SCIENCE OF COMMON THINGS. 



Situations in which no dew is deposited. 



Cold condensing the vapor of the air wlien near the 

 point of saturation. 



1067* Why do mist and fog vanish at sunrise 1 } 



Because the condensed particles are again changed 

 into invisible vapor by the heat of the sun. 



1068 Why is dew most abundant in situations most exposed? 



Because the radiation of heat is not arrested by 

 houses, trees, hedges, or any other thing. 



1069 Why is there scarcely any dew under a leafy tree ? 



1. Because the thick foliage of a tree arrests the radi- 

 ation of heat from the earth ; and 



2. A leafy tree radiates some of its own heat towards 

 the earth in consequence of which the ground under- 

 neath a tree is not sufficiently cooled down to chill the 

 vapor of the air into dew. 



1070 Why is there never much dew at the foot of walls and hedges? 



1. Because they act as screens to arrest the radiation 

 of heat from the earth ; and 



2. They themselves radiate some portion of heat 

 towards the earth ; in consequence of which the ground 

 at the foot of walls and hedges is not sufficiently cooled 

 down to chill the vapor of the air into dew. 



1O7*1 Why is there no dew on a windy night % 



1. Because the wind evaporates the moisture as fast 

 as it is deposited ; and 



2. It disturbs the radiation of heat, and thus dimi- 

 nishes the deposition of dew. 



1O7*!3 Why are valleys and hollows often thickly covered with dew, 

 although they are sheltered f 



Because the surrounding hills prevent the agitation 

 of the air, but do not overhang and screen the valleys 

 sufficiently to arrest the radiation from their surfaces. 



1O7*3 Why does dew fatt more abundantly on some substances than on 

 others? 



Because some substances radiate heat more freely 

 than others, and therefore become much cooler in the 

 night. 



