146 ELEMENTARY SCIENCE 



at the top of the room. It is also due to convection that 

 great movements of air occur in the earth's atmosphere 

 and produce thereby profound effects upon weather and 

 climate (see Fig. 2); of this matter there will be more 

 discussion in the chapter on seasons and weather. 



QUESTIONS 



1. Why is hotting water always at the same temperature? 



2. What is the "heat of fusion" of ice, and how would you mea- 



sure it ? 



3. Explain why evaporation is a cooling process. 



4. Why do we feel more uncomfortable on hot moist days than 



on hot dry days? 



5. Why will drinking water keep cool if put into a porous vessel ? 



6. Why does it take longer to freeze a deep body of water than a 



shallow one? 



7. What are convection currents? 



