BOOK III. 



FIXED STRUCTURES. 



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303 



G3/T 



1553. The frame on props, in construction, resembles the Alderstone pit, excepting that 

 the superstructure is a frame instead of masonry. Such frames are much used about 

 London to grow pines, the back being enclosed by walled hurdles, supported by the 

 props as stakes, and round the hurdles linings of dung are applied. 



1554. There are a great many varieties of this species of frame : that adopted at Ed- 

 monton for the culture of pines will be noticed in treating of that fruit. 



SECT. II. Fixed Structures. 



1555. Fixed structures consist chiefly of erections for the purpose of improving the 

 climate of plants by shelter, by supplying heat, and by exposing them to the influence 

 of the sun. The genera are walls and espalier rails, of each of which the species are 

 numerous. 



1556. Garden-walls are formed either of brick, wood, stone, or earth, or brick and 

 stone together ; and they are either solid, flued, or cellular, upright or sloping, straight 

 or angular. 



1557. Brick, stone, or mud walls consist of three parts, the foundation, the body of the 

 wall, and the coping. The foundation should be somewhat broader than the body of 

 the wall, and of depth proportionate to the quality of the sub-soil, or intended plan of 

 culture. In some cases where it is intended that the roots should have free access to 

 both sides of the wall, it should be placed on arches (Jig- 235.), or piers, with plank- 

 stones, the soffit of the stone or under crown of the arch being within 6 inches, or 1 foot of 

 the surface, and the openings, smaller or larger, according to the power of the materials 

 to resist the pressure of the wall. The arch should be a segment of a circle, or an ellip- 

 sis, and the piers (a, a, a) proportioned to the qualities of the foundation and the super- 

 structure. Where the body of the wall commences, there will be a set-off or rebate of 

 one or two inches on each side, which should be commenced below the ground's surface, 

 both for the sake of appearance, and to prevent the alternate action of the air and rain 

 from rotting the mortar in the rebate. The body is generally carried up of the same 

 width to the coping ; but where the walls are high, say 18 feet, it may taper equally on 

 both sides to 14 inches ; in doing which, great care must be taken by the bricklayer to 

 make good joints. To facilitate this, some architects have bricks formed of a smaller 

 size for the upper part of the wall. It is not settled among gardeners whether the cop- 

 ing should project at all ; or if it projects, how much, and what proportion on each side. 

 Nicol is of opinion it need not project at all, and that there is no occasion, as is gene- 

 rally done, to bevel the coping stones to the north, or less useful side of the wall, to 

 throw off the rain in that direction. Walls without copings have two advantages in their 

 favor ; the first is, that no insects are harbored in the angle, under the coping, as is 

 generally the case ; and the second, that trees are more readily trained over from one 

 face of the wall to the other, a practice which has been found to induce a fruitful state 

 in trees, which had never produced fruit before. There is also some saving in extent 



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