EVAPORATION, BOILING TEMPERATURE, ETC. 11 



Drop a few drops of alcohol or gasoline upon the cloth. Watch for 

 a change in temperature. Repeat the experiment. Does the ther- 

 mometer record the lowest temperature when the liquid is first 

 dropped upon it or a little later? How do you account for this 

 fact? Draw a conclusion regarding the effect of evaporation upon 

 temperature. 



Wet your hand and hold it out of the window where the wind can 

 strike it. What is the sensation? 



Hunters usually wish to approach their game from the 

 leeward side, that is, so that the wind will blow from the game 

 toward the hunter. Why do they wish to do so? In order 

 to tell the direction of the wind when it is too light to be ob- 

 served readily by ordinary means the hunter often wets one 

 finger at his mouth and then holds it high above his head where 

 the wind can strike it. Try the experiment when you are out of 

 doors and decide how it is that this tells the direction of the 

 wind. 



Why do we often feel chilly if we sit down with damp cloth- 

 ing on? Why does fanning one's self produce the cooling 

 effect that it does? Is the cooling effect of fanning increased 

 or decreased by the fact that one has been perspiring freely? 



In regions having a dry, hot, windy climate like Arizona 

 and New Mexico, it is found that butter can be kept hard 

 and sweet for long periods simply by setting the dish contain- 

 ing the butter into a larger dish containing water and then 

 spreading over the butter a piece of soft absorbent cloth, such 

 as a clean towel, so that its corners and edges dip down into 

 the water. Explain. 



In the same regions drinking water is generally kept dur- 

 ing the summer months in a large, porous, earthen vessel 

 called an OLLA (o-ya), which is hung out of doors, usually under 

 a porch or tree. The olla being porous, its outer surface is 

 constantly covered with a film of water. Evaporation taking 

 place over so large a surface cools the water within the vessel 

 far below the temperature of the surrounding air. 



13. Laws of Evaporation. From these and similar experi- 

 ences we draw the following conclusions: 



