74 



RETENTION OF HEAT. 



from the sun, often becomes much hotter than the 

 suiTOunclmg atmosphere. The soil is frequently 

 raised to a temperature of one hundred degrees 

 Fahrenheit, and sometimes to one hundred and fifty. 

 This fact explains the reason for the immense 

 growth of trees at such times, if moisture is also 

 present, while the air is only about eighty-five de- 

 grees. 



The exposure of the soil has an influence 

 upon this quality, as the degree of heat will be 

 much higher if it is protected from cold winds. Its 

 color is also a great consideration, as the darker 

 earths wdll, of course, absorb the most. But the 

 power of retaining this heat after it is absorbed is 

 a different matter. Schlibler's tables will again 

 supply us with a valuable guide. 



TAUJ.E E. 



