THE LEAVES 91 



117. The air is colder on a frosty night near the 

 ground than it is a few feet above it. If one ther- 

 mometer is hung near the surface of the ground on 

 a quiet night in mid autumn, and another ten to 

 twenty-five feet higher in the branches of a tree, 

 it will be found that it may be five to ten degrees 

 colder near the ground than it is in the tree tops. 

 This is due to the fact that the soil cools very 

 rapidly, and the layer of air resting on it is also 

 cooled, while the upper air is comparatively warm. 

 On account of this fact the leaves and buds on the 

 lower branches of a tree may be frosted, while those 

 on the upper part are untouched. The low-growing 

 shrubs and herbs will be frosted before the taller 

 trees. If, however, a great movement of wind from 

 the northward covers the country with cold air, it 

 will result in a general freeze which affects all alike. 

 The farmer and the fruit raiser prevent damage to 

 their crops from frosts by covering the plants with a 

 shield which will prevent the loss of heat by the 

 ground and by the plants, by building fires to heat 

 up the layer of cold air, or by making smudge fires 

 which add heat, smoke, and moisture to the air, 

 making a fog blanket that prevents the loss of heat 

 as effectually as a covering of cloth might do it. 



