118 Batute's /UMracles. 



leaves of all kinds, hay and straw, while they 

 are poor conductors of heat, are excellent 

 radiators. In another chapter we have re- 

 ferred to this quality of straw, that is taken 

 advantage of by the inhabitants of hot coun- 

 tries in the manufacture of ice and in our own 

 land for storing it. 



Perhaps everyone who has lived in the coun- 

 try has noticed that on a summer's morning 

 when the grass is laden with dewdrops a gravel 

 walk or a dusty road will be perfectly dry. 

 This is due to the fact that the gravel will re- 

 tain heat and not radiate it, for a much longer 

 time than grass or green leaves. Dew begins 

 to form upon the grass very soon after the sun 

 is set because the moment the sun's rays are 

 withdrawn the heat is rapidly radiated by the 

 blades of grass, which cools the earth under it 

 and the air above and surrounding it, so that 

 if the air is anywhere near the moisture satu- 

 ration point on cooling at the surface of the 

 ground it will readily give up a part of its 

 moisture, which condenses in drops upon the 

 blades of grass. 



If the night is still and clear and there is 

 much moisture in the air, the dew will be 

 heavy, but if the night is cloudy there will be 

 little or no dew formed. The clouds form a 

 screen between the earth and the upper regions 

 of the atmosphere, which prevents the heat 

 from radiating to a sufficient extent to form 



