ARID PORTIONS OF SOUTH AUSTRALIA. 



27 



above was of clay with an admixture of small stones. Soils of other 

 compositions, especially of other physical properties, would give other 

 results. Thus, Hilgard (1906 : 306) , quoting Wollny, states that different 

 soils have summer and winter temperature properties as follows: In 

 summer the sandy soils are warmest, with humous, lime, and loam soils 

 following in the order named. In winter the following order in this 

 regard obtains: humous, lime, loam, and sandy soils. Sandy soils, 

 at least the superficial layers, in summer and in the desert, become 

 intensely hot, according to Hilgard, but at the same time they allow the 

 existence of moisture at a depth of 10 to 12 inches below the surface. ' 

 Clay soils in the same regions, "being usually in a compacted condi- 

 tion, will show a lower surface temperature and will be warmer and 

 drier at a depth at which sand will still retain abundant moisture and 

 be comparatively cool." 



Certain additional features concerning the temperature of the soil 

 should be mentioned. It should be said that the position of the sur- 

 face relatively to the incident heat rays is of some importance. Thus, 

 as is well known, the sun of winter is less effective than that of summer, 

 and slopes may be warmer or colder, depending on their relation to the 

 direction of the rays of heat impinging their surfaces. Only surfaces 

 lying at an angle of 90° to the incident rays receive the maximum heat. 

 When the angle is 30° the amount of heat is about half the maximum, 

 and it rapidly falls with sharper angles (Cannon, 1915^: 213), so that at 

 15° from the incident rays it is only about 8.5 per cent of the maximum. 



Fig. 86. — Average yearly temperatiire of Australia, after Hunt. 



