523. The mode of heating a plant cabinet is sometimes a matter of difficulty, en account of the small space to be heated, end the large surface exposed to the external air. We have already mentioned the practicability of heating such places from the kitchen fire, or from a fire or boiler placed in another story; and suggested, at the same time, that the necessary pressure on the boilers, or the tubes, rendered this mode of heating by no means advisable. It is also a fact, that in some places sufficient warmth may be given to the plant cabinet by opening the door of the sitting-room communicating with it the last thing before the family retire to bed, and leaving it open all night On the whole, we are of opinion that the best mode of heating plant cabinets, ox small green-houses attached to dwellings, is by some source of heat on their own level; and not from any source either above or below. If no fine has been built in the wall of the house suitable for carrying off the smoke from any stove or fireplace made in the plant cabinet, a tube of cast or sheet iron, or of earthenware, may be partially sunk into the outer face of the wall of the house; and disguised by a projection so designed as to be consistent with the architectural character and effect of the elevation.

In some places, tubing of this sort may be placed against the wall, and covered with an architectural case of boards, metal, or slates, painted in imitation of stone; and sometimes these projections admit of being disguised by common ivy, or the Virginian creeper. At all events, no architect of the slightest degree of ingenuity will be at a loss to discover the proper situation for a circular flue of 3 in. in diameter; and no builder who has any regard either for appearances, or the free ascent of the smoke, in such a flue, will ever put it up without a casing to give it architectural effect, and to serve as a nonconductor, and thus to preserve what may be called a lining of heat round it, to favour the ascent of the smoke. The situation for the flue having been fixed on, the next thing is to determine the mode of heating; and this, we are of opinion, ought, in most cases of small plant cabinets, to be by a hot-water stove placed within the cabinet, and heated by a register fire-pot within, like that of Or. Arnott The fuel used may be coke on ordinary occasions; anthracite when a greater heat was wanted; and, perhaps, charcoal in the most severe winter nights, when the heat required was very considerable.

A stove of this kind, properly constructed, may be kept burning night and day, regulating the admission of air to the fire according to the heat required. For this purpose, the stove may either have a hand regulator, as in the imitations of Dr. Arnott's stoves, a thermometer one being unnecessary; or, in order to insure a draught, the air may be brought to the stove by a leaden pipe of 1 in. in diameter within, from a lover level, either immediately under the house, or from the open air; or from any place from which it is desirable to extract the air for the purpose of ventilation. In all these cases, the air admitted to the fire may be regulated by a common stopcock, like that in use for common water-pipes. We think it net unlikely that the smoke, or products of com-bustion, where the kind of fuel we have recommended is used, might be conveyed away in a horizontal direction, or perhaps even downwards to a drain, in a tube of net mere than double the diameter of that used for supplying air to the fire; but, never having seen this mode put in practice, we cannot venture to recommend it.

We have seen the common- smoke-flues of hothouses discharge their smoke horizontally, but it is always attended with a waste of heat. 524. Heating with hot-water. - The mode in which water is heated by a small store is now so well known to ironmongers, that it seems scarcely necessary to describe it Supposing the fire-pot, or fireplace, surrounded by firebrick, to occupy a cubic foot in the centre; then enclose this on three aides and over the top with a square or circular double cylinder, water-tight. The side not cased with water must contain the furnace door for supplying fuel, the ash-pit door for withdrawing the ashes, and an opening immediately under the top, or cover, for the insertion of the tube to convey away the smoke. On any of the three sides cased with water two tubes must be joined, one at the bottom, and the other at the top; and these may be conducted on a level to any distance from the,stove that may be desirable, being joined at the farther extremity, either by a vertical tube, or by the ends of the horizontal tabes being inserted into an open cistern.

This being done, and the fire lighted, the circulation will go on in consequence of the difference in specific gravity between cold water and hot water.

525. A very simple apparatus of this kind is shown in the section, fig. 350.: a is the fire-pot; b is the furnace door; c, the ash-pit door; d, the situation of the pipe which supplies air; e, the nozzle to which the smoke-pipe is attached, and which may be turned in any direction, except downwards, that may be suitable to the situation; f f, the casing of water which surrounds the boiler; g, the upper pipe; h, the under pipe; and i, the cistern which forms the junction between both pipes at the farther extremity. Stoves and pipes of this description may be placed under the stage of plants, so as to be completely concealed from the eye; a portion of the stage being made to separate from the rest, and to draw out, so as to admit the attendant to supply fuel, etc In order to save time, and to look well by never requiring the plants to be taken off the shelves, the moveable portion of the stage may be on casters and small wheels, the latter running in grooves as far as the area of the stage extends, and the casters serving to make it run easily on the paths where the grooves would be unsightly.

