PESTS loi 



the moisture in the air, even when this is invisible 

 moisture, so that radiation takes place more energeti- 

 cally in a high and dry situation, than in a damp and 

 low one. Yet it is in the latter where trees suffer 

 most from night frosts. The explanation of this 

 apparent anomaly is, that the air which is cooled in 

 the elevated situation, being made heavier by the 

 cooling, flows down into the lower parts of the 

 ground, and accumulates there ; while, in the higher 

 situations, warmer air is drawn in to take the place 

 of that which has flowed away, and we get there a 

 constant, gentle circulation, the cold layers never 

 attaining the same depth as they do in the hollows, 

 and generally not extending far enough from the 

 ground to reach the blossoms. The laws which govern 

 the flow of the cold and heavy air are identical with 

 those governing the flow of water, and, in selecting 

 a site for a plantation, attention to air drainage is just 

 as important as attention to water drainage ; any 

 situation favouring the presence of stagnant air is one 

 of the worst for fruit planting. Occasionally, however, 

 there may be conditions which somewhat modify 

 this rule, for if there is anything in the neighbourhood 

 capable of raising the temperature of air, the evil effect 

 of the position may be mitigated. A considerable 

 body of water will do this ; and it is found that trees 

 in a low situation near such a body of water suffer 

 less from frost than those which are some little distance 

 away. 



The trees themselves, just like the earth, lose heat 

 by radiation, but it is hardly possible that such a com- 

 paratively minute object as a fruit blossom could ever 



