THIRD GRADE FALL WORK 129 



In this connection a lesson on dew and conditions under 

 which it is formed will be appropriate. Have the children 

 describe dew. Where is it found? On what things does 

 it form? On what things does it not form? When is it 

 formed? Does it form on nights that are clear? cloudy? 

 warm? cool? windy? calm? Where does it come from? 

 What is the effect of a wind upon the formation of dew? 

 What is the effect upon dew of extreme heat ? of extreme 

 cold ? What good does it do ? Does it ever do harm ? 



Discuss the heavy dews of the tropical regions. Breathe 

 upon the cold window pane, or perform other simple experi- 

 ments to show how water comes from vapor. 



NOTE. Dew is formed when warm moist air comes in contact with 

 cool substances that condense the moisture. Plants lose their heat more 

 rapidly than stones and other common objects, and therefore condense 

 more moisture generally. In the arid regions where the ground is 

 irrigated, grasses, rather than tall trees, catch the dew, since the mois- 

 ture comes chiefly from the ground near them. In tropical forests, 

 however, the trees drip with dew condensed from the upper currents 

 of the moist air from the ocean. 



LESSON V 



FROST 



When the night is very cold, what forms instead of dew? 

 Examine with a lens some frost crystals. Are they frozen 

 drops of dew? 



Explain that the crystals are formed gradually from the 

 vapor, which does not condense first into drops and then 

 freeze into ice; the frost crystals, at first very small, grow 

 gradually larger by the addition of more vapor condensed 

 and frozen simultaneously. 



NAT. STUDY 9 



