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Section of proposed Pit on the line a b of fig. 30.
The arrows indicate the course of the circulating current. siderable value. There is nothing in the plan which lays claim to novelty; it professes merely to be a combination of various expedients, already severally in use, but which have not, it is believed, been hitherto applied to any very efficient purpose.
Fig. 29. is a section of the proposed pit, which is supposed to be built entirely above ground; brick on edge with hollow walls. This mode of construction is not essential to the plan, but its economy recommends it whenever a pit is to be built anew.
Fig. 30. is a plan of the same, and a b is the line of the section shown in fig. 29. The following details will be best understood by keeping both plan and section in sight. .
At certain intervals, say every 3 ft. 8 in. or 4 ft., corresponding with the rafters which support the lights, a hollow pilaster (P P in fig. 30.) is carried up inside the pit, projecting from the wall, the thickness of one brick only, and so constructed as to have no communication with the general cavity of the wall; and an aperture is left in each of these pilasters, both at top and bottom, inside the pit. Along the centre of the pit, also above ground, is constructed a flue of 4-inch work, 2 ft. wide, and 18 in. deep, having an aperture at bottom corresponding with each aperture of the pilasters on the front, or lower, side of the pit, and an aperture just below its upper edge corresponding with each pilaster along the back; and these apertures are respectively to be connected by means of small brick Hues, or, cheaper and better, by 6-inch draining-tiles, with the apertures in the pilasters with which they correspond. The space between the flue and the back wall may be filled up with earth or rubbish, and trodden firm before the cross flues are laid ; that between the flue and the front wall should be filled with brickbats, stones, or with
coarse dry rubbish. The heating-pipes, which should be not less than a pair of 4-inch pipes for a pit whose lights are 6 ft. long, are to be laid as usual, on small piers about 9 in. above the floor of the flue, which should be paved with bricks or tiles, or else trodden very firm. The flue is then to be covered in with
-inch slate (Valencia slate is best and strongest) in slabs. The apertures of the flues leading to the back pilasters must be 3 in. below the top of the flue, to retain a stratum of heated air in contact with the slate; and the pilasters should correspond with the rafters of the pit, thus interfering least with the general arrangement. The boiler, for which purpose one of the conical boilers described p. 132. is admirably adapted, must be placed in an ash-pit, sunk at one end of the pit ; unless, indeed, hot water can be said on from some existing apparatus near at hand.
In finally preparing the pit for the operations of culture, the space between the flue and the front of the pit is to be filled, and the whole surface of the pit over the flue is to be covered, with stones or brickbats free from dirt, and coarse gravel, diminishing in size upwards till they be of the size of pigeon's eggs or rather less. The depth of this layer over the flue should not exceed a foot; over this should be placed a layer of turf with its grassy side downwards, where it is intended to cultivate melons or cucumbers; for pines or plants in pots, fine gravel or coal-ashes should be employed, in which the pots might be plunged. The pit is now complete. When the fire is kindled and the pipes heated, the air in the flue or chamber c being heated, will rise to the top of the flue, and imparting some of its heat to the slate above, will proceed up the cross flue and pilaster p, and be poured out into the pit; its place below being supplied by the cooler air from the front, or lower, side, descending by the hollow pilaster in front to be heated in its turn by the pipes below. The slate, being an excellent conductor, will transmit abundance of bottom heat. It will, of course, take a few days, after the apparatus is set at work, before the mass of rubbish, &c., becomes thoroughly warmed, after which it will be subject to little variation.
All moisture in the pit, produced by watering or otherwise, will gradually sink down to the central flue, towards which the floor of the pit should slope from both sides. On arriving at this flue the moisture will be immediately taken up by the air heated by the pipes, and, being brought up as it circulates, will be deposited by the cooling air in the pit above. The arrows in fig. 29. indicate the course of the circulating air.
By this arrangement, it will be perceived, 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 ft. wide and 3 ft. high at back, and 1 ft. 3 in. in front, above the mould, the whole atmosphere of such a pit would pass through the flues once in every two minutes, when the apparatus was in full work, thus producing a constant and vigorous circulation. Further, if it be desired to change a portion of the air continually, small apertures to the outward air may be made in the front descending flues; 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 various crevices. The internal areas of the pilaster flues should not be less than 6 in. square, which they may be according to the proposed plan; and the cross fues and the apertures into the pit must have the same area. If it be found that, with this area or aperture, the heat produced by the pipes is brought up too rapidly, not leaving 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 of 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 apertures of the size above recommended be employed, there will not be 2o 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 semicylindric 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, I believe, be found sufficient for cucumbers, melons, or pines, in a 6-foot pit. My experience shows me they will produce 30° of temperature, which, as such pits are always covered at night in frosty weather, would be quite sufficient, except for very early forcing; for which purpose two pair (i. e. four rows) of 3-inch pipes would be preferable.
