ICE, WATER, AND STEAM 



121 



body of the bather, thus 

 giving him the sensation 

 of cold. As soon as all the 

 water had evaporated from 

 the suit this withdrawal of 

 heat ceased and the sur- 

 rounding air temperatures 

 were felt. 



Ether and alcohol evap- 

 orate so much more rapidly 

 than water that they feel 

 very cold when poured over 

 the hand. Ether may pro- 

 duce so low a temperature 

 that water will be frozen 

 by its use. Surgeons some- 

 times use ether to partly 

 freeze the surface of a por- 

 tion of the body upon which 

 a surgical operation is to be 

 performed, since the frozen 

 tissues are so numb that the 

 pain is lessened. 



In many hot, dry coun- 

 tries the water from wells 

 is usually warm and needs 

 cooling. Often ice cannot 

 be had, and other methods 

 of cooling must be adopted. 

 The water is put into 

 porous earthenware jars or 

 into canvas bags, and these 

 are placed where the dry 

 winds can blow over them. The water soaks through the 

 canvas or the porous jar and evaporates so rapidly in the 



FIG. 61. Wet-bulb and dry-bulb 

 thermometers 



The bulb of one of the thermometers is 

 covered with a cloth, which is kept moist 

 by the water in the glass reservoir with 

 which it is connected. Evaporation cools 

 the wet-bulb thermometer, which regis- 

 ters the lower temperature 



