SCIENCE OF COMMON THINGS. 185 



Where hoar-frost does not accumulate. Frostwork on windows. 



The thick fog which invested the earth during the 

 night (being condensed by the cold frost of early morn- 

 ing) is congealed upon every object with which it comes 

 in contact. 



1221 Why is there little or no hoarfrost under shrubs and shady trees f 



1. Because the leafy top arrests the process of radi- 

 ation from the earth ; and 



2. Shrubs and trees radiate heat towards the earth ; 

 and therefore the ground beneath is rarely cold enough 

 to congeal the little dew which rests upon it. 



1SSS Why does hoarfrost very often cover the ground and trees, when 

 the water of rivers is not frozen ? 



Because it is not the eifect of cold in the air, but 

 cold on the surface of the earth (produced by excessive 

 radiation), which freezes the dew condensed upon it. 



1223 Why is the hoarfrost upon grass and vegetables much thicker 

 than that upon lofty trees ? 



Because the air (resting on the surface of the ground) 

 is much colder after sunset than the air higher up in 

 consequence of which more vapor is condensed and 

 frozen there. 



1224 What is the cause of the pretty frostwork seen on bedroom win- 

 dows in winter-time? 



The breath and insensible perspiration of the sleeper 

 (coming in contact w T ith the ice-cold window) 2x0, frozen 

 by the cold glass, and, crystallizing, form those beautiful 

 appearances seen on a winter morning. 



1225 Are all the figures of frostwork formed in accordance ivith certain 

 fixed laws ? 



All these figures are limited ~by certain laws, and the 

 lines which bound them form among themselves no 

 angles but those of 30, 60, and 120. 



1226 If you fracture thin ice by allowing a pole or weighi to fall upon 

 it, will the lines of the fracture have anything of regularity ? 



Yes ; the fracture will generally present a star with 

 six equidistant radii, or angles of 60. 



122 7 Why is a glass or earthen vessel apt to break when hot water is 

 poured into itf 



Because the inside of the glass is expanded by the 



