62 'ftature'a /HMracles. 



there can be but little evaporation. One of 

 the effects of evaporation is to withdraw heat, 

 and so to produce cold in the substance from 

 which the evaporation takes place. 



If we put water into a vial and drop regu- 

 larly upon it some fluid that evaporates readily 

 it will extract the heat from the vial and the 

 water in it to such an extent that in a short 

 time the water will be frozen. In hot coun- 

 tries ice is manufactured on a large scale upon 

 the principle that we have just described. 

 Water is put into shallow basins, excavated in 

 the earth, over which is placed some substance 

 like straw that readily radiates heat, and on 

 the straw are placed porous bricks, that are 

 kept wet, thus furnishing a very large evapor- 

 ating surface. In this way the process of 

 evaporation is carried on very rapidly and 

 the heat is extracted from the water to such an 

 extent that it freezes, often forming ice in one 

 night over an inch in thickness, and this in 

 the hottest climates on the globe. Evapora- 

 tion cannot go on in places where the air is al- 

 ready saturated with moisture. When the air 

 is dry evaporation is very rapid, but as it be- 

 comes more and more filled with moisture the 

 evaporation is checked to the same degree. 

 This fact accounts for the difference of bodily 

 comfort that we experience at different times 

 in the year when the temperature is the same. 

 Sometimes we are very uncomfortable al- 



