50 THE BOOK OF NATURE STUDY 



frost may be expected, a light covering should be put upon 

 cherished plants in the garden, to show that by stopping radia- 

 tion this protects against frost. 



In a region of market gardens also observations should be 

 encouraged on the extent to which gardens at different levels 

 are attacked by frost. Sometimes in the same garden it will 

 be found that the lower branches of a blossoming tree are black- 

 ened while the upper ones are untouched, this showing that the 

 chilling of the air is due to loss of heat by the ground, and is 

 greatest near the ground. Again, as a general rule the gardens 

 which lie at the bottom of a valley suffer more from frost than 

 those on the slope above, this being partly due to the tendency 

 cold air has to accumulate at the bottom of valleys in calm 

 weather. The upland gardens, on the other hand, feel of course 

 the effect of strong and cold winds much more than those on 

 a lower level. 



In a good many of the examples mentioned above, care 

 should be taken not to force an explanation on the child before 

 the need for one is felt, but the facts should be borne in mind 

 by the teacher, because they suggest directions in which observa- 

 tions may be made. 



