Air Drainage on a Hillside. 53 
is due to the draining off or settling away of the 
cold air, which is densest, and therefore heaviest. 
It pours down the valleys of hilly and mountainous 
countries, and as its vapor condenses it gives rise to 
the valley fogs and clouds. It lies in the low places 
upon the farm, and there may cause frost. A person 
riding across an undulating country upon a still sum- 
mer night can scarcely fail to notice the chillier air 
of the depressions. This escape of the cold air is’ 
the secret of much of the success of fruit-growing 
on rolling and sloping land; and this fact explains 
Fig. 1. The frosty belt below a wood upon a hillside. 
the importance of giving great attention to the selec- 
tion of the site and aspect when setting a planta- 
tion of the tenderer fruits. 
Features of such little apparent importance as 
not to appeal to the fruit-grower often exert great 
influence upon the quiet movements of air. A fre- 
quent case is this: A strawberry field is upon a 
gentle slope, and on the upper side is a wood. In 
time of frost, the only injury occurs in a belt two 
or three rods wide just against the wood, in the 
very place where the greatest immunity was ex- 
pected. This is because the slight bodily movement 
of the air down the hillside and over the forest 
