l6o RELATIONS OF SOIL TO HEAT 



of Bavarian forests to a depth of 4 feet is, in June, July, and 

 August, nearly 7 F. less than that of similar soil covered 

 by turf and freely exposed to the sky. In winter, the mean 

 temperature of the two soils was nearly the same. The 

 forest soil was thus on the whole distinctly cooler than the 

 grass land. This coolness of a forest soil doubtless favours 

 that accumulation of humus which is so characteristic of soils 

 of this description. Had the forest soil been compared with 

 arable, instead of with grass land, the differences in the range 

 of temperature observed would have been much more con- 

 siderable ; the turf was, indeed, itself an efficient protection 

 to the underlying soil. 



The range of temperature of soil permanently covered with 

 grass is much less than that of soil not so protected ; it is, 

 indeed, a common observation, that winter frosts do not 

 penetrate to such a depth under turf as they do in bare 

 soil. 



The temperature of the soil may be considerably affected 

 by other circumstances besides the amount of solar radiation. 

 Every circumstance affecting the general temperature of a 

 locality, as altitude, and prevalence of hot or cold winds, 

 will clearly affect the temperature of the soil. Prominent 

 among these circumstances is the neighbourhood of large 

 masses of water. As the temperature of such masses of water 

 is far more constant than the temperature of either the atmo- 

 sphere or the soil, the neighbourhood of such masses will 

 generally considerably diminish the extremes of heat and 

 cold. The moist and cloudy state of the atmosphere arising 

 from the presence of much water will also act in the same 

 direction. The range of temperature on an island is thus 

 distinctly less than on a continent. 