There is one portion of the above-suggested plan so evidently borrowed from an arrangement employed by Mr. Penn of Lewisham, at Wilmot's of Isleworth, and probably elsewhere, that it appears due to Mr. Penn both to acknowledge the source from which it is derived, and to point out, at the same time, wherein I conceive the proposed plan an improvement upon his.
Mr. Penn lays his pipes in flues, or tunnels, as above described, either at the front or back of the house, and causes the air to circulate in tunnels passing under the house. The arrangement is ingenious, and the circulation very complete; but considerable expense is incurred, and it does not appear that any great advantage is attained by it. It is stated that a very salutary circulation of air is obtained; but precisely the same circulation takes place in every house heated by pipes, only more freely if the pipes are not encased: any one who has steamed a house by syringing the pipes can bear testimony to the rapid circulation of the atmosphere, ascending along the roof, and descending by the back, till it returns across the house, or along the floor to the pipes.
Now, in the arrangement of the pit above suggested, there is a reason for encasing the pipes in a flue; viz., to obtain from them bottom heat, an advantage neither obtained nor sought in Mr. Penn's arrangement. A collateral advantage is the getting rid of the pipes, which are always exceedingly in the way in a small pit, burning up the plants near them. A third great advantage is, the constant supply of moisture brought up from below by the air, all the moisture of the pit draining down to the pipe flue; and a fourth, the regular circulation, an advantage which I am not disposed to despise, though I do not think it of such value as to be worth much expense in attaining.
In conclusion, I would remark that any existing pit may be fitted up on the principle above recommended, simply by building a centre flue, and forming the cross and ascending flues of draining-tiles.
It may be objected to this plan, that the ascending flues should be at the front rather than at the back, lest the upper part of the pit should be warmer than the front. The extreme rapidity of the circulation, already alluded to, is a complete answer to this objection; but, further, any other arrangement of the circulation than that above suggested would be prejudicial, and lead to precisely the opposite results to those intended. In the first place, instead of a great, there would be but a small, difference between the height of the ascending and descending columns, and, consequently, a much slower circulation. In the next place, if the descending flues were at the bottom of the back instead of at the front, and the heated air entered in front, it would immediately rise to the upper part of the pit, and there accumulate its heat, while the whole surface occupied by the plants would be covered by the coldest air. Moreover, the earth at back being always higher than the front, the coldest air would not flow away to be reheated, but would lie all along the front of the pit, and scarcely circulate at all ; whereas, by the arrangement proposed, the coldest air will always descend to be heated, while the influx of warm air, constantly encountering the warmest air which has risen to the top, will beat it down, and mingle it with that below. This is, perhaps, theory, but it is theory based on experience, and I have no hesitation in saying that such would be the effects.
I am induced to make these remarks, because the objection combated above is very likely to occur to any intelligent person perusing this plan ; and has, in fact, already been made to me by one to whose judgment I should, without good cause to show to the contrary, willingly defer.
It would prolong a paper already of unseemly length, to point out the manifold convenience of such a pit. Its application to pines and Orchídeæ, strawberries, &c., is obvious: it may also be employed as a preserving pit ; for, when the apparatus is in full work, the bottom heat alone, with the assistance of a mat, would keep out any frost from a part devoted to geraniums : while, the air-flues of that part being closed, the heating power of the pipe corresponding with them would be expended with accumulated force of circulated air in the forcing-pits which were at work, supplying them with more than their average amount of heat at a season when they most require it.
Sevenoaks, February 9. 1840.
Art. XIV. Botanical, Floricultural, and Arboricultural Notices of
the Kinds of Plants newly introduced into British Gardens and Plantations, or which have been originated in them; together with additional Information respecting Plants (whether old or new) already in Cultivation: the whole intended to serve as a perpetual Supplement to the “ Encyclopædia of Plants," the “ Horlus Britannicus," the