66 ON THE CONSTRUCTION OF WALLS. 



the superstructure. The drier and looser, indeed, that 

 the materials can Be laid together, and the greater 

 number of cavities and interstices that can be left in the 

 foundation, the better adapted will it be to admit the 

 roots of the vines, which delight to ramble amongst 

 such materials, in preference to growing in even the 

 richest soil. 



Blacke-ning the surface of a wall, is productive of a 

 considerable increase of heat as long as the sun shines 

 upon it, out during the night, and such part of the day 

 as the surface is in the shade, it will make the wall 

 colder. This arises from the black-coloured surface 

 parting with its heat immediately the sun's rays are 

 withdrawn. With respect, therefore, to walls racing 

 the east or west, the surfaces of which, even in the 

 height of summer, do not receive the solar rays more 

 than one-third of every twenty-four hours, the colour- 

 ing of them black will be injurious rather than other- 

 wise, inasmuch as the intensity of the cold increases 

 in proportion to the sun's absence. Hut when the as- 

 pect is due south, or very nearly so, the surface of a 

 wall may be blackened with advantage, as the duration 

 of the sun's absence as compared with his presence, in 

 this aspect, is more equally balanced throughout the 

 summe'r months ; and the increase of heat, therefore, is 

 more than equivalent to that of the cold ; the former 

 being, on a clear day, and when the sun is on the 

 meridian, frequently from ten to twenty degrees more 

 than that of the surface of an unblackened wall. 



Lime-washing the surface of a vine wall every year, 

 will be found very advantageous in keeping it clean, 

 and free from insects and the growth of moss. Walls 

 newly-built may be exempted from this operation 

 during the first three or four years after their erection, 

 but in every subsequent year it is almost indispensable. 

 When the surface of a wall is covered with the foliage 



