. The book of the garden. Gardening. COVERING GLASS ROOFS. 551 thought of by self-styled hothouse- builders, who, in too many cases, show as slight an acquaintance with the rules of carpentry as they do of the principles upon which hothouse-building should be based. Indeed, looking at many specimens of their handiwork, one would think their ideas were stereotyped to error. When the parapet walls are built of brick or rubble stone, inch-iron dowels should be built in them, at 10 feet distances, let into the wall not less than 12 inches, and upwards into the wall-plate at least to the extent of
. The book of the garden. Gardening. COVERING GLASS ROOFS. 551 thought of by self-styled hothouse- builders, who, in too many cases, show as slight an acquaintance with the rules of carpentry as they do of the principles upon which hothouse-building should be based. Indeed, looking at many specimens of their handiwork, one would think their ideas were stereotyped to error. When the parapet walls are built of brick or rubble stone, inch-iron dowels should be built in them, at 10 feet distances, let into the wall not less than 12 inches, and upwards into the wall-plate at least to the extent of half their thickness: by this means the superstructure and the walls will be firmly knit together. If the parapets are of ashlar, the dowels should be sunk 6 inches into the top course. Top wall-plates in lean-to houses have hitherto been little attended to; they have in general been merely a 4 or 6 inch square batten, built into the solid wall at top, and surrounded with bricks or stones and mortar. To these the top ends of the rafters have been nailed or otherwise fastened. The ridge beams of span-roofed houses may be regarded as wall-plates, as they answer a similar purpose. Fig. 787 Fig. shows an improved form, with its coping, and the manner in which the top ends of the rafters are secured to it. Such ridge beams are only to be used where the roof is composed of rafters and sashes, the top ones of which are to slide up and down for ventilation. Their form is, however, different when ventilation is effected at that part, and has been else- where explained. § 5.—COVERING THE ROOFS OF GLASS HOUSES AND PITS, FOR THE EXCLU- SION OF COLD OR THE RETENTION OF HEAT. The advantages arising from covering glass roofs appear to have been to a cer- tain extent known, and partially acted upon, perhaps as early as the beginning of the last century. At all events, speak- ing from our own personal knowledge, we have seen hothouses covered during win- ter fifty years ago ; and, judg
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