STEAM AND VAPORISATION. 



B 



colder a substance kept quite under water, or 

 one out of the water but kept wet with it, he 

 would probably decide for the former. He 

 would be wrong, as he has often been before. 



The point is illustrated in Fig. 35, which 

 represents the lower part of two thermometers, 

 one of which has its bulb in the water, while 

 the bulb of the other 

 is just above the 

 water, but is kept 

 moist by some cotton 

 wool or wick, which 

 is tied round, above 

 and below the bulb, 

 and allowed to dip 

 into the water be- 

 neath. 



The thermometer 

 A always indicates 

 the lower tempera- 

 ture of the two in 

 steady weather. 



This effect depends upon the principle 

 which we noted in the last section, that it re- 

 quires heat-energy to convert water into 

 vapour, and consequently the water which is 

 supplying the heat necessary to vaporise a part 

 of itself is thereby cooled. 



In steady weather the water in the cistern of 

 our illustration is practically at the same tem- 

 perature as the air, and that is the temperature 



149 



FlG. 35. VAPORISATION DEPEND- 

 ING ON DRYNESS OF AIR: 

 THE HYGROMETER. 



