION. 



for the purpose of measuring the dryness of 

 the air is called a hygrometer, and is frequently 

 placed in parks for public observation. The 

 hygrometer, however, has its bulb B exposed to 

 the air instead of being covered by water ; 

 and thus it more quickly takes the temperature 

 of the air for comparison with the other ther- 

 mometer. 



We have placed the bulb in water in order 

 to illustrate the paradox that a thing which is 

 sunk in cold water may be less cooled than one 

 which is in the air and only wetted by the water 

 at a distance. 



