78 BERTHA'S VISIT TO HER 



of Dr. Wells, it appears that the heat which the 

 earth receives from the sun in the day is returned 

 or radiated back again from the earth during the 

 night, and is dispersed in the sky ; the surface 

 of the earth thus becomes cold from its sudden 

 loss of heat, and congeals the dew. The cold 

 produced by this radiation of heat from the 

 earth, is always less if any substance be inter- 

 posed between it and the sky ; not only a solid 

 body, but even a fog, or clouds, have this effect, 

 because they intercept the heat, and perhaps 

 again send back a portion of it to the earth ; and 

 this, he added, is the reason why a bright clear 

 night is generally colder than a cloudy night. 



I asked my uncle if that was also the reason 

 that such slight substances, as straw or mats, are 

 found to protect tender plants from cold ? 



" Yes," said he ; " I used to wonder how such 

 thin, open things as Russia mats could prevent 

 plants from becoming of the same temperature 

 as the atmosphere ; but when I learned that all 

 bodies at night give out their heat by radiating 

 it, unless some covering be interposed, which 

 acts, not by keeping out the cold, but by pre- 

 venting their heat from flying off, then I per- 

 ceived the reason of what before had appeared to 

 me to be almost useless." 



He described several experiments he had tried 

 to satisfy himself on this subject. He found 

 that even a cambric handkerchief was sufficient ; 



