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



allowing room for one trained tree between every two piers, have also 

 been found beneficial from the shelter afforded by the piers, which at 

 the same time greatly strengthen the wall, and admit of its being built 

 thinner. In general, however, a straight wall, without projections of 

 any kind, is most convenient, most suitable for training, and for pro- 

 tecting by temporary copings, and most agreeable to the eye. 



The materials of walls are brick, stone, mud, earth, concrete, and 

 wood; but the first is by far the best. Brick retains warmth, in 

 consequence of its much greater porosity than stone ; forms a very 

 strong wall with comparatively little substance, from the rectangular 

 shape of the bricks, and the firmness with which mortar adheres to 

 them ; and it is the best of all walls for training on, from the small 

 size of the bricks and the numerous joints between them. Add also, 

 that from the porosity of the bricks, nails may even be driven suffi- 

 ciently far into them to hold branches, as securely as nails driven into 

 the joints. Stone walls are good in proportion as they approach to 

 brick walls. For this reason, if the stone is not naturally porous and 

 a bad conductor of heat, the walls should be built of extra thickness, 

 and the stones should not be large, nor so rough as to make coarse 

 joints. The warmest walls of this kind are such as are of sufficient 

 thickness to allow of the interior of the wall being built without mortar, 

 in consequence of which much air is retained, and heat is not readily 

 conducted from the warm side of the wall to the cold side. A stone 

 wall, with a facing of bricks on the warm side, forms the next best 

 wall to one entirely of brick ; and, next to this, a stone wall stuccoed 

 or plastered over with a mixture of stone lime and sharp sand, or coated 

 over with Roman cement of good quality. Walls formed of earth or 

 mud are still better non-conductors than brick walls ; but though they 

 are warm, yet as surfaces for training trees on they are attended with 

 several disadvantages. They cannot conveniently be built high, and 

 whatever may be their height, they require the coping to project 

 farther than is beneficial to the plants trained on them at any other 

 season than in early spring ; and they require a trellis on which to 

 fasten the plants. Nevertheless the vine and the peach have been 

 successfully grown against such walls at various places in the neigh- 

 bourhood of Paris, though they are now rapidly giving way to stone 

 walls. These walls are commonly built without mortar, excepting to 

 close the outside joints, or to plaster over the surface of the wall as a 

 substitute for a trellis, which is always used when this is not done. 

 The grapes at Thomery, near Fontainebleau, are chiefly grown on 

 trellised walls of this kind ; and the peaches at Montreuil, near Paris, 

 are chiefly on stone walls stuccoed. Concrete walls are likewise used 

 in different parts of France. Walls formed of boards are frequent in 

 the north of Europe, where timber is abundant ; but, except when the 

 boards are five or six inches in thickness, they are very cold. In 

 Holland, and more particularly in Sweden, when such walls form the 

 backs to hothouses, they are thatched from top to bottom. In Britain, 

 were it not for the expense of the material, boarded walls might, in 

 many cases, be adopted instead of brick ; more especially in the case, 



