Walls are usually built in panels, from fifteen to thirty feet in length, one brick thick, with pillars for the sake of adding to their strength, at these specified distances ; the foundation a brick and a half thick. The plan of Mr. Silverlock, of Chichester, is worthy of adoption, since if well constructed, it is equally durable, and saves one-third of the expense. Walls so constructed are stated to become dry after rain, much more rapidly than a solid wall of the same or any other thickness, and there appears not a shadow of a reason why it should not ripen fruit equally well.

He forms the wall hollow, nine inches in breadth, by placing the bricks edgewise so as to form two facings, they are laid in good mortar, and the joints carefully finished. They are placed alternately with their faces and ends to the outsides, so that every second brick is a tie, and in each succeeding course, a brick with its end outwards is placed on the centre of one laid lengthwise on either side. The top of the wall must be covered with a coping of stone or bricks, projecting two inches. It is strengthened at every twenty feet, by piers of fourteen inch-work, built in the same manner, with bricks laid on edge. The mode of constructing the piers, obviating the disadvantages arising from training branches round their sharp angles, which often causes them to gum, recommended by the Rev. T. Cul-ilum, of Bury St. Edmonds, is to have their corners bevelled. He also advises the copings to project much further than they are usually made to do, even as much as twelve inches; but his reasoning refers more immediately to the management of wall fruit.

It is a practice sanctioned by economy, to build the wall half brick thick, on a nine inch foundation, and to compensate for its want of strength, a waved form is given. Both the small-ness of its substance and its form, are found, however, to be inimical to the ripening of fruit.

In every instance a wall should never be lower than eight feet. The thickness usually varies with the height of the wall, being nine inches, if it is not higher than eight feet; thirteen and a half inches, if above eight and under fourteen feet; and eighteen inches, from fourteen up to twenty feet.

Fruit trees will succeed quite as well against a stone wall as against a brick one, although the former is neither so neat in appearance, nor can the trees be trained in such a regular form upon it as upon the latter. The last disadvantage may be in a great measure remedied by having a wooden or wire trellis affixed to it. - Gnrd. Chron.

If it be desirable that the roots of the trees should benefit by the pasturage outside the wall, it is very common to build it upon an arched foundation.

Colour has very considerable influence over a body's power of absorbing heat. If a thermometer on a hot summer's day, be exposed to the sun, it will indicate a temperature of about 100°; but if the bulb be blackened with Indian ink or the smoke of a candle, it will rise from ten to twenty degrees higher. The reason for this is that the polished surface of the glass reflects some of the sun's rays, but the blackened surface absorbs them all. Blue absorbs all but the blue rays - red all but the red - green and yellow all but those of their own name - and white reflects all the rays. The lightest coloured rays are the most heating, therefore light coloured walls, but especially white, are the worst for fruit trees. The thermometer against a wall rendered black by coal tar, rises 5° higher in the sunshine, than the same instrument suspended against a red brick structure of the same thickness; nor will it cool lower at night, though its radiating power is increased by the increased darkness of its colour, if a proper screen be then employed. - Johnson's Princ. of Card.

Inclined or Sloping Walls have been recommended, but have always failed in practice. It is quite true that they receive the sun's rays at a favourable angle, but they retain wet, and become so much colder by radiation at night than perpendicular walls, that they are found to be unfavourable to the ripening of fruit.

"The Flued-wall or Hot-wall," says Mr. Loudon, is generally built entirely of brick, though where stone is abundant and more economical, the back or north side may be of that material. A flued wall may be termed a hollow wall, in which the vacuity is thrown into compartments, to facilitate the circulation of smoke and heat, from the base or surface of the ground, to within one or two feet of the coping. Such walls are generally arranged with hooks inserted under the coping, to admit of fastening some description of protecting covers, and sometimes for temporary glass frames. A length of forty feet, and from ten to fifteen high, may be heated by one fire, the furnace of which, being placed one or two feet below the surface of the ground, the first course or flue will commence one foot above it, and be two feet six inches, or three feet high, and the second, third, and fourth courses, narrower as they ascend. The thickness of that side of the flue, next the south or preferable side, should for the first course, be four inches or brick and bed, and for the other courses, it were desirable to have bricks cast in a smaller mould: say for the second course three, for the third two and three quarters, and for the fourth two and a half inches in breadth.

This will give an opportunity of beveling the wall, and the bricks being all of the same thickness, though of different widths, the external appearance will be everywhere the same." - Enc. Gard.

Mr. Paxton has the following excellent observations upon Conservative Walls, or walls so constructed as to shelter trees trained against them from winds, and Other natural modes of rapidly lowering the temperature: -

"In forming a conservative wall, it is necessary that it should have a south or a south-western aspect. It is also desirable, in order to give it an ornamental appearance, that there should be promi-nent parts at certain intervals, or that the whole should be divided into recesses and projections. The latter, by being of limited dimensions, would serve for the display of the more hardy plant, and also afford additional shelter to the remaining portions. If, moreover, the whole be surmounted by an appropriate coping, its beauty will be greatly enhanced.

"Much has been said of the conservative wall at Chatsworth, the leading characteristics of which are a practical illustration of the opinions now advanced - large retiring compartments, covered with a neat trellis, and relieved by occasional small stone projections or piers; and as the wall stands on a steep slope, each of the piers is raised considerably higher than the one below it, thus constituting as it were a series of very broad ascending steps on the top.

"The advantage of having a slight wooden trellis against the wall, instead of fastening the plants to it in the usual way, need scarcely be pointed out. Independently of its superior appearance, which is a point too frequently neglected in such matters, the greater ease with which the branches can be attached to it, and removed or altered at any time, is quite sufficient to give it the preference, while the destruction and defacement of the wall consequent on the use of nails, and the injury they often occasion to the shoots of the plants, give a value to any system by which they can be discarded. The extra expense of the trellis is too trifling to be regarded.

"It has been found, in attempting to grow exotics against open walls, that whatever tends to preserve the border in which they are planted comparatively dry, during the winter, does more towards sheltering them from the frost than extensive protection of any other kind.

"As the fluids of plants are, for the most part, imbibed through the roots, and as the heat of vegetable bodies escapes mainly in proportion to the fluids they contain, protection to the medium in which they grow is perhaps even more necessary than to the stems and branches. It will therefore be seen, that the portion of the border where the roots lie must be covered, and that, if the canvas or other protection actually given to the wall does not extend over the border, a coating of dry litter should be spread over it, as soon as severe weather commences, and be retained on it until the spring.

"The wall is composed of alternate prominent and retiring compartments. Each of the former includes two stone pillars, which stand out a little beyond all the remainder, and are to be left uncovered; while, between these, is a division, over which is extended a trellis for supporting the hardiest sorts of climbers and those that demand no protection.

"The recesses are capable of being covered in cold weather with glazed sashes, which can be placed out of sight in a moment, whenever it is safe to remove them, by sliding them behind the other divisions. In these recesses the tenderest green-house plants may be cultivated, and trained against a trellis.

"Thus are combined a handsome architectural elevation, and the means of having some of the finest exotic plants exposed in summer, without danger, and in a condition incomparably more healthy and attractive than they ever attain in the green-house.

"If glazed sashes are too expensive, canvas, or other screens, can be readily substituted; but, in that case, the plants will suffer from being kept in darkness during the winter. Where it is thought preferable, another set of rails, on the outside of those for the sashes, can be prepared, to carry some kind of covering in rigorous weather. Where this is done, the necessity for fire heat will be trifling; in fact it might be entirely dispensed with."- Paxton's Magazine of Botany.