WALLS AND TRELLISES. 163 



will be more intensely heated under the blackened 

 surface, and the night (or time of cooling) being short, 

 it may not have returned to the temperature of the 

 air, before it is again subjected to an increase of heat. 

 If the time of cooling were long enough, that part of 

 the wall under the blackened surface, might become 

 actually cooler than the part not blackened, and thus 

 the extremes of heat and, cold be greater than when 

 the wall was left with its usual surface. In the sum 

 mer time, however, the wall is not only more in 

 tensely heated, but probably retains a great portion 

 of the heat during the night. 



&quot; Horticulturists will decide which of these two 

 causes is efficient in producing the effect I have 

 stated, or whether both may not cooperate ; it is not 

 for me to presume to do so, though I should be 

 inclined to think, that in this climate, the intensity 

 had more influence than the uniformity.&quot; 



Chas. Harrison, gardener at Wortley Hall, York 

 shire, gives the following directions for blackening 

 walls : 



&quot; When the leaves have fallen in the autumn, I take 

 the earliest opportunity to loosen the tree from the 

 wall and to prune them ; the wall is then colored 

 with coal-tar, mixing with every gallon of the tar one 

 pint of linseed oil, in order to prevent it having a 

 shining surface when dry. It is more necessary to 



