2024. Boarded or wooden walls (fig. 582. a) are variously constructed. One general rule is, that the boards of which they are composed should either be imbricated or close-jointed, in order to prevent a current of air from passing through the seams; and in either case they should be well nailed to the battens behind, in order to prevent them from warping from the sun. When well tarred and afterwards pitched, such walls may last many years. They must be set on stone posts, or the main parts or supports formed of cast iron. Nicol informs us (Kalendar, p. 149.) that he has 'constructed many hundred lineal feet of wooden walls, which recline considerably towards the north (fig. 582. b), presenting a surface at a better angle with the sun than if they were upright. They are placed on sloping ground, and range in five ranges or lines, due east and west, at the distance of 7 yards from each other, the southmost being 5 ft. high, and the northmost 7 ft., composed of imbricated boards, pitched over to give them durability; the supports are set on (not in) blocks of stone, which are sunk in the earth, and firmly laid on solid foundations, 3 ft. under the ground level.'