THE REASON WHY. 97 



" Behold, I will put a fleece of wool in the floor ; and if the dew be on the fleece 



only, and it be dry upon all the earth beside, then shall I know 



that thou wilt save Israel. * * * 



CHAPTER XIX. 



396. Why are cloudy days colder than sunny days ? 

 Because the clouds intercept the solar rays in their course 



towards the earth. 



397. Why are cloudy nights warmer than cloudless nights ? 



Because the clouds radiate back to the earth the heat which the 

 earth evolves ? 



Because, also, the clouds radiate to the earth the heat they have 

 derived from the solar rays during a cloudy day. 



398. Why is the earth warmer than the air during sun- 

 shine ? 



Because the earth freely absorbs the heat of the solar rays; but 

 the air derives comparatively little heat from the same source. 



399. Why does the earth become colder than the air after 

 sunset ? 



Because the earth parts with its heat freely by radiation ; but 

 the air does not. 



400. Why do glasses, mats, or screens, prevent the frost 

 from billing plants ? 



Because they prevent the radiation of heat from the plants, and 

 also from the earth beneath them. 



401. Why are the screens frequently covered with dew on 

 their exposed sides ? 



Because they radiate heat from both their surfaces. A piece of 

 glass, laid horizontally over the earth, would radiate heat both 

 upwards and downwards. But on its lower surface it would 

 receive the radiated heat of the earth, while from its upper surface 

 it would throw off its own heat and become cool. Therefore dew 

 would be deposited upon the upper, but not on the under surface. 



402. Why does dew rest upon the upper surfaces of leaves ? 



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