In the kitchen-garden Mr. Paxton is introducing a new mode of covering glass cases, whether frames, pits, or low houses, during the night. This is simply by having a thatched roof of somewhat larger dimensions than the frame, pit, or house to be covered, resting on side walls, and independent of those which support the glass; the lower edge or base of this roof slides on a railway, which extends at either or at both ends of the house, so as to afford space for the roof to stand on in the day time, or when it is not wanted. The advantages of this mode of covering are, that more heat can be retained than by mats or boards; and that the covering and uncovering can be effected with less labour, and almost instantaneously. The house on which the experiment is about to be tried is for growing orchidaceous plants, and is being heated by Mr. Penn. It is span-roofed, and stands in the direction of north and south: fig. 67. is a section, in which a is the glass roof, and b the thatched roof. The situation is low, and being on a level with the river, is incapable of drainage beyond a certain depth; in consequence of which, a water-tight cast-iron box, or caissoon, is sunk, and in it the furnace and boiler are built. This practice is common in Holland even for dwelling-houses; the lower rooms of which, being often below the level of the adjoining canals, have a thick flooring and thick side walls of masonry, built with cement, which completely exclude water. The sash-bar used in this orchidaceous house has side gutters for collecting the drip, as shown in the section, fig. 68., which is of the full size. The glass is intermediate between the two thicknesses, shown in fig. 65. p. 572., and is 6 in. wide, in panes not exceeding 40 in. in length; the cost of which, glazing included, is about 1s. 4d. a square foot. In Germany and Holland, plant structures with upright glass in front, as in the Botanic Gardens at Munich and Leyden, are covered with great rapidity by letting down rolls of straw mats, as noticed in our vol. for 1830; and where the glass roof slopes, hinged shutters, suspended from the back wall by cords and pulleys, are instantly let down, and as quickly pulled up, as in the Botanic Gardens at Carlsruhe. These modes, however, are not so well adapted for this country, where few houses have opaque roofs and only front glasses; and equally few those elevated projections called bonnet roofs, shown in fig. 69., in which a is the point where the shutters are hinged.
