ON THE CONSTRUCTION OF WALLS. 61 



need be gone to, nor should any more cement be used 

 in putting the materials together, than is necessary to 

 make the foundation sufficiently strong and firm to 

 support the superstructure. The drier and looser, in- 

 deed, 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. 



Blackening the surface of a wall^ is productive of 

 a considerable increase of heat as long as the sun 

 shines upon it, but during the night, and such part of 

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

 Avail colder. This arises from the black-colored sur- 

 face parting with its heat, immediately the sun's rays 

 are withdrawn. With respect, therefore, to walls 

 facing the east or icest^ the surface of which, even in 

 the height of summer, do not receive the solar rays 

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

 coloring them black will be injurious rather than 

 otherwise, inasmuch as the intensity of the cold in- 

 creases in proportion to the sun's absence. But when 

 the aspect 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 summer 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 unblack- 

 ened wall. 



Linie-icashing the siu'face of a vine wall every 

 year, will be found very advantageous in keeping it 

 clean, and free from insects and the growth of moss. 

 Newly-built walls may be exempted from this opera- 

 tion during the three or four years after their erec- 

 tion, but in every subsequent year it is almost indis- 

 pensable. When the surlace of a wall is covered with 

 6 



