152 THE TEMPERATURE OF THE SOIL [chap. 



flat ; if the ground slopes to the south, the same beam is 

 spread over the smaller area represented by AC ; if the 

 ground slopes to the north, it is spread over the larger 

 area represented by AD. During the winter half-year, 

 also, the southern slope will have a longer duration of 

 sunlight than the northern slope. 



FlG. 13. Distribution of the Sun's Rays on Southerly and 

 Northerly Slopes. 



Though in a general way the temperature both of 

 the air and the soil decreases with elevation above 

 sea-level, yet it is well known that the severest frosts 

 occur locally at the bottom of valleys and hollow places. 

 This is particularly noticeable in the sudden night frosts, 

 characteristic of early autumn and late spring, which 

 are so dangerous to vegetation ; it is usual to find the 

 tenderer plants of our gardens, such as dahlias, cut 

 down by frost on the lower levels long before the 

 gardens on the hill are affected. Spring frosts, again, 

 will often nip the early potatoes in the valleys when 

 the higher lands are untouched. Fruit plantations 

 should not be set in the valleys, for no crop suffers 

 more from these unseasonable snaps of cold ; so clearly 

 is this fact recognised, that in some fruit - growing 

 districts only land above a certain elevation is regarded 

 as suitable for fruit, and commands a higher rent in 



