110 



ELEMENTS OF GENERAL SCIENCE 



steam. At the end of half an hour's vigorous boiling, a kettle 

 of water will have the same temperature as at the beginning of 



the period, but the amount 

 of water hi the kettle will 

 have decreased. The effect 

 of the heat, therefore, has 

 been to evaporate the water, 

 but it has had no effect 

 upon the temperature. The 

 flame or stove has lost the 

 heat which has passed into 

 the water, but the water is 

 not hotter because of it. 



The total amount of heat 

 that disappears when a gram 

 weight of water changes 

 into steam is the same 

 whether the steam forms 

 rapidly, as in boiling, or 

 slowly, as in evaporating 

 from wet clothing. In the 

 latter case there is usually 

 no source of intense heat, 

 such as the flame, and the 

 heat which disappears dur- 

 ing evaporation is taken 

 from the water itself and 

 from the surrounding ob- 

 jects (fig. 59). Since heat 

 disappears from the water 

 and its surroundings, they 

 are cooler. In the case of 

 the bather on the beach, 

 the heat which was used in 

 evaporating the water from 



FIG. 59. Wet-bulb and dry-bulb 

 thermometers 



The hiilb of one of the thermometers is 

 covered with a cloth, which is kept moist 

 hy the water in the glass reservoir with 

 which it is connected. Evaporation cools 

 the wet-bulb thermometer, which regis- 

 ters the lower temperature 



