146 WALLS, ESPALIER-RAILS, AND TRELLIS-WORK. 



" No matter of what material the wall be made, it will be desirable to 

 whiten its surface and keep it white. Black and dark-coloured sur- 

 faces absorb heat in the daytime, and give it out again during the 

 night in the form of radiant heat ; from which facts we might draw 

 the conclusion that walls for training fruit-trees against should be 

 black, or at any rate of a dark colour. Direct experiment was, how- 

 ever, necessary to settle this question, and M. Vuitry, who employs 

 his leisure in arboriculture, has communicated the results of his ex- 

 periments in this direction to M. du Breuil, which leave no doubt as 

 to the proper colour to be chosen for walls against which fruit-trees are 

 to be trained. He has proved 1st. That a thermometer hung during the 

 day with its face turned towards a white wall, at a distance from it equal 

 to that of a fruit-tree trained against it i.e., about an inch and a 

 quarter always showed a mean temperature of nearly 6 deg. Fahr. 

 higher than one hung against a black wall under precisely similar 

 circumstances. 2nd. That during the night the difference of tem- 

 perature shown by these two thermometers was inappreciable. 

 Contrary therefore to the opinions entertained by many persons, it 

 seems to be evident that the walls must be whitened when we wish 

 to give the trees trained against them the maximum amount of heat 

 to be obtained from the particular climate and aspect. Indeed, it is 

 precisely the plan that has already been pursued by the fruit-growers 

 of Montreuil for peach-trees, and of Thomery for their vines, it 

 having been frequently remarked that trees trained against white Avails 

 were healthier than those nailed to more or less dark-coloured ones. 

 This result is easily explained, for not only does the lighter colour 

 reflect more heat back to the trees, but by this means they receive a 

 greater quantity of light ; and it is well known how greatly vegetation 

 is stimulated by these agencies. Walls of a light tint are advantageous 

 in another way, for they not only reflect light and heat on the parti- 

 cular trees trained against them, but also on the others in their imme- 

 diate neighbourhood." ('The Parks, Promenades, and Gardens of Paris.') 

 Flued walls are either built entirely of brick, or with one side of 

 brick and the other of stone ; the latter being the north side of ea st- 

 and-west walls. In the case of north -and-south walls which are to be 

 flued, the thickness is equal on both sides, and the wall is built entirely 

 of brick. The flues, which are generally from six to eight inches wide, 

 commence about one foot above the surface of the border ; the first 

 course is from two to three feet high, and each successive course is a 

 few inches lower, till the last flue, within a foot of the coping, is about 

 eighteen inches high. The thickness of that side of the flue next the 

 south should, for the first course, be four inches, or the width of a 

 brick laid flatways; and for the other courses it is desirable to have 

 the bricks somewhat narrower, on account of the heat being less 

 powerful as the smoke ascends. All the bricks, however, whatever 

 may be their width, must be of the same thickness, in order to pre- 

 serve uniformity in the external appearance of the wall. As, where 

 garden walls are to be built, a large supply of bricks is requisite, no 

 difficulty need occur in getting such a quantity as might be requisite 



