20 ELEMENTS OF GENERAL SCIENCE 



This explains why it is that in summer a wind wliich is 

 coming across a large body of water is cooler than one which 

 conies across the land, for in summer the land is warmer than 

 the water. In the winter the reverse is true. In the same 

 way, a wind from the north is likely to be colder than one 

 from the south. 



When the sun is not shining directly upon a part of the 

 earth, as at night or on a cloudy day, the earth's surface is 

 continually losing heat to the surrounding space and there- 

 fore becoming cooler. The heat which passes off from objects 

 upon the earth warms any transparent medium through which 

 it passes much more than it does when the sun's rays pass 

 through such a medium. This is well illustrated by a green- 

 house. Here the rays of the sun pass through the glass with 

 little loss, but the heat which is given off by the objects in 

 the greenhouse, after they have been warmed by the sun, 

 does not easily pass through, the glass. The heat is therefore 

 retained in the greenhouse. 



