54 The Principles of Fruit-growing. 



strikes obliquely downward from the edge of the 

 wood -top, and leaves a narrrow belt of dead air 

 against the timber (as at A in Fig. 1, page 53.) 



The atmospheric drainage is marked only in still 

 air. Winds mix up the air, and bring it all to a 

 comparatively uniform condition. The slightest ob- 

 stacles may sufficiently retard the movement to leave 

 their impress in the distribution of a light frost. A 

 rail fence, a stone wall, a row of bushes, a slight 

 elevation of land, the earth thrown out of a ditch, 

 all of these are obstacles to drainage of cold air 

 when they extend across a slope. In some cases, 

 there may be a difference of ten degrees in tempera- 

 ture in as many feet of elevation . A dense row of 

 trees standing diagonally across a slope may convey 

 away the cold air which settles down against it, 

 and thereby prevent injury to plants on the lower 

 levels. It has been suggested that in certain hilly 

 regions, levees a few feet high be built diagonally 

 across the slopes, with ditches or moats above them 

 to hold water, the evaporation of which would tend 

 to raise the dew-point. 



The range of elevation through which atmos- 

 pheric drainage acts beneficially to the f ruit - grower 

 is limited. A fall of a few feet in a plantation is 

 often sufficient for the very best protection from 

 light frosts ; and a fall of one or two hundred feet 

 may be regarded as the general maximum through- 

 out which the benefit may be observed, for very 

 high elevations are, as we have seen, bleaker and 

 colder in sum -temperature than comparatively low 