In plant cabinets where there is no stage the pipes may be concealed by rockwork, or by some other suitable contrivance; or the caring of water may be enlarged, so as to form a reservoir of heat sufficient for the demands of the house, without any pipes. In some cases, the stove and reservoir of water might be covered with a small stage of plants, with rockwork, with sculpture, or with statuary; or the stove might be rendered ornamental in its form, so as to be regarded as a handsome piece of furniture: but this last plan would, we think, render the plant cabinet too much like a living-room.

526. A portable hot-water apparatus for plant cabinets is shown in fig. 351., which was invented by Mr. Joshua Major of Knowstrop, near Leeds.

This apparatus maybe made of tin or copper; the latter, though more expensive at first, being from its durability much the cheaper in the end. Charcoal is employed as fuel for this apparatus: oil lamps and gas have been tried instead of charcoal, but with not nearly so powerful an effect. When charcoal is used, it is necessary to employ pipes to conduct the effluvium arising from it out of the place to be warmed; and it will be advisable, in order to abstract all the heat possible from this smoke-pipe before it reaches the outside of the house, to have it of a considerable length. In order to render the smoke-tubes suitable for any situation, it is necessary to have elbow pipes, like those used for turning corners, and several lengths of straight pipes, by means of which the piping may be lengthened, and turned in any direction that may be required. The largest-sized apparatus should not be more than 8 ft. long; as, if longer, it would be inconvenient to move about The size of the one which Mr. Major found the most useful is as follows: - The whole height of the centre portion of the apparatus, comprising the boiler, etc, is 15 in., and the width 5 1/4in. by 7 1/2in.; the fire-pan is 5 3/4in. by 4 1/2in., and 3 1/2in. deep; surrounded on three sides by a boiler, in the form of a casing, half an inch in diameter, which becomes more spacious upwards, as the fireplace diminishes.

The opening necessary for the reception of the fire-pan, and for supplying it with fuel, is 6 in. wide by 5 1/2 in. deep. At the top of this opening the fireplace begins to taper, and, consequently, the water in the boiler expands more immediately over the fire; the smoke-pipe takes its regular width (1 1/2 in.) in the boiler, about an inch below where the lid unites; the horizontal water pipes (fig. 325. a) are each 23 in. long, by 2 in. in diameter; the end pipes (b) are 141/4 in. high, by 3 in. in diameter; a feeder (c) is added, in case it should be thought better to have the lid fixed tight on the boiler. In order to promote the circulation of the water, small holes are to be perforated in the top of the lids (dd), which are also intended to be fixed tight. The apparatus may either be placed on the floor of the house to be warmed, or raised by bearers, or suspended by wire or cord; the two latter methods making the fire to burn more freely.

527. A mode of heating employed at Strathfieldsaye, is shown in fig. 352. The apparatus consists of a stove (originally Dr. Arnott's) and two copper cylinders. The stove contains two copper boilers 1 ft. deep, and 3 in wide, which form the fire-box of the stove, out of which the boiling-water flows by the top pipe into the cylinder, and returns by the lower pipe into the bottom of the boiler. The cylinders have each thirty tubes, one inch in diameter, extending through their whole length, and among which the water flows; so that the stove and the two cylinders all radiate heat equal to their surfaces, and warm the air that passes between them. The stove is 18 in. square, and 3 fit. 9 in. high, including the ornamental cap at the top, which is four inches deep. The cylinders are 18 in. in diameter, and the same height as the stove. The apparatus consumes a bushel of coke every day, half being given in the morning, and half at night. The water is given through a covered valve near the top of each cylinder, as shown in the figure. There is a small pipe for evaporation at the back of each cylinder. The ornamental caps are moveable, and conceal the tubes of the cylinder, and the feed-hole of the stove.

The smoke escapes by a tube at the back of the stove, communicating with a flue built in the wall. The apparatus has a very neat appearance, and two of them are found sufficient to heat a conservatory 67 ft. long, 27 ft. wide, and 21 ft. high, so as to preserve the plants from injury by cold or damp. For other modes of heating, see Loudon's Horticulturist, p. 194 to p. 218, where the subject will be found fully discussed.