bottom heat, with the humidity requisite for such structures.

"Fig. A is a section of the pit supposed to be built entirely above the ground level; the walls are hollow, and built of brick on edge, to economize material."

4

As this pit is planned for England, where the weather is milder than here, it will be well for us to build thicker walls, by setting the bricks flat.

"Fig. B is the groundplan, a b marking the sectional line. The details are as follows: Under each rafter is carried up hollow pillars c, c, c, c, projecting only four inches within the pit, and having no communication with the general cavities in the side walls. In these a cavity is left at a the top and bottom; along the centre of the pit a flue of brick in bed d is

B

carried 2 feet wide, and 14 in depth, with openings in its bottom, e, corresponding with those in the pillars along the front of the pit, and also with those in the back; the openings in the latter being just below the levels of the tops of those in front. These openings are all to be connected by flues or tile-drains; the hotwater pipes are to be laid along the centre, as shown in the plan and section, but somewhat elevated above the floor on which they are placed, and covered over about 9 inches above their upper surface, with slate slabs, three-fourths of an inch thick. The communicating openings between the flue in which the pipes are laid and

the hollow pillars of the back wall, should be 3 inches below the top of the said flue, to act as chambers to retain heated air. The boiler is to be placed at one end of the pit, unless the pit is so situated as to be heated from a boiler adjoining; the space ƒ, between the hot-water pipe flue and the back wall, are to be made solid with earth and rubbish, so as to form a good foundation for the connecting openings or flues to be laid on top of them."

"In this climate, and to economize heat, it would be better, instead of filling this space with rubbish for a foundation, to carry through it, in solid brick-work, the chimney flue from the boiler; the smoke, etc., would heat it thoroughly, and go far toward warming the pit in an economical manner."

"The space between this flue and the front wall should be filled with hard, dry, open materials.

"In fitting this pit for the reception of plants, the space between the hot-water pipe flue, and the front wall of the pit, as well as the whole surface over the flue, should be covered, as shown in the section, with stones, brickbats, or coarse gravel, to the depth of not less than 12 inches.

"Over this, if for Melons and Cucumbers, is to be placed a layer of turf, with the green side undermost, and over it the necessary compost, h. If the pit is intended for Pines, or other plants in pots, then gravel or coal ashes may be substituted."

"By this arrangement three things are attained, bottom heat, top heat, with an atmosphere constantly moist, and finally a constant circulation of air. Unless I am much mistaken, supposing the pit to be 6 feet wide and 3 feet high at back, and 1' 3" in front, above the mould, the whole atmosphere of such a pit would pass through the flue once in every two minutes, when the apparatus was in full work, thus producing a constant and vigorous circulation; farther, if it be desirable to change a portion of the air continually, small apertures to the outward air may be made in the front descendingflues; a small portion of fresh air will then pour in continually, mingling with the descending air, and ascending heated into the pit, supplying the place of that which will escape through unputtied laps and other crevices."

"The internal areas of the pilaster-flues should not be less than

6 inches square, which they may be according to the proposed plan; and the cross-flues and apertures into the pit must have the same area. If it be found that, with this area, or apertures, the heat produced by the pipes is brought up too rapidly, not having sufficient bottom heat, and overheating the top, these apertures may be diminished. If they be too small, an inequality will arise between the temperature of the back and front of the pit, caused by an excessive difference between the ascending and descending air; the total heat brought up will be the same; for as the heat increases the velocity with which the air will ascend will increase also. But if an aperture of the size above recommended be employed, there will not be two degrees difference between the front and back of the pit. In order the better to disperse the ascending currents, it may be well that the aperture, instead of opening directly into the pit, should have a semi-cylindrical draining-tile, placed in front of it, to throw its draught right and left. But this is an unnecessary refinement; a pair of 4 inch pipes will be found sufficient for Cucumbers, Melons, or Pines, in a 6 feet pit."

You remember that these two pipes are to be laid in a chamber just above the floor; the only drawback to the perfect action of this arrangement is, that the air is liable to lack the great moisture needed in a pit; by enclosing a portion or the whole of the pipes in gutters or troughs, as previously shown, to be kept full of water, moisture will be supplied in abundance. These troughs may connect directly with the pipes, and thus ensure their being constantly full.

"Should the temperature afforded by 2 four-inch pipes be insufficient, 4 may be used, but if a nozzle-pipe were attached to the boiler, and the entire flue in which the pipes are enclosed were made water-tight, and used for a tank, an ample supply of both heat and moisture would be afforded."

There are some apparent but unreal objections made to this plan by McIntosh, and afterwards refuted by himself, which I shall not quote, but shall leave it with his further endorsement; viz., that experience has proved this to be a very superior pit.

* "Pits might to a very considerable extent be heated on the tank "Book of Garden," p. 450.

system, at little expense, if arranged as shown in the cut. Figs. A and B are ground plans and sections of such a set of pits; they show how a large range of pits may be heated by a single fire in a very economical manner. The ground plan is intended to represent four distinct pits, each 50 feet in length, and 6 feet wide in

the clear; the boiler is to be placed in the middle (dotted lines), at b, or as better seen in section B, with the stoke hole under the level of the walk or passage between them; the floors are to be rendered perfectly level and firm with a coating of concrete, and afterwards covered with cement over their whole breadth, as

are

also the side walls to the

height of 6 or 7 inches. Down the middle of each tank under each pit, let

a line of bricks, set on edge, be placed, leaving the ones nearest the extreme e ls out; this will give a flow and return current, the line of bricks forming the outer edge of each tank being kept 3 inches clear of the side walls, to prevent the loss of heat by absorption or otherwise; over the tanks thus formed, thick slate or thin pavement is to be laid, and closely joined with cement, extending from back to front of pit. Apertures at every 5 or 6 feet may be cut in them, to which iron or earthenware tubes may be attached of 4 inches diameter, and furnished with stoppers, to be taken, out for the admission of moist heated air into the atmosphere of the pit, and replaced when that is not required. Cut C, a, tank; b, pit; c c c, tubes of iron leading from tank to top of earth in pit, and open air; these tubes would not only moisten and warm the air

when opened, but would radiate heat into the earth around them at

"Similar provision should be made over the apertures a, between the outer edges of the tank and the side walls, to allow the heated air in them free access to the pit, and as the air from such openings is not over moist, the apertures may be left uncovered. The back walls may be 3 feet high above the ground line, that is, allowing 8 inches for depth of water and top covering, and 3 feet 10 inches from top of covering (of tank) to the glass.

"The front wall above the ground line should be 2 feet 6 inches high. Circumstances will always direct this, as the height of pits must be suited to the purposes for which they are intended. The boiler will supply all or part (of the tanks), as may be required, and this can be regulated by stop cocks placed in the pipes which connect the boiler with the tanks, and these again can be subdivided by sluices, as in the adjoining cut.

"By this means we should have 1200 feet of surface heated by one fire, and for the purpose of growing young plants in pots, forcing French Beans, Strawberries, Asparagus, Salads, etc., such pits would be invaluable. The walls should be 9 inches thick for durability, and also to retain heat and exclude cold, and if they are built hollow so much the better, and the