The construction of glass houses has undergone much improvement during the last twenty years. The change from the angular to the curvilinear shape, introduced a simple and superior method of forming glazed roofs. Not that the curved outline does, in itself, possess any important advantages; but the system of glazing with large glass in slight fixed rafters, not only admitted more light, but was cheaper than the older method of heavy rafters and sliding sashes. The expense consequent upon the formation of curved rafters, did not, in effect, render these houses less expensive than the old method, but there was the gain of more light and less opacity, and this, I am rather inclined to believe, is the only superiority such houses possess; and, that angular houses constructed on the same principle are much cheaper and equally efficient.

I have for several years adopted this system of glazing roofs, and as it is the cheapest and best mode of erection that I know of, I annex a description of the manner I usually have them put up; so that those who have been deterred from erecting graperies and green-houses at eight and twelve dollars per foot in length, may be enabled te do so at one-half of these rates.

Single roofed cold graperies may be put up in a rough but substantial manner at even lower rates than these, but if the best American glass is used (as it ought always to be) and the wood work planed and painted outside, they will cost from four to six dollars a foot in length, varying in price according to the height and width of the building, and the material used for foundation.

Fig. 1, is a section of rafter one-half of full size. These are made out of sound, clean, inch thick best white pine boards, sawn out in strips three inches wide, and prepared for glazing. They are then fitted on the bottom and top wall plates, and permanently fixed. Their distance apart will depend upon the size of glass; I find 10 X 14 a convenient sized pane; to suit this the rafters will require to be 14 1/2 inches apart from centre to centre. In glazing, the concave surface of the glass should be turned outwards, to throw the water into the centre of the pane. There will be little or no leakage or drip in a bouse glazed in this way. Fig. 2, represents part of a roof fixed for glazing. About the middle of the roof a purlin is ran across and supported by uprights and posts; any degree of strength can thus be given. Ventilation is secured by hinged shutters or glazed sashes near the top of the roof, and by openings in the front wall below the glased portion. Ample space for airing ought in all cases to be provided. A span roofed house should have means of ventilating to the extent of a three feet opening the whole length of the house.

The top ventilators shut down on the rafters, they are hinged to the ridge, and to prevent leakage, they shut into a slight groove, as shown in Fig. 3. Houses constructed in this manner are not only neater in appearance, but are better adapted as plant habitations, than those built with heavy rafters and sliding sashes.

Fig. 1.

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Fig. 2.

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Fig. 3.

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