182 



WALLS, ESPALIER-RAILS, AND TRELLIS- WORK. 



such a quantity as miglit be requisite for the flued walls made of any con- 

 venient width. To prevent the risk of overheating the trees by the flues, 

 trellises are sometimes applied against them for training on ; but where the 

 wall is properly constructed, and only moderate fires kept, they are unneceS' 

 sary. A great improvement in flued walls has been made by Mr. Shiells, 

 gardener at Erskine House, Renfrewshire, who, though the garden is in one 

 of the worst climates of Scotland, has been singularly successful in ripening 

 grapes, figs, peaches, &c., on these walls without the aid of glass. Mr. 

 Shiells places the furnace, as usual, at the back of the wall, about eighteen 

 inches from it, and two feet below the surface of the ground. To prevent 

 the roots of the trees on the south side of the wall from being injured by the 

 heat, a wall of four-inch brickwork is carried up opposite the furnace with 

 a two-inch cavity between them. From the furnace the smoke and heated 

 air enter the wall at c, in fig. 121, over which, at a, there is a damper, by 



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Fig. 121. Longitudinal section of a Jlued wall. 



means of which the heat throughout the whole wall is regulated. When 

 this damper is drawn about four inches, a sufficient portion of the smoke 

 and heated air pass through the two under flues to produce the necessary 

 degree of heat in them ; while another portion of the smoke and heat rises 

 directly to the third flue, by which it, and the fourth or upper flue, are 

 heated a little more than the two lower ones. This Mr. Shiells considers 

 a great advantage, because the upper part of the wall is more exposed to the 

 cold air, and less benefited by the reflection of heat from the ground than the 

 lower part ; besides, the shoots there are generally more luxuriant and 

 spongy, and would be later in ripening than those on the lower part of the 

 wall, if they did not acquire an extra degree of artificial heat. Sometimes, 

 therefore, it is desirable to warm only the upper part of the wall, and this is 

 readily done by withdrawing the damper, when the whole of the smoke and 

 heated air will rise direct to the third flue ; and thus, more especially if only 

 a small fire is made, the desired result will be obtained without warming the 

 lower part of the wall at all. By reducing the communication between the 

 first and the second flue at a, to about thirty square inches, the damper may 

 be dispensed with : because in that case a sufficient portion of the heat would 

 rise direct through this opening to the third flue, and so heat as effectually 

 the upper part of the wall as the lower part ; but by retaining the damper, 

 the heat can be regulated more eff^ectually. The depth of the first or lowest 

 flue is two feet six inches; of the second, two feet ; of the third, two feet 

 three inches ; and of the fourth, one foot six inches : the width of all of 

 them is seven inches and a half. The bottom of the lowest flue is about one 

 foot above the surface of the ground, and the top of the upper flue within 

 seven inches of the coping : the total thickness of the wall is about one 

 foot nine inches; viz., the width of a brick in front, the length of a brick 

 behind, and the remainder for the width of the flue. About two yards of 

 the front of the wall at the warm end of the flues is built rather thicker on 



