Fig. 355. is the plan, and fig. 356. a cross section of a span-roofed plant cabinet, with the plant stage in the middle. The stage may either be finished at the end next the entrance with right angles, as at a, or rounded off, as at b; and the further extremity both of the sides of the house, and the end of the stage, may either be semicircular or square, at pleasure. If it were square, a portion of the stage might be made to draw out right and left, to admit the attendant to the fireplace; and if semicircular, as in the plan, the semicircle might be formed into two quadrants, each turning on a pivot at the angle, and opening outwards to a sufficient extent to admit the operator to the stove, as shown in fig. 355. at c. One advantage of this arrangement is, that the stove can never be seen by a visiter; because the attendant, when he is at work at c, cannot escape from that place without shutting at least one of the quadrants. Another advantage is, that, while both quadrants are open to the fullest extent, as at dd, the pots being on the shelves, any person entering the cabinet will rather consider the appearance as the peculiar termination of the stage, than as a convenient arrangement for getting at the stove. There are props for plants at e.

The water-pipes are shown at f, in fig. 356., and the arched rods to the props at g. In severe weather mats can be stretched along the whole extent of the roof, attached to these arches. The entrance from the house is at A, fig. 355.

530. Ventilation is the only remaining point of importance, connected with plant cabinets or small green-houses, which remains to be touched on, and this is highly essential in every situation. In houses 20 or 30 feet long, and from 10 to 15 feet broad, the sashes of the roof should he made to slide, so that the upper ones may be let down at pleasure, and the lower ones drawn up; or they may be hinged at the upper end, and made to lift up. In addition to this, the upright sashes should either be made to open outwards, by being hinged at one side (which, in general, is the best mode) or at the top; or they may slide in two grooves, so that the one can be pushed past the other. These modes are applicable to green-houses 20 or 30 feet in length; but, for those under 20 feet in length, it will generally be sufficient to have one or more small openings in the roof or sides. It must, however, never be forgotten that the atmospheric air forms an important part of the food of plants; and that unless they have a sufficient quantity, they become sickly, and will not produce either fruit or flowers.

One of the simplest and most elegant modes of having an opening in the roof is by having a cap to be raised by means of a vertical rod, with a line and pulley, as shown in fig, 357. In this figure, which is a cross section of a span roof, a represents the cap; b, the rod by which it is raised; c, a cross piece of iron, in which the rod b works; d, the cord passing over the pulley e, for raising and lowering the rod; and f, the rafter (forming part of the span roof) which supports the whole. The rod and bars of the cap are generally of iron; and the panes of glass small, to lessen the risk of breakage. Sometimes the cap is balanced by a weight attached to a cord which passes over a pulley fixed on the other aide of the rod opposite to e; and which, by counterbalancing the greater part of the weight, and leaving little more than the friction by the rod passing through the bar c and the rafter f to be overcome, renders it easy to raise a cap of the largest size, either by hand, or by a self-acting apparatus to be hereafter mentioned.

Though we have shown in fig. 357. this cap on the ridge of a span roof, yet it may be constructed with equal ease on the upper part of the slope of any pent roof, or even in any part of that slope, by introducing on the upper side of it a cross bar, or a flashing of lead, to throw off the rain to the two sides. Air may also be admitted through the side sashes, by hinging a flap in the upper part of any of the sashes, with a lever and pulley to open it, as shown in fig. 358. In this figure g is a lever, which, when pulled by the cord A, raises the flap at pleasure to any point not beyond i. Whenever ventilators of this kind are used, they ought to be placed in the highest part of the house; because as the hot-test air always ascends, it consequently collects there, and will pass off by any opening with greater rapidity than it would through an opening on a lower level, admitting a counter current of fresh air to supply its place.

531. When the sloping sashes of a span roof are made to slide, the operation of opening them may be greatly facilitated by balancing them with a weight in the manner shown in fig. 359. In this figure, a represents cords attached to two opposite sashes, and passing over pulleys fixed on the ridge-piece; after which, under the stage (c), they are joined together at b, which represents a weight attached to a pulley, and which weight is sufficiently heavy to balance the two sashes.

By this arrangement, either sash or both sashes may be let down at pleasure, to any length desired. When they are drawn closely up, the weight (b) is within 2 in. of the floor

(d); but, if it were necessary, an opening might be made in the floor for the descent of the weight to the extent of 2 or 3 feet On inspecting the figure, it will appear evident that a cap, such as that described in fig,

357., may be balanced and raised in a similar manner, as shown in fig.

360., and as practised in the case of chandeliers in churches and ballrooms. In this figure, the weight (e) may have a basket or saucer attached to it, so as to admit of disguising it, by surrounding it with pots of hanging plants, In this case, however, there should be heavier and lighter weights to hook on at pleasure, so that, with the addition of the pots of plants, the total weight should be no more than just sufficient to balance the cap.