WALLS, ESPALIER-RAILS, AND TRELLIS-WORK. 139 



of walls built in the direction of north and south, because in them the 

 air is of nearly the same temperature on both sides ; whereas in an 

 east and west wall, the heat produced by the sun on the south side is 

 being continually given out to the much colder north side. Boarded 

 walls two or three centuries ago afforded the only means, in the 

 neighbourhood of London, of forcing the cherry, the only fruit which 

 at that time was attempted to be produced out of season. The boarded 

 wall or fence was placed in the direction of east and west, the cherries 

 planted against it on the south side, and casings of hot dung placed on 

 the north, close to the boards. To derive the full advantage from the 

 south side of an east and west wall, it ought to be of greater thickness 

 than a south and north wall under the same circumstances ; because, 

 from the much greater cold of the north side, the south side is con- 

 tinually liable to have the heat abstracted from it in that direction. A 

 south and north wall, on the other hand, can never become so hot on 

 either side as an east and west wall does on the south side; and as it 

 receives its heat equally on both sides, so it loses it equally. Where 

 an east and west wall is thin, and consequently cold, it might become 

 worth while, when it was desirable to retain as much heat on the south 

 side as possible, to thatch it on the north side during the winter and 

 spring months. The great advantage of covering with some pro- 

 tecting material the north sides of walls in spring, when trees are in 

 blossom, may be inferred from the case of trees trained against 

 dwelling-houses, which invariably set their blossoms better than trees 

 against unprotected garden-walls. 



The height of garden-walls may vary according to the object in 

 view, but it is rarely necessary to be more than twelve or fifteen feet, 

 or less than six feet. In kitchen-gardens the highest wall is generally 

 placed on the north side, as well to protect the garden from north 

 winds as to admit of a greater surface for training on exposed to the 

 full sun, and to form, if necessary, a back sufficiently high for forcing- 

 houses. The east and west boundary walls are commonly made two 

 or three feet lower than the north wall, and the south wall somewhat 

 lower still. The usual proportions in a garden of three acres are 17, 

 14, and 12 ; for gardens of one acre, 14, 12, and 10; that part of the 

 north wall against which the forcing-houses are placed being in small 

 gardens raised somewhat higher than the rest. Twelve feet is found 

 to be a sufficient height for peach and apricot trees ; but for pears and 

 vines it may be one-half more ; and indeed for vines there is scarcely 

 any limit. An attempt has been recently made to introduce walls of 

 glass, which the projectors conceived to possess such advantages as 

 would cause them to supersede ordinary walls. This, however, they 

 are not likely to do, as they reflect much less heat than a common brick- 

 wall, or one of any kind of cement and with its surface whitewashed. 



We now proceed to give the following instructions for the erection 

 of walls : 



The foundations of garden-walls should be at least as deep as the 

 ground is originally dug or trenched. The wall is sometimes sup- 

 ported on arches ; but this is not in general desirable, more especially 



