THE SEASON WHY. 



87 



" The Lord is my shepherd, I shall not want. He maketh me to lie down in green 

 pastures." PSALM xxm. 



The earth, which during- the day received heat from the solar 

 rays, radiates the heat back into the air, and therefore becomes 

 itself colder. All the various objects upon the face of the earth 

 also radiate heat in a greater or lesser degree. And dew will be 

 found to be deposited upon the surfaces of such bodies in proportion 

 to the fall of their temperature through radiation. 



348. Why is there little or no dew when the nights are 

 cloudy ? 



Because clouds act as secondary radiators ; and when the earth 

 radiates its heat towards the clouds, the clouds again radiate it 

 back to the earth. 



Fig. 3. ILLUSTRATING THE POBMATION OF DEW. 



If plates of glass be laid over grass-beds, as in the engraving Pig. 3, no dew 

 will be deposited on the grass underneath the glass plates, although all around 

 the grass will be completely wetted. The explanation is that the glasses, being 

 radiators of heat, act in the same manner as the clouds, returning the heat to 

 the bodies underneath them, and preventing the formation of dew thereon. 



349. WTiy does dew form most abundantly an cloudless 

 nights ? 



Because the heat which is radiated by the earth does not return 

 to it. The temperature of the earth, and the air immediately upon 

 its surface, is therefore lowered, and dew is formed. 



It has been observed that sheep that have lain on the grass during the 

 formation of dew have their backs completely saturated with it, but that under- 

 neath the line where their bodies turn to the earth, their coats will be dry. In 

 the same manner glass globes suspended in the air, on dew forming nights, will 

 be found loaded with globules of dew upon the top, but there will be no 

 appearance of moisture underneath. 



