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 fruit- 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 
