Where horse-dung is employed, a trench should be dug, of a width and depth proportioned to the size of the frames intended to be used ; and which, in dry ground, ought to be a foot, or a 'foot and a half, deep; but, if the soil be wet, it should net exceed six inches. The dung is then to be spread even and smooth 0n every part of the bed, laying the finer manure on the surface: if the bed be intended for planting out cucumbers, a hole should be made, about ten inches broad, and six inches deep, in the middle of the place destined for each light, and then filled up with good fresh earth. The bed is next to be covered, to the depth of four inches, with the earth taken out of the trench, and the frame fixed over it, to remain till the earth become warm, which commonly takes place in the course of 3 or 4 days after the bed is made; when the cucumbers may be planted.
In case the hot-bed be designed for other plants, it will not be necessary to make holes in the dung ; but, after levelling the surface, good earth ought to be spread over it, to the depth of three or four inches ; the frames and glasses being put on as before. In making such beds, the dung should be settled close with a fork ; and if it be full of long litter, it must be trod down equally in every part In the first week, or ten days, after the hot-bed is made, the glasses should be slightly covered during the night, and cautiously opened in the day time, to give vent to the steam; but, as soon as the heat abates, the covering should be increased by mats or straw; and, when the bed becomes cold, fresh dung should be applied to its sides.
Hot-beds made with tanners' bark are preferable to those above described, especially for tender exotic plants and fruits; as they require a more equal warmth than can be produced by horse-dung. The method of making them is as follows: a trench is dug about three feet deep, if the ground be dry; but, if the soil be wet, it ought not to exceed the depth of a foot, and should be raised two feet-above the ground. Their size must be in proportion to the frames intended to cover them; though they ought to extend at least 10or 12 feet in length, and six feet in width. The trench should be lined with bricks on each side, to the height of three feet, and filled in the spring with fresh tanners' bark, which should be previously thrown up into a round heap, in order to drain for three or four days. When the tan is laid on, it ought to be gently beaten down with a dung-fork; for, if it be trodden in, it will be prevented from heating, as it settles too close. The frame and glasses are now to be fixed ; and, in the course of ten days or a fortnight, the bud will grow hot, when pots or plants of seed may be plunged in it; care being taken that the bark be not compressed. These beds will preserve a proper temperature of heat for three or four months, which may be continued two or three months longer, by adding a load or two of fresh bark, as often as the warmth begins to decrease.
Frames vary in size, according to the plants they are destined to cover. If designed for ananas or pine-apples, the back should be three feet high, the lower part fifteen inches: when the bed is intended for taller plants, the frame must be made proportionally higher; if for seeds only, it will not be necessary to employ frames more than fourteen inches in height at the back, and seven in the front. Thus, the heat will be increased, and the growth of the plants considerably promoted.