PLANTS, WITH GLASS ROOFS. 109 



rigid economy both in heating and lighting. The form of plant-houses, 

 therefore, must be determined by the object in view, and the means at com- 

 mand. For early and for late forcing, narrow houses with upright glass, or 

 glass at a very steep slope, are preferable, as giving but a small volume of air to 

 be heated, and as admitting the sun's rays at a right angle, atthose seasonswhen 

 he is low in the horizon, and above it only for a short time. For summer 

 forcing the angle of the roof may be larger, and of course its slope less steep ; 

 for greenhouses and plant stoves, in which plants are to be grown all the year, 

 there should be a portion of the roof with the glass very steep, or upright 

 front glass, for admitting the sun's rays in winter. The roofs of such houses 

 may be at a large angle, say from 35 to 45 with the horizon, which is more 

 favourable for throwing off rain, and also for resisting 

 hail, than a flatter surface. For growing herbaceous 

 plants and } r oung plants, and for the general pur- 

 poses of propagation, whether by seeds, cuttings, or 

 laj'ers, a low flat house, in which the glass shall be 

 near to all the plants, as in pits and frames, is the 

 most convenient form ; though, when fruits are to be 

 ripened in such houses in the winter season, the 

 flatness of the glass, and consequent obliquity of the 

 sun's rays to it, is a great disadvantage. Hence, 

 when such plants can be conveniently grown in pots, 

 as in the case of strawberries, or bulbous or other 

 flowers, it is desirable to have very steep glass, and 

 to place the plants on shelves immediately within it, 

 as practised by Mr. Wilmot, and other market-gar- Fig .' 127 . ste ~~ fed houte 

 deners, in such structures as fig. 127 ; or, when the for winter forcing of plants 

 plants are climbers, as the cucumber and melon, in pott. 

 training them up trellises parallel to the glass, and at a short distance 

 within it, as in Ayres* cucumber-house. 



483. Curvilineal roofs. The ordinary form of the roofs of plant-houses is 

 that of a right-lined plane, like the roof of any other building, but they have been 

 also formed with curvilineal roofs, which, as compared with roofs having up- 

 right glass with standards and wall-plates, more especially when the sash bar 

 is of iron, admit much more light. The ends of plant-houses are generally 

 vertical planes, but in curvilineal houses they are sometimes of the same 

 curvature as the front, which adds greatly to their beauty, as well as being 

 favourable to the admission of the sun's rays, morning and evening, and to 

 the transmission of diffused light when the sun does not shine (282). The 

 only disadvantages attending curvilineal ends to plant-houses, is, that the 

 doors cannot be placed in these ends without some intricacy of construction ; 

 but when such houses are placed against walls, as in fig. 128, they may be 



e 



Fig. 128. Curvilineal glass roofs. 



entered through a door made in the wall, to a recess taken from the back 

 shed, as shown by fig. 129, in which a, a, represent the plans of portions of 



o 



